Annual Report 2016 - Jülich Aachen Research Alliance (JARA) Annual Report 2016 Annual Report 2016...

Annual Report 2016

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AN INITIATIVE OFAN INITIATIVE OF

JARA-FITJülich Aachen Research Alliancefor Fundamentals ofFuture Information Technology

OfficeForschungszentrum Jülich GmbH52425 JülichGermany

Phone: ++49-2461-61-3107Email: [email protected]

Publication Details

JARA-FITJülich Aachen Research Alliancefor Fundamentals of Future Information TechnologyAnnual Report 2016

Published by:Forschungszentrum Jülich GmbH, 52425 JülichRWTH Aachen University, 52062 Aachen

Publication supported by the Excellence Initiative of the German federal and state governments.

Editors:Dr. Wolfgang SpeierManaging Director JARA-FITForschungszentrum Jülich GmbH52425 JülichGermanyPhone: ++49-2461-61-3107

Prof. Dr. Stefan TautzScientific Director JARA-FITPeter Grünberg Institute – Functional Nanostructures at Surfaces Forschungszentrum Jülich GmbH52425 JülichGermanyPhone: ++49-2461-61-4561

Prof. Dr. Matthias Wuttig Scientific Director JARA-FIT I. Institute of Physics A RWTH Aachen University 52074 Aachen Germany Phone: ++49-241-8027155

Contact:Dr. Wolfgang [email protected]

Layout:Ulrike Adomeit Silke Schilling

Year of publication: 2017 Cover pictures refer to the following selected research reports (From left to right)

E. Mlynczak et al., Fermi surface manipulation by external magnetic field demonstrated for a prototypical ferro-magnet, p. 119 | M. dos Santos Dias et al., Orbital magnetism as a fingerprint of topological magnetic structures, p. 49 | S. Heedt et al., Ballistic transport in InAs nanowire quantum point contacts, p. 33 | Y. Xiao et al., Spin-wave and electromagnon dispersions in multiferroic MnWO4 as observed by neutron spectroscopy, p. 87

2017

JARA-FIT Annual Report 2016

JARA-FIT

Jülich Aachen Research Alliance for

Fundamentals of Future Information Technology

Annual Report 2016

Forschungszentrum Jülich

RWTH Aachen University


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Dear Reader,

JARA, the Jülich Aachen Research Alliance, was founded ten years ago by signing a contract between

the RWTH Aachen University and the Forschungszentrum Jülich. Very recently, both institutions and

JARA took the opportunity to celebrate this occasion during a JARA Day. For JARA-FIT, the section

dedicated to the fundamentals of future information technology, the founding year also marked an exciting

year since the nobel prize for physics 2007 was awarded to Peter Grünberg.

JARA-FIT has expanded over this period of time in dedicated fields by means of a joint strategic planning

of research, coordinated teaching activities and infrastructure development. Today, JARA-FIT is a

collaborative and cross-disciplinary effort of 32 institutes and chairs. We are active in joint appointment

procedures, joint infrastructure planning, and the establishment of joint research groups located at both

sites, thus harmonizing the general strategy of both institutions in our research area. Important examples

of the success of JARA-FIT are the establishment of the DFG-funded Collaborative Research Centre

“Nanoswitches”, the DFG-funded Research Training Group “Quantum many-body methods in condensed

matter systems”, ten ERC grants received by its scientists, the participation in the EU Flagship Graphene

as well as the establishment of infrastructure such as the Helmholtz Nanoelectronic Facility (HNF) in

Jülich or PICO at the jointly operated Ernst Ruska Center for Electron Microscopy. Moreover, new

institutes were founded: the Virtual Institute for Topological Insulators and, very recently, two dedicated

JARA-Institutes in the field of energy-efficient information technology and quantum information.

Quantum information is a good example of how the research in JARA-FIT has developed and the field is

quickly progressing. Back in 2007, scientists of JARA-FIT had just acquired the DFG-funded Research

Unit “Coherence and relaxation of electron spins” bundling the activities in physics of spin-based quantum

systems. This has brought great insights into the spin relaxation and coherence of various systems

including semiconductors and carbon-based materials. With the recruitment of renowned scientists and

the foundation of the Institute for Quantum Information the topic has now extended, both experimentally

and theoretically, towards a scalable solid-state quantum computing. In 2017, the institute has launched a

project that aims to take our existing state-of-the-art results in solid state qubits and extend them to the

creation of multi-qubit systems aiming to demonstrate many aspects of the scalability of such systems. As

highlighted below, this brings together the physics-based quantum scientists with the colleagues from

electrical engineering, a stronghold of JARA-FIT, as well as with experts from Karlsruhe. In fact, one can

observe a world-wide upsurge of activities towards the realization of condensed matter quantum

information devices. Even major companies such as Google and IBM have joined the global effort and the

European Union has announced the launch of the Quantum Technologies Flagship. JARA-FIT is taking

an active role in this exciting endeavour.

We hope that the present report summarises some of the most interesting findings we have achieved in

the last year and gives you a flavour of the enthusiastic research undertaken within JARA-FIT.

Stefan Tautz Matthias Wuttig Wolfgang Speier Scientific Director Scientific Director Managing Director


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Contents

JARA‐FIT Highlights ...................................................................................................................... 5

General Information................................................................................................................... 13

JARA-FIT Members ...................................................................................................................... 14

JARA-FIT Institutes ...................................................................................................................... 16

Selected Research Reports ......................................................................................................... 25

Structural characterization of ultrathin topological insulator Sb2Te3 /Bi2Te3 stacks ....................... 27

Protected by time‐reversal and mirror symmetry: the dual topological insulator Bi1Te1 ................ 29

Search for the mysterious SiTe – an examination of the binary Si−Te system using first principles‐based methods ................................................................................................................ 31

Ballistic transport in InAs nanowire quantum point contacts ......................................................... 33

Lazarevicite‐type short‐range ordering in ternary III‐V nanowires ................................................. 35

Complementary Si nanowire GAA TFETs inverter with suppressed ambipolarity ............................ 37

Reconfigurable multi‐gate transistors ............................................................................................. 39

Phase diagram of Eu magnetic ordering in Sn‐flux‐grown Eu(Fe1‐xCox)2As2 single crystals ............. 41

Study of free charge carrier distributions in electrically contacted InAs nanowires with infrared s‐SNOM .............................................................................................................................. 43

GeFe3N0.6: an itinerant nitride with a frustrated magnetic ground state ........................................ 45

Suppressing relaxation in superconducting qubits by quasiparticle pumping ................................. 47

Orbital magnetism as a fingerprint of topological magnetic structures ......................................... 49

Dynamic stability of topological spin structures governed by Landau‐Lifshitz‐Gilbert equations ... 51

Topological orbital ferromagnetism driven by chiral spin textures ................................................. 53

Self‐energy effect on interaction‐driven phase transitions in mono‐ and bilayer graphene ........... 55

Competing pairing channels in the doped honeycomb lattice Hubbard model ............................... 57

Defects in graphene nanostructures ............................................................................................... 59

High quality CVD graphene devices fabricated by a dry transfer method ....................................... 61

Flexible graphene devices for extracellular measurements............................................................. 63

Dual‐SQUIDs with graphoepitaxial step‐edge Josephson junctions ................................................ 65

GeSn microdisk lasers on Si ............................................................................................................. 67

Adiabatic demagnetization refrigeration for ultra‐high vacuum: Magnetothermal investigation of Gd(HCOO)3 and YbPt2In ............................................................. 69

Electrical properties of gold nanowire arrays made by microstructured hydrogel templates ........ 71

Fabrication of organometal halide perovskite layers via chemical vapor deposition .................... 73

Field‐induced self‐assembly of iron oxide nanoparticles ................................................................. 75

KKRnano: Towards precise large‐scale density‐functional calculations .......................................... 77

Quantum interference effects in molecular spin hybrids ................................................................. 79

Molecular characteristics of V18O42 nanoclusters in solution and adsorbed on the Au(111) surface ............................................................................................................................... 81


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On the formation of ZnO patches on a ZnPd/ZnO catalyst during methanol steam reforming ...... 83

Néel‐like domain walls in ferroelectric Pb(Zr,Ti)O3 single crystals .................................................. 85

Spin‐wave and electromagnon dispersions in multiferroic MnWO4 as observed by neutron spectroscopy.................................................................................................................................... 87

Neutron diffraction to distinguish between symmetry lowering and Renninger effect: An example of multiferroic Ba2CoGe2O7 .......................................................................................... 89

Mott transition and spin‐orbit effects in Ca2RuO4 ........................................................................... 91

Surface atomic structure and growth mechanism of monodisperse 100‐faceted strontium titanate zirconate nanocubes .......................................................................................................... 93

An electric field triggered thermal runaway model for volatile resistive threshold switching in NbO2 ............................................................................................................................................ 95

On the interrelation of gradual and abrupt SET switching in valence change memory cells .......... 97

Dynamic modelling of the RESET process in valence change memory cells .................................... 99

Dynamics of the metal‐insulator transition in donor‐doped SrTiO3 .............................................. 101

Investigating the resistive switching properties of ultrathin TaOx films with Scanning Tunneling Microscopy ................................................................................................................... 103

Quantifying redox‐induced Schottky barrier variations in memristive devices via operando spectromicroscopy with graphene electrodes ............................................................................... 105

Hafnium carbide formation in oxygen deficient hafnium oxide thin films .................................... 107

Space charge effects at complex oxide interfaces ......................................................................... 109

Impact of oxygen exchange reaction at the ohmic interface in Ta2O5‐based ReRAM devices ...... 111

Impact of defect occupation on current‐voltage characteristics in amorphous Ge2Sb2Te5 ........... 113

Study of time resolved plasma dynamics in a hollow cathode Z‐pinch EUV source ...................... 115

Strain compensated ZnSe/CdSe/ZnSe quantum wells ................................................................... 117

Fermi surface manipulation by external magnetic field demonstrated for a prototypical ferromagnet .................................................................................................................................. 119

Challenges of neuromorphic computing ....................................................................................... 121

Stochastic optimization of a function‐specific nanoelectronic 1R1S‐based binary associative connection structure ..................................................................................................................... 123

Publications .......................................................................................................................... 125


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JARA-FIT Highlights

Helmholtz Association launches project on scalable solid state quantum computers headed by JARA-FIT

The Helmholtz Association has selected a project on “Scalable Solid State Quantum Computing” as one of its new innovative topical research projects with strategic importance for the development and strengthening of cooperation within the Helmnholtz Association. The project aims to establish the conditions for future multi-qubit systems. In order to realize such systems with potentially millions of qubits in the long run, new technologies are required so that the qubits can be precisely controlled in such large numbers. Forschungszentrum Jülich, the Karlsruhe Institute of Technology and RWTH Aachen University are all involved in the project, which is being provided with € 6 million in funding by the Helmholtz Association for three years. The project is headed by the JARA Institute for Quantum Information.

While many of the basic requirements for realizing quantum processors have been demonstrated, the scale required for practical applications is several orders of magnitude larger than that of current experiments. Making a quantum computer is an immense scientific and technological challenge that needs to be addressed in a concerted, interdisciplinary and long term effort requiring a substantial critical mass. This project aims to initiate that process by bringing together quantum scientists and engineers with highly relevant expertise. The central scientific theme will be to take our existing state-of-the-art results in solid state qubits and extend them to the creation of multi-qubit modules whose size is large enough to demonstrate many aspects of the ultimately scalability of these systems. In the project this will be followed up for two major solid-state qubit types, based on superconducting (Josephson) as well as semiconducting (spin) devices. Assemblies of relatively small numbers of devices will already be capable of enacting quantum simulations, presaging the large-scale applications that we foresee in this area. However, to reach qubit systems of the next level of scale (beyond100s of qubits), new techniques will be needed that come directly from the disciplines of circuit engineering and systems architecture. A work package will bring together a team which will focus on the problem of constructing workable electronic control systems. A demonstrator involving the operation of 50 qubits is targeted. Both low-temperature and room temperature solutions will be sought, with attention to the necessity of reducing the power consumption of this control system by many orders of magnitude compared with current possible solutions. Every work package will be strongly supported by the world-leading theory and simulation capabilities at the involved institutions, continuing our tradition of strong theoretical guidance at all levels of the experimental and engineering investigations.

Delft, Aachen and Jülich join forces to build scalable quantum technologies

Quantum science is rapidly evolving towards greater technological maturity throughout the world. To remain at the forefront of this exciting field, collaborations are crucial. The QuTech institute in Delft as well as Forschungszentrum Jülich and RWTH Aachen University have intensified their collaboration through an official agreement. The partners aim to enhance scientific and technical developments in the field of solid-state quantum computing and to conduct joint campaigns within the framework of the EU flagship on quantum technologies.

The signed agreement represents a step towards establishing strong collaborations between Europe's key players in solid-state quantum information processing and high-performance computing. The partners will combine their skills and knowledge with respect to understanding and fabricating quantum systems with a large number of controllable and reliable qubits. They will also cooperate in the field of scalable solid-state quantum computing to pool their unique skills concerning scalable quantum information processing, the information technology of the future.

© TU Delft / Judith de Keijzer


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JARA-FIT Science Days 2016

The JARA-FIT Science Days serve the purpose of providing a forum for intense scientific exchange among JARA-FIT scientists and initiation of new interdisciplinary projects. Participants are PhD students, PostDocs and scientists from RWTH Aachen University and Forschungszentrum Jülich. Traditionally, the JARA-FIT Science are held in in a retreat in Schleiden, a small town in the Eifel mountains close to Aachen and Jülich. The Science Days in 2016 brought 110 scientists together for two days, on 11 and 12 November 2016. The two days were filled with tutorial lectures, presentations and a poster session,

with special focus on topics including “Quantum Information and Computing”, “Neuromorphic Computing” or “Advanced 2D Materials”. The sessions were organized in order to foster discussion in these currently highly active fields, also with a strategic perspective for the development of JARA-FIT. The programme of the Science Days was developed by Hendrik Bluhm (RWTH), Detlev Grützmacher (Jülich & RWTH), Christoph Stampfer (RWTH & Jülich), and Wolfgang Speier (JARA-FIT).

During the Science Days poster prizes were awarded to: Christoph Bäumer for his work on "Elucidation and quantification of valence changes in memristive

SrTiO3 devices" (1st poster prize) Daniel Rosenbach for his work „Induced superconductivity in lateral topological Josephson junctions

with (Bi0.07Sb0.93)2Te3 interlayer” (1st poster prize) Marcus Liebmann for his work on “Probing variations of the Rashba spin-orbit coupling at the

nanometer scale” (2nd poster prize) Rebecca Liffmann for her work on “Polydiacetylene Stabilized Goldnanoparticles - Extraordinary High

Stability and Integration into a Nanoelectrode Device” (2nd poster prize) Nils von den Driesch for his work on “Light Emitting Diodes from Group IV GeSn Alloys” (2nd poster

prize)

48th IFF Spring School Topological Matter - Topological Insulators, Skyrmions and Majoranas

Around 290 early-career scientists from Germany and abroad attended the 48th IFF Spring School on “Topological Matter - Topological Insulators, Skyrmions and Majoranas” held at Forschungszentrum Jülich from 27 March to 7 April 2016. Topology is the branch of mathematics that deals with properties of spaces that are invariant under smooth deformations. It provides newly appreciated mathematical tools in condensed matter physics that are currently revolutionizing the field of quantum matter and materials. Topology dictates that if two different Hamiltonians can be smoothly deformed into each other they give rise to many common physical properties and states are homotopy invariant. Thus, topological invariance, which is often protected by discrete symmetries, provides some robustness that translates into the quantization of properties; such a robust quantization motivates the search and discovery of new topological matter. So far, the mainstream of modern topological condensed matter physics relies on two profoundly different scenarios: the emergence of the complex topology either in real space, as manifested e.g. in non-trivial magnetic structures or in momentum space, finding its realization in such materials as topological and Chern


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insulators. The latter renowned class of solids attracted considerable attention in recent years owing to its fascinating properties of spin momentum locking, emergence of topologically protected surface/edge states governed by Dirac physics, as well as the quantization of Hall conductance and the discovery of the quantum spin Hall effect. Historically, the discovery of topological insulators gave rise to the discovery of a whole plethora of topologically non-trivial materials such as Weyl semimetals or topological superconductors, relevant in the context of the realization of Majorana fermions and topological quantum computation. At the same time, the physics of skyrmions with complex magnetic real-space topologies is rapidly moving to the centre of attention in spintronics owing to the bright prospects of skyrmionic materials related to topological protection and robust dynamics. The discovery of skyrmions in various geometries (bulk, thin films, interfaces), the complex interplay of their properties with their topology, the fascinating aspects of their dynamics and transport properties are believed to result in skyrmions and other topological spin structures as basic building blocks for information manipulation and storage. Overall, the expanding manifold of materials, phenomena and concepts, which are born from the combination of ideas and methods of topological characterization and geometrical analysis with the most advanced developments in modern solid state physics, marks one of the most exciting moments in the history of physics related to a paradigm shift in our understanding of matter.

The spring school covered an interdisciplinary spectrum of topics ranging from the theoretical and experimental physical fundamentals of the research area to state-of-the-art analytical methods as well as current research findings and projects. More than 30 experts in the field, coming from Jülich, Cologne, Aachen, and other German and international research institutions, held around 50 hours’ worth of lectures on the topic during the course of the spring school. Campus tours in Jülich complemented the programme and permited the early-career researchers to also learn about experimental facilities and laboratories of related scientific disciplines. The first of these spring schools was held in 1970 and has since then been organized by several institutes of Forschungszentrum Jülich, with a different physical focus every year. The name of the spring school refers to the former Institute of Solid State Research (Institut für Festkörperforschung, IFF) which initially launched the project and organized a total of 41 spring schools. The book with lecture notes is available through the Verlag des Forschungszentrum Jülich (ISBN ISBN 978-3-95806-202-3) as well as by Open Access.

Autumn School on Correlated Electrons: Quantum Materials - Experiments and Theory

More than 100 young researchers working in the field of strongly correlated materials visited Jülich during the week from 12 to 16 September 2016. Continuing the successful tradition, the 6th Autumn School on Correlated Electrons offered lectures by internationally recognized scientists aimed at bringing young investigators quickly up to speed for pursuing original research of their own. The format of the lectures plus ample time for discussions provided a thorough introduction to modern areas of research. The School was led by Eva Pavarini from the Institute for Advanced Simulation (IAS) and Erik Koch from the Jülich Supercomputer Centre (JSC).

The lectures addressed the physics of strongly-correlated materials from the theoretical and the experimental side. Experimental lectures highlighted the capabilities and achievements of spectroscopic methods using light, x-rays, NMR and tunnelling. Theoretical lectures ranged from model-building and mean-field approaches to non-perturbative methods and applications to real materials. Students enthusiastically took the opportunity to interact with the scientists that taught at the school. In addition, a poster session allowed them to present their projects and to expand their network in the global research community, represented, besides Germany and other EU countries, by participants from Turkey, Iran, South Africa, India, China, Singapore, Argentina, the USA, and Canada. Many of the lecturers were generously supported by the DFG Research Unit 1346 "Dynamical Mean-Field Approach with Predictive Power for Strongly Correlated Systems”.


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Moreover, some international participants received travel awards from the Institute for Complex Adaptive Matter, ICAM. To enhance the impact of the courses, comprehensive lecture notes were published as a book through the Verlag des Forschungszentrum Jülich (ISBN 978-3-95806-159-0). These lecture notes will help to fill the acute gap between introductory textbooks and the research literature as evident from the high demand also arising from outside the school. To be as widely accessible as possible, the book has been made available via Open Access.

US-German Networking Activity on Microscopy in Materials Science and Engineering

A symposium on ”Advanced and In-Situ Microscopies in Materials Science and Engineering” was held as part of US-German Networking Activities on Materials Science and Engineering at the MSE 2016, held in Darmstadt from 27 to 29 September 2016. The symposium organisers were Rafal E. Dunin-Borkowski and Joachim Mayer from the Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons in Forschungszentrum Jülich and RWTH Aachen University together with Wolfgang Jäger from the Christian-Albrechts-University in Kiel (Germany) and Guillermo Solórzano from the PUC Pontifical Catholic University Rio de Janeiro (Brazil). The symposium provided a forum for researchers who are interested in applying advanced methods of electron microscopy and spectroscopy, including aberration corrected electron microscopy and in situ characterisation methods, to materials research and development for electronics, optics, magnetics and for energy and the environment. In 5 exceptionally-well-attended sessions, a wide spectrum of topics was covered, including nanostructured functional materials, soft matter, materials in bioscience and structural materials for industrial engineering. The symposium also provided a platform for further initiatives to form research collaborations and partnerships. Current research topics were highlighted in keynote presentations given by leading invited experts from different institutions from the US and from European institutes. The scientific presentations addressed topical research fields that are of fundamental importance for understanding the chemical and physical properties of materials and evaluating their potential for technological applications, including: (1) Advanced materials research using high-resolution TEM (HRTEM), spectroscopy in the TEM and atom probe tomography: impact of aberration-corrected HRTEM, spectroscopic investigations of plasmon properties in nanoscopic and mesoscopic metals, nanoscale heat conduction using in-situ thermal measurement techniques and correlative microscopy of internal interfaces in semiconductor materials and devices; (2) Dynamic and time-resolved TEM: atomic resolution dynamic observations of grain boundaries, rapid nanoscale materials processes with high time resolution by in-situ TEM and in-situ electrochemistry in the TEM; (3) Environmental (E)TEM, low voltage EM and in-situ scanning (S)TEM: in-situ and environmental (E)TEM for material reactions, low voltage EM for quantum materials research and in-situ TEM studies of fundamental processes in thin films using chip-based heating systems; (4) Functional nanomaterials and devices: Resolving the interplay of nanostructures and mechanical properties by advanced EM, structural control of graphene and further two-dimensional materials for new functional materials, localised electrical resistance of supported rGO measured using in-situ TEM and complex metal hydride systems for hydrogen storage; (5) Metallic alloys and composite materials: application of aberration-corrected electron microscopy to the characterisation of phase transformations and microstructural evolution in complex metallic alloys, size focusing effect in the precipitation of core/shell nanophases in aluminum alloys, in-situ TEM of core-shell formation in Bi-Na-Ti-O/Sr-Ti-O nanoparticles and analytical (S)TEM of sensitive composite organic-inorganic materials.


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1st Sino-German Symposium on Advanced Electron Microscopy and Spectroscopy in Materials Science

The 1st Sino-German Symposium on Advanced Electron Microscopy and Spectroscopy in Materials Science was held in and supported financially by the Chinesisch-Deutsche Zentrum für Wissenschaftsförderung (CDZ) in Beijing. The 4-day symposium was organised by Rafal Dunin-Borkowski from the Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons in Forschungszentrum Jülich and Rong Yu, Jing Zhu and Xiaoyan Zhong from the National Center for Electron Microscopy in Beijing at Tsinghua University. The event attracted more than 130 participants from more than 30 research institutes and universities, including more than 90 students and young scholars. During 15 scientific sessions that comprised more than 40 presentations given by invited experts from China, Germany, Singapore and the United States, the participants discussed current developments and challenges in advanced electron microscopy and spectroscopy, including aberration-corrected electron microscopy, in situ characterisation methods and their applications to current and future materials science problems and to the processing of materials for the future development of materials and devices.

A wide spectrum of methodological and materials research topics was covered, including (1) Imaging and spectroscopic methods of aberration-corrected high-resolution and scanning transmission electron microscopy (TEM) and related novel developments, such as low-voltage electron microscopy and dynamic and time-resolved TEM, (2) in situ and environmental transmission electron microscopy involving nanometre-scale investigations of materials reactions and processes at different temperatures in ultraclean environment, in gases, or in liquids, (3) the development of novel methods and instrumentation for the in situ manipulation and measurements of nanomaterials, as well as (4) applications of electron microscopy and spectroscopy methods to advanced materials research on structural and functional materials, including two-dimensional materials, soft materials, materials for applications in bioscience, nanostructured functional materials, devices, metallic alloys, composite materials and structural materials for industrial engineering. The symposium presented insights into many current research efforts in materials for energy technology, nanotechnology, future nanoelectronics, transport, product developments, soft matter in structural biology, medicine and the environment. The novel developments in instrumentation and materials that were presented documented the huge progress that has been made possible by applying the methods of aberration-corrected electron microscopy to the characterisation and understanding of novel materials and devices and their properties. Based on the presentations, it was clear that the use of correlative approaches that involve the application of different techniques to the same problem can be used to provide an improved understanding of the fundamental properties of structures and mechanisms on the atomic and molecular scale.

PICO 2017 - Frontiers of Aberration Corrected Electron Microscopy

PICO 2017, the fourth Conference on Frontiers of Aberration Corrected Electron Microscopy, took place at Kasteel Vaalsbroek between 30 April and 4 May 2017. The meeting was attended by more than 150 delegates including company representatives and a good number of international colleagues. Organisers put together an oral programme of 46 scientific keynote lectures. About the same number of contributions were scheduled for poster presentations. The event was organised by the Ernst Ruska-Centre in Aachen and Jülich and supported by industry, including Thermo Fisher Scientific, CEOS GmbH, DENSsolutions BV, Gatan GmbH, JEOL (Germany) GmbH, Hitachi High-Technologies Europe GmbH, NanoMEGAS SPRL, Protochips Inc., and Nion Company. The scientific programme of PICO 2017 contained an wide range of presentations focusing on recent advances in methods and applications for the study of structural and electronic properties of solids by the application of advanced electron microscopy techniques. Topical issues of aberration corrected electron microscopy research on (1) novel techniques and approaches, (2) advanced methods and instrumentation, (3) interface phenomena and lattice imperfections as well as (4) dynamic phenomena and in-situ techniques were highlighted in keynote presentations given by leading invited experts. Conference proceedings have been published in a special issue of Ultramicroscopy.


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VITI Meeting 2017 in Shanghai

The 2017 meeting of the Virtual Institute for Topological Insulators took place in Shanghai on 20 and 21 April 2017. During the workshop 40 participants discussed different research aspects regarding two- and three-dimensional topological insulators. Special focus was laid on topological insulator superconductor hybrid structures. During the meeting the participants had a chance to visit labs at Jiaotong University in Shanghai and at SIMIT. The VITI is a Virtual Institute where different experimental groups from Forschungszentrum Jülich, RWTH

Aachen University, Würzburg University and the SIMIT in Shanghai are collaborating closely in the field of material development, characterization and electron transport. The VITI is financed by the Helmholtz Association.

Helmholtz funds "Nanoprober made in Jülich"

The Helmholtz Validation Fund funds projects that show particular promise to transfer research results to market-ready products. The project “Nanoprober for the semiconductor industry” developed by JARA-FIT scientist Bert Voigtländer of the Peter Grünberg Institute – Functional Nanostructures at Surfaces has been chosen for funding. Within this project, a unique, combined AFM/SEM nanoprober developed in Jülich, will be validated for it’s use in semiconductor failure analysis. Together with the project partner Global Foundries (Dresden), probing of individual transistors from current technology nodes will be performed. The project budget is € 0.85 million in funding over the next two years. After successful validation a spin-off company will be established.

Smart sensors for food packaging

An interdisciplinary team of young scientists from Bioelectronics at the Peter Grünberg Institut-8 and Institute of Complex Systems-8 consisting of Alexey Yakushenko, Jan Hendrik Schnitker, and Marcel Grein have developed a cheap electrochemical sensor for food packaging which can determine the freshness of food in real time. The innovation is based on the possibility to construct (ultra) low-cost sensors by making use of novel printed electronics techniques and special functional inks. The idea is to digitalize food logistics through in-package electronics and to bring a “lab on a chip” for individual packages. Based on the printed, chip-based sensor attached to the package, the product freshness can be monitored in real-time. Even more, the food-safe sensor can be in direct contact with the packaged contents and thus could perform physical and (bio-) chemical measurements such as temperature, pH as well as the fill level. The team receives funding by the Helmholtz Validation Fund and claimed second place at the AC² competition 2016 for new start-ups.

Phyphox – Scientists from JARA-FIT turn smartphones into a physics lab

The RWTH Aachen University hosted the 108th federal congress of the association for the promotion of the STEM education (MNU) from 6 to 10 April 2017. The association is one of Germany’s largest subject teacher associations, which is active in 18 federal associations all over the country. It represents the interests of teachers for Mathematics, Biology, Chemistry, Physics, Engineering, Astronomy and Computer Science of all school forms. During the congress, more than 160 talks and workshops were presented and it was visited by around 1000 teachers from all over Germany. JARA-FIT member Christoph Stampfer gave the opening talk for physics and introduced the app PHYPHOX (PHYsical PHOne eXperiments) which was developed by Sebastian Kuhlen at the 2nd Institute of Physics at RWTH

Aachen University. The app uses the various sensors integrated in modern smartphones, such as accelerometers, gyroscopes, and pressure sensors, as a basis for a broad range of experimental measurements and gives pupils, students and interested citizens the opportunity to conduct experiments on their own and to get hands-on experience with data acquisition and analysis. The app is available for free on Android and iOS; more details can be found on the project web-site:

www.phyphox.org


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Awards for young scientists from JARA-FIT

Young scientists of the member institues of JARA-FIT have been honored for their work:

Christoph Bäumer, PhD. student at the Peter Grünberg Institute-7 of Forschungszentrum Jülich, has been awarded with the Jülicher Excellence Prize 2017 for his work on spectromicsocopy of memristive materials utilizing graphene electrodes. His thesis is entitled “Spectroscopic characterization of local valence change processes in resistively switching complex oxides” and was conducted within the SFB 917 Nanoswitches.

Torsten Rieger, PhD. student at the Peter Grünberg Institute-9 of Forschungszentrum Jülich, is among the recipients of the Helmholtz Doctoral Prize 2016. His thesis is entitled “Growth and structural characterization of III-V semiconductor nanowires”. The materials researcher received the award for the research field of key technologies during the annual conference of the Helmholtz Association.

12th JARA-FIT Lab Course Nanoelectronics in Jülich

This year, 35 students participated in our one week 12th JARA-FIT Lab Course Nanoelectronics. Addressees were master students of physics, electrical engineering, chemistry, and materials science. The idea of the Nanoelectronics lab course is to bring students into contact as early as possible with real top-level research equipment used by Jülich JARA-FIT scientists. The Lab Course introduced the students also to current research topics of JARA-FIT. The students received introductory lectures, performed one experiment per day in small groups, and were able to discuss scientific issues as well as carrier issues with their JARA-FIT supervisors. The students came from Aachen as part of their master courses, and additionally several students came from all over Germany.

Books published by JARA-FIT scientists

Thomas Schäpers, scientist at the Peter Grünberg Institute-9 of Forschungszentrum Jülich and Virtual Institute for Topological Insulators (VITI), has written a book on Semiconductor Spintronics published by de Gruyter. His textbook covers ferromagnetism in nano-electrodes, spin injection, spin manipulation, and the practical use of these effects in next-generation electronics. Based on foundations in quantum mechanics and solid state physics the textbook guides the reader to the forefront of research and development in the field, based on repeated lectures given at the RWTH Aachen University. The book is the first comprehensive introduction into the rapidly evolving field of spintronics including chapters on low-dimensional semiconductor structures, magnetism in solids, diluted magnetic semiconductors, magnetic electrodes, spin injection, spin transistor, spin interference, spin Hall effect, quantum spin Hall effect, topological insulators, and quantum computation with electron spins. [De Gruyter Textbook, ISBN 978-3-11-042544-4]

JARA-FIT member Rainer Waser at the Peter Grünberg Institut-7 of Forschungszentrum Jülich and Institut für Werkstoffe der Elektrotechnik 2, RWTH Aachen University, has published together with his colleague Daniele Ielmini, Department of Electrical Engineering, Information Science and Bioengineering at Politecnico di Milano (Italy) a book on Resistive Switching – From Fundamentals of Nanoionic Redox Processes to Memristive Device Applications (Wiley-VCH, 750 pages, full color). The book introduces readers to the wide topic of redox-based resistance switching, providing the knowledge, tools, and methods needed to understand, characterize and apply resistive switching memories. The book focuses on the phenomena of nanoionic redox processes where ionic motion over nanoscale dimensions in two-terminal elements leads to local redox phenomena and, in turn, affects the resistance of the elements. Starting with those materials that display resistive switching behaviour, the book explains the basics of resistive switching as well as microscopic physics of switching mechanisms and models. An in-depth discussion of memory reliability is followed by chapters on memory cell structures and architectures, while a section on logic gates and neuromorphic computing circuits rounds off the text. The book addresses materials scientists, electrical engineers and physicists dealing with memory research and development. [Wiley-VCH, ISBN 978-3-527-33417-9]


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General Information


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JARA-FIT Members

Prof. Dr. St. Appelt, Lehrstuhl für Technische und Makromolekulare Chemie, Lehr- und Forschungsgebiet Niederfeld-NMR (Methoden der NMR),

RWTH Aachen University Zentralinstitut für Engineering, Elektronik und Analytik - Systeme der Elektronik,

Forschungszentrum Jülich Prof. Dr. H. Bluhm, JARA-FIT Institut für Quantum Information, RWTH Aachen University and

Forschungszentrum Jülich II. Physikalisches Institut – Quantum Technology Group, RWTH Aachen University

Prof. Dr. S. Blügel, Peter Grünberg Institut/Institute for Advanced Simulation – Quanten-Theorie der

Materialien, Forschungszentrum Jülich Prof. Dr. T. Brückel, Peter Grünberg Institut/Jülich Centre for Neutron Science – Streumethoden,

Forschungszentrum Jülich Prof. Dr. D. P. DiVincenzo, JARA-FIT Institut für Quantum Information, RWTH Aachen University and

Forschungszentrum Jülich Peter Grünberg Institut/Institute for Advanced Simulation – Theoretische Nanoelektronik, Forschungszentrum Jülich

Prof. Dr. R. Dronskowski, Lehrstuhl für Festkörper- und Quantenchemie und Institut für Anorganische Chemie,

RWTH Aachen University Prof. Dr. R. Dunin-Borkowski, Peter Grünberg Institut – Microstrukturforschung, Forschungszentrum Jülich

Ernst Ruska-Centre für Mikroskopie und Spektroskopie mit Elektronen Prof. Dr. D. Grützmacher, Peter Grünberg Institut – Halbleiter-Nanoelektronik, Forschungszentrum Jülich Prof. Dr. S. Grün, Institut für Neurowissenschaften und Medizin, Computational and Systems Neuroscience

Forschungszentrum Jülich Prof. Dr. C. Honerkamp, Institut für Theoretische Festkörperphysik, RWTH Aachen University Prof. Dr. L. Juschkin, Lehr- und Forschungsgebiet Experimentalphysik des Extrem-Ultraviolett,

RWTH Aachen University Prof. Dr. P. Kögerler, Institut für Anorganische Chemie (Molekularer Magnetismus), RWTH Aachen University

Peter Grünberg Institut – Elektronische Eigenschaften, Forschungszentrum Jülich Prof. Dr. U. Klemradt, II. Physikalisches Institut B, RWTH Aachen University Prof. Dr. J. Knoch, Institut für Halbleitertechnik, RWTH Aachen University Prof. Dr. P. Loosen, Lehrstuhl für Technologie Optischer Systeme, RWTH Aachen University

Fraunhofer-Institut für Lasertechnik, Aachen Prof. Dr. S. Mantl, Peter Grünberg Institut – Halbleiter-Nanoelektronik, Forschungszentrum Jülich Prof. Dr. M. Martin, Institut für Physikalische Chemie, RWTH Aachen University Prof. Dr. J. Mayer, Gemeinschaftslabor für Elektronenmikroskopie, RWTH Aachen University

Ernst Ruska-Centre für Mikroskopie und Spektroskopie mit Elektronen Prof. Dr. R. Mazzarello, Institut für Theoretische Festkörperphysik, RWTH Aachen University Prof. Dr. V. Meden, Institut für Theorie der Statistischen Physik, RWTH Aachen University Prof. Dr. Chr. Melcher, Lehrstuhl I für Mathematik, RWTH Aachen University Prof. Dr. W. Mokwa, Institut für Werkstoffe der Elektrotechnik 1 – Mikrostrukturintegration,

RWTH Aachen University


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Prof. Dr. M. Morgenstern, II. Physikalisches Institut B, RWTH Aachen University Prof. Dr. T. Noll, Lehrstuhl für Allgemeine Elektrotechnik und Datenverarbeitungssysteme,

RWTH Aachen University (till 31 August 2016) Prof. Dr. A. Offenhäusser, Peter Grünberg Institut/Institute of Complex Systems – Bioelektronik,

Forschungszentrum Jülich Prof. Dr. E. Pavarini, Peter Grünberg Institut/Institut for Advanced Simulation, Forschungszentrum Jülich Prof. Dr. R. Poprawe Fraunhofer-Institut für Lasertechnik, Aachen Prof. Dr. G. Roth, Institut für Kristallographie, RWTH Aachen University Prof. Dr. H. Schoeller, Institut für Theorie der Statistischen Physik, RWTH Aachen University Prof. Dr. U. Simon, Institut für Anorganische Chemie, RWTH Aachen University Prof. Dr. C. Stampfer, II. Physikalisches Institut A, RWTH Aachen University

Peter Grünberg Institut – Halbleiter-Nanoelektronik, Forschungszentrum Jülich Prof. Dr. C. M. Schneider, Peter Grünberg Institut – Elektronische Eigenschaften, Forschungszentrum Jülich Prof. Dr. T. Taubner, I. Physikalisches Institut A, RWTH Aachen University Prof. Dr. S. Tautz, Peter Grünberg Institut – Funktionale Nanostrukturen an Oberflächen,

Forschungszentrum Jülich Prof. Dr. B.M. Terhal, Institut für Quanteninformation, RWTH Aachen University Prof. Dr. A. Vescan, Lehr- und Forschungsgebiet GaN-Bauelementtechnologie, RWTH Aachen University Prof. Dr. R. Waser, Institut für Werkstoffe der Elektrotechnik 2, RWTH Aachen University

Peter Grünberg Institut – Elektronische Materialien, Forschungszentrum Jülich Prof. Dr. M. Wegewijs, Peter Grünberg Institut – Theoretische Nanoelektronik, Forschungszentrum Jülich Prof. Dr. S. Wessel, Institut für Theoretische Festkörperphysik, RWTH Aachen University Prof. Dr. J. Witzens, Institut für Integrierte Photonik, RWTH Aachen University Prof. Dr. M. Wuttig, I. Physikalisches Institut A, RWTH Aachen University


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JARA-FIT Institutes

Zentralinstitut für Engineering, Elektronik und Analytik: Systeme der Elektronik (ZEA-2), Forschungszentrum Jülich & Institut für Technische und Makromolekulare Chemie (ITMC), RWTH Aachen University

St. Appelt

Our research revolves around spin order generation, the manipulation, measurement and transfer of spin order by combining Hyperpolarization technology with Nuclear Magnetic Resonance (NMR) spectroscopy. Starting from states with high spin orders, like nuclear singlet states or highly premagnetized (hyperpolarized) spin systems, we investigate the field dependancy of the complexity, and thus information content, of corresponding NMR spectra in very low magnetic fields. Our research includes chemical synthesis as a means for substrate generation and optimization of spin order transfer, the development and construction of new hardware for mobile NMR spectroscopy as well as the investigation of the underlying quantum mechanical principles of coupled spins in low magnetic fields.

Peter Grünberg Institut / Institute for Advanced Simulation – Quantum Theory of Materials, Forschungszentrum Jülich

S. Blügel

The Institute for Scattering methods develops and uses scattering methods (neutron- as well as synchrotron x-ray-scattering) to investigate ordering phenomena and the corresponding fluctuations and excitations in (nano-) magnetic and highly correlated electron systems. We relate this microscopic information to macroscopic physical properties and functionalities to obtain an understanding of the underlying mechanisms and to optimize material systems for possible applications in future information- or energy-technologies. Research ranges across a wide spectrum, from novel quantum materials through frustrated and topological magnets, magnetic nanoparticles and thin film heterostructures to multiferroic and magnetocaloric materials.

JARA-FIT Institute for Quantum Information, RTWH Aachen University and Forschungszentrum Jülich

H. Bluhm and D.P. DiVincenzo

This new institute combines the forces of theoretical and experimental research in quantum information science, with the overarching goal of making key advances towards to the achievement of large-scale quantum computation. But many principles of quantum information are investigated here. On the theory side, new principles for the implementation of quantum computation in noisy systems, particularly Fermionic many-body systems are studied. This includes particularly the investigation of Majorana qubits realized in semiconductor nanowires. Protocols for error correction codes and fault tolerance in quantum computation are investigated. New applications of the theory of quantum entanglement are developed. Both theory and experiment focuses on highly coherent two-level quantum systems in semiconductor quantum dots for quantum information processing, exploring the physics governing these devices as well as pushing forward their technological development. Key topics include high fidelity control, decoherence measurements and multi-qubit circuits.

II. Physikalisches Institut, Quantum Technology Group, RWTH Aachen University

H. Bluhm

Operationally, the quantum technology group is the experimental part of the JARA-Institute for Quantum Information. In addition to the quantum computing related activities mentioned above, it is pursuing scanning SQUID microscopy at ultra-low temperatures for magnetic imaging and ultra-sensitive magnetic measurements on mesoscopic structures.

Peter Grünberg Institut / Jülich Centre for Neutron Science - Streumethoden, Forschungszentrum Jülich

Th. Brückel

At the Institute of Scattering Methods, we focus on the investigation of structural and magnetic order, fluctuations and excitations in complex or nanostructured magnetic systems and highly correlated electron systems. Our research is directed at obtaining a microscopic atomic understanding based on fundamental


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interaction mechanisms. The aim is to relate this microscopic information to macroscopic physical properties. To achieve this ambitious goal, we employ the most advanced synchrotron X-ray and neutron scattering methods and place great emphasis on the complementary use of these two probes. Some of our efforts are devoted to dedicated sample preparation and characterization from thin films and multilayers via nano-patterned structures to single crystals for a wide range of materials from metals to oxides.

Peter Grünberg Institut / Institut for Advanced Simulation – Theoretische Nanoelektronik, Forschungszentrum Jülich

D. P. DiVincenzo, group leaders G. Catelani, T. Costi, E. Pavarini, M. Wegewijs

The behavior of interacting electrons in nano-scale structures is a primarly focus. The Kondo effect, involving the interaction of an isolated spin impurity with conduction electrons, or the formation and transport of high-spin complexes forming spin quadripoles, are particular areas of expertise. Novel computational techniques permit accurate calculations with thousands of atoms, and in complex multi-functional perovskites. Correlated electrons also form the basis of the physical creation of qubits, and the coherence and dynamics of such qubits, and mutiqubit systems, is being investigated.

Lehrstuhl für Festkörper- und Quantenchemie und Institut für Anorganische Chemie, RWTH Aachen University

R. Dronskowski

The chair is specialized in the fields of synthetic and quantum-theoretical solid-state chemistry, bordering with materials science, physics, and crystallography. In detail, we synthesize novel, sometimes extremely sensitive, compounds (nitrides, carbodiimides, guanidinates, intermetallics, small molecules etc.) and elucidate their compositions and crystal structures by means of X-ray and neutron diffractional techniques. The characterization of their physical properties such as electronic transport and magnetism also plays an important role. We regularly perform solid-state quantum-chemical calculations from first principles to yield the electronic structures and to extract the important chemical bonding information needed to thoroughly understand the interplay between chemistry and physics (LOBSTER code). In particular, ab initio steel, phase-change materials, phase prediction, theoretical thermochemistry, and finite-temperature vibrational properties (ab initio ORTEP) are being studied. In addition, we are engaged in constructing the POWTEX time-of-flight neutron diffractometer at Garching.

Peter Grünberg Institut – Mikrostrukturforschung & Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich

R. E. Dunin-Borkowski The institute works on topical fields in solid state physics. Strategically, two directions are followed: first, to make key contributions to the development and application of ultra-high-resolution and in situ transmission electron microscopy, with a strong focus on aberration-corrected electron optics for the highest spatial resolution quantitative imaging of structural, spectroscopic and functional properties and, second, to synthesise selected materials and to study their physical properties. Examples of materials systems that are studied are high temperature superconductors and novel complex metallic alloys. The high temperature superconductors provide the basis for the institute's work on SQuID sensors. The head of the institute is co-director of the Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons.

Institute of Neuroscience and Medicine - Computational and Systems Neuroscience (INM-6) and Institute for Advanced Simulation – Theoretical Neuroscience (IAS-6), Forschungszentrum Jülich

S. Grün together with M. Diesmann, A. Morrison, M. Helias, S. van Albada, A. Stein

The institute is specialized in the integration of experimental data on the structure and the dynamics of the brain into mathematical models and in overcoming bottlenecks in simulation technology and scientific workflows. The group “Statistical Neuroscience” led by Sonja Grün focuses on the development and application of methods to analyze multi-channel activity data in close contact to experimental groups. A focus is the connection between neuronal activity data recorded on different temporal and spatial scales and on the structure of correlations of spiking activity. The group “Theoretical Neuroanatomy” headed by van Albada focuses on the collation and analysis of microscopic and macroscopic anatomical data, informing large-scale dynamical models of the mammalian brain at cellular and synaptic resolution that are simulated using supercomputers. Comparison of the model dynamics with experimentally measured activity further constrains the inferred connectivity. The group “Computational Neurophysics” headed by


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Markus Diesmann focuses on bottom-up approaches in order to integrate physiological and anatomical data into models. This also requires the development of simulation technology for neural networks. The group “Functional Neural Circuits” led by Abigail Morrison investigates mechanisms underlying neural computation through the development of models on the level of networks of spiking neurons. It applies a predominantly top-down approach to discover functional constraints on structure, plasticity and dynamics, particularly with respect to learning and memory. The secondary focus is on simulation technology for high-performance computers. Recent research indicated that a much deeper understanding of the correlation structure of neuronal networks may be possible by the import of theoretical tools of modern physics into neuroscience, and a systematization of the theory of neuronal networks. To this end, the group “Theory of Multi-scale Neuronal Networks” (Helias) focuses on the investigation of mechanisms shaping the correlated and oscillatory activity in neuronal networks with structured connectivity on several spatial scales. This requires the development of quantitative theoretical descriptions, adapted from statistical physics, combined with direct simulations of neuronal networks at cellular resolution. The Bernstein Coordination Site (BCOS) headed by Alexandra Stein, located at the University of Freiburg, is an administrative unit that coordinates the activities of the national Bernstein Network Computational Neuroscience.

Peter Grünberg Institut – Halbleiter-Nanoelektronik, Forschungszentrum Jülich

D. Grützmacher

The institute’s research activities are based on its competence in semiconductor heterostructure and nanostructure research, both in fundamental and device physics as well as in material and process development. They address two major fields. (1) Energy efficient information technology (Green-IT). Here compound semiconductors and group IV alloys are employed for innovative devices, to exploit novel physical phenomena and thereby contribute to progress in future optical communication, data storage and advances in nanotechnology. (2) Exploring Quantum Systems on the Nanoscale. Special emphasis is put on nanostructures consisting of semiconductors, topological insulators and other layered materials as well as hybrid structures of them with magnetic and superconducting materials for the conceptual development of devices for quantum information technology.

Institut für Theoretische Festkörperphysik, RWTH Aachen University

C. Honerkamp, R. Mazzarello, S. Wessel

The research groups in this institute study many-particle interactions in solids, ranging from quantum effects in magnetic systems over electron correlation effects leading to unconventional superconductivity and magnetism to the dynamics of structural phase transitions. Recent work has focused on interaction effects in graphene systems, topological insulators, pnictide high-temperature superconductors and chalcogenide phase-change materials. The powerful theoretical methods employed and developed here comprise quantum Monte Carlo techniques, the functional renormalization group, density-functional theory and molecular dynamics.

Lehr- und Forschungsgebiet Experimentalphysik des Extrem-Ultraviolett, RWTH Aachen University

L. Juschkin

The research in the field of extreme ultraviolet (EUV) radiation is a major contribution for nanoelectronics and future developments in information technology. At the Chair for Experimental Physics of EUV different aspects related to the EUV radiation are investigated ranging from generation and characterization of EUV, to wave propagation and light-matter interaction as well as developing new methods and applications. In combination of EUV interference lithography and the self-organized growth of nanostructures novel materials are prepared, and their properties are analyzed. Moreover, in cooperation with the Fraunhofer Institute for Laser Technology in Aachen different concepts of EUV sources are investigated. On the application side, a series of measurement procedures for which the specific features of EUV radiation can be used, for example, the EUV microscopy and spectroscopic reflectometry, are investigated.

II. Physikalisches Institut (IIB) – Röntgenstreuung und Phasenumwandlungen, RWTH Aachen University

U. Klemradt

Our research is centered at the investigation of nanoscale structures and fluctuations, with focus on nanoparticles, polymer-based nanocomposites and ferroic materials. Of particular interest are phase transitions in smart materials like shape memory alloys. The main experimental tools are X-ray scattering and acoustic emission spectroscopy. We use both laboratory tubes and international synchrotron facilities


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for X-ray experiments. Core techniques are small angle X-ray scattering (SAXS), grazing incidence scattering (GISAXS and reflectometry), and photon correlation spectroscopy using coherent X-rays (XPCS).

Institut für Halbleitertechnik, RWTH Aachen University

J. Knoch

The institute carries out research on semiconductor technology and device with a special focus on low power and energy harvesting technologies with the long-term vision of energy autonomous systems. To be specific, we work on nanoelectronics transistor devices based on Si- and III-V nanowires as well as on carbon nanotubes and graphene particularly aiming at a realization of so-called steep slope switches that enable a significant reduction of the power consumption of highly integrated circuits. In addition, the institute has broad experience in the science and engineering of Si wafer-based solar cells and also performs research on Si-based third generation photovoltaic cells. A combination of our know-how in micro- and nanotechnology with the solar cell technology is used to investigate and realize novel concepts for energy harvesting and storage based e.g. on efficient direct solar water splitting.

Institut für Anorganische Chemie – Molekularer Magnetismus, RWTH Aachen University & Peter Grünberg Institut – Elektronische Eigenschaften (Molekularer Magnetismus), Forschungszentrum Jülich

P. Kögerler

The Molecular Magnetism Group focuses on the chemistry and fundamental physics of discrete and networked magnetically functionalized inorganic molecules. Based on its experience with the control and understanding of magnetic characteristics of purely molecular origin, the group synthesizes magnetic materials based on transition metal clusters that exhibit a complex interplay of charge transport and static/ dynamic magnetic properties such as phase transitions, hysteresis, or quantum tunneling. To functionally combine magnetic state switching and charge transport in systems for FIT spintronic devices, the molecule-surface interface is addressed, in particular employing pre-synthesized contact groups for precise electrical access to an individual molecule in e.g. a gated environment.

Lehrstuhl für Technologie Optischer Systeme, RWTH Aachen University (RWTH-TOS) & Fraunhofer-Institut für Lasertechnik ILT, Aachen

P. Loosen

Extreme ultraviolet radiation (XUV, 1-50 nm, or EUV at 13.5 nm) enables new optical, analytical and manufacturing technologies because of its characteristic interaction with matter, its short wavelength and recent progress on light sources and optical components (e.g. EUV lithography). XUV tools are already deployed by the semiconductor industry, which significantly pushes the further development of XUV technology. Future applications which will support scientific progress in a variety of fields such as nanoelectronics or biotechnology are also within the scope of our research. Activities include structuring on a nanometer scale using interference lithography, XUV microscopy for imaging of dynamic processes or at-wavelength inspection of multilayer mask-blanks for hidden defects, and characterization of thin film coated surfaces using grazing-incidence reflectometry.

Institut für Physikalische Chemie (IPC), RWTH Aachen University

M. Martin

The institute’s research activities are based on its competence in the physical chemistry of solids with a special emphasis on defects and diffusion in inorganic solids, in particular oxides. Within JARA-FIT two major fields are addressed. (1) Ionic transport: transport of oxygen ions in the bulk, across and along grain boundaries and in space charge zones is investigated by means of secondary ion mass spectrometry (SIMS), density functional theory and Monte Carlo simulations. (2) Electronic transport: amorphous and highly non-stoichiometric oxides are investigated concerning correlations between structure, electrical conductivity, and electronic structure with a view to applications in resistive switching.

GFE – Gemeinschaftslabor für Elektronenmikroskopie & Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, RWTH Aachen University

J. Mayer

GFE is a central facility of RWTH Aachen University and has state-of-the-art equipment in the fields of transmission electron microscopy, scanning electron microscopy, electron microprobe analysis, focused ion beam instruments and atomic force microscopy. GFE provides services for a large number of


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institutes from RWTH Aachen University and a broad range of industrial companies. In the field of information technology, GFE participates in research projects on nonvolatile memories and on nanoscale CMOS devices. The head of the GFE is co-director of the Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons and coordinates the RWTH user activities and contribution to the Centre.

Institut für Theorie der Statistischen Physik, RWTH Aachen University

V. Meden, H. Schoeller, M. Wegewijs

The members of the institute are investigating the spectral and transport properties of low-dimensional quantum systems in contact with heat and particle reservoirs. The research focuses on the development of many-body methods for strongly correlated mesoscopic systems in nonequilibrium (quantum field theory and renormalization group in nonequilibrium) as well as on the application to experimentally realizable physical systems like semiconductor quantum dots, quantum wires (e.g. carbon nanotubes), and molecular systems.

Lehrstuhl I für Mathematik, RWTH Aachen University

Ch. Melcher

The research at our institute has a focus on nonlinear partial differential equations from mathematical physics and materials science. We are particularly interested in the emergence and dynamics of patterns and topological solitons in models from micromagnetics and Ginzburg‐Landau theory. Using tools from functional and multiscale analysis, our aim is to capture the qualitative behavior of solutions to such complex theories and, if possible, to identify simpler models, whose behavior is easier to understand or simulate.

Institute of Materials in Electrical Engineering I, RWTH Aachen University

W. Mokwa

The institute´s research activities are focused on the development of micro systems for medical and life science applications. Main activities lie on coupling of biological systems to technical systems, development of "intelligent" implants and prostheses and micro fluidic systems for biotechnology and medical diagnostics. For the development of these systems silicon and thin film technologies, silicon micromechanics, micro electroplating, soft lithography as well as sophisticated packaging technologies are used in a clean room of about 600 m2.

II. Physikalisches Institut (IIB) – Rastersondenmethoden, RWTH Aachen University

M. Morgenstern

The research group develops scanning probe methods working in particular at low temperatures down to 0.3 K and in high magnetic fields up to 14 T in order to investigate the electronic structure of interacting electron systems and systems relevant for nanoelectronic applications. Thereby, we exploit the advantage of mapping the electronic structure down to the atomic scale at an energy resolution down to 0.1 meV, but also use the scanning probes for the excitement of the systems under study, which is probed with ps time resolution. Current topics of interest are topological insulators and Majorana fermions, electronic and mechanic properties of graphene, quantum Hall physics in graphene and III-V-materials, confined wave functions in quantum dots, nanomagnetic systems, and phase change materials.

Chair of Electrical Engineering and Computer Systems, RWTH Aachen University

T. Noll (till 31 August 2016)

The group is conducting research on architectural strategies, circuit concepts and design methodologies for highly integrated circuits in nano-scale CMOS as well as potential post-CMOS technologies. The focus is on circuits for applications of high-throughput digital signal processing and special emphasis is placed on the issues of reliability and energy-efficiency.

Peter Grünberg Institut / Institute of Complex Systems – Bioelektronik, Forschungszentrum Jülich

A. Offenhäusser

Biological signal processing and their utilization requires investigations of correlated biological events with high spatiotemporal resolution. Our research is focused on the development of bioelectronic devices and


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tools which exploit biology in conjunction with electronics encompassing for example, biomaterials for information processing, sensors, actuators, and biomedical devices. A key aspect is the interface between biological materials and electronics. The two main themes are “biosensing” and “neuroelectronics”.

Institute of Crystallography, RWTH Aachen University

G. Roth

The institute's research profile covers the topics synthesis, structure and properties of novel materials. The synthetic activities include the preparation of new or crystal-chemically modified compounds with interesting properties in bulk poly- or single-crystalline form. Crystal and magnetic structures are studied by powder and single crystal X-ray as well as neutron diffraction methods (outstation at FRM-II/Garching) with special emphasis on complex, defect dominated systems such as partially disordered, incommensurately modulated structures and composite crystals. Among the materials recently studied are superconductors (modulated CaAlSi), fullerenes (C70 high pressure polymer), spin-chain-compounds (vanadates and cuprates) and pyroxene-type multiferroics.

II. Physikalisches Institut (IIA), RWTH Aachen University

C. Stampfer

Our research activities are focused on (i) carbon-based quantum electronics, (ii) semiconductor-based spin-electronics, and on (iii) topological insulators. For instance, we focus on studying electronic and mechanical properties of carbon and Bi2Se3-based systems that have critical dimensions on the nanometer scale. Such structures approach the atomic scale and the ultimate limit of solid state miniaturization. In particular we investigate systems based on nanostructured graphene (a monoatomic sheet of graphite) and carbon nanotubes. Current interests include (i) developing advanced processing technologies for fabricating novel nanodevices, (ii) understanding new and interesting transport phenomena that arise in these devices, and (iii) learning how to control and detect the charge, spin and mechanical degrees of freedom in these systems. Potential applications include ultra-fast electronics, new spin-based nanoelectronic device concepts and applied quantum technology.

Peter Grünberg Institut – Elektronische Eigenschaften, Forschungszentrum Jülich

C. M. Schneider

The institute is engaged in the study of electronic and magnetic phenomena in novel materials and is one of the birthplaces of spintronics. Present research concentrates on the fundamental aspects, properties, and control of spin textures, spin transfer, and spin dynamics in a wide range of material classes down to the molecular level. The activities include the development of novel synchrotron- and laser-based microscopy and spectroscopy techniques for the study of static properties and highly dynamic processes in condensed matter systems. Further important research fields comprise nanomagnetism and molecular spintronics, which may form a bridge to quantum information processing.

Institut für Anorganische Chemie (IAC), RWTH Aachen University

U. Simon

Our research is devoted to functional metal and metal oxide nanostructures. One focus is designated to the wet chemical preparation and characterization of tailored ligand stabilized metal nanoparticles of different geometries, i.e. nanospheres, nanorods and hollow nanospheres, as well as distinct nanoparticle assemblies. On the one hand the prepared nanostructures are investigated with respect to applications as molecular probes, i.e. photoaccoustic imaging, or actuators in biomedicine. On the other hand the utilization as fundamental building blocks in nanoelectronic devices is surveyed. Molecules exhibiting distinct functionalities, e.g. conducting or isolating (particles exhibiting diode-like characteristics), or molecules allowing self-organization, e.g. DNA, leading to precisely controllable nanoparticle networks (microgels) are applied. Our characterization involves conventional techniques as IR, NMR, UV-vis, DLS as well as local probe measuring techniques, and investigations related to the uptake mechanism into cells and the impact on relevant biological systems is performed.

A further topic deals with the wet chemical synthesis of metal oxide nanostructures, which are applicable as sensor materials, new cathode materials for Li-ion batteries or as resistive switching elements. In the latter context the development of chemically-based bottom-up approaches for the fabrication of resistively switching nanostructures is explored by in situ SEM. This topic aims at understanding the switching and structural consequences of the resistive switching process by using individual nanoparticles as model systems. Furthermore, self-assembly and surface patterning techniques are applied to produce long range order of nanoparticles.


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I. Physikalisches Institut (IA), Metamaterialien und Nano-Optik, RWTH Aachen University

T. Taubner

Our research is focused on the development and application of new infrared imaging and spectroscopic techniques with enhanced resolution and sensitivity. Specifically, we use and further develop scattering-type Scanning Near-field Optical Microscopy (s-SNOM) and related concepts like superlenses for imaging and spectroscopy below the diffraction limit of light. The use of infrared light enables us to gain information on the local chemical composition, the structural properties and the distribution of free carriers in individual nanostructures at nanoscale resolution. Additionally, we explore the use of resonant nanostructures to enhance the sensitivity of infrared spectroscopy and to create actively tunable optical devices by combining them with phase-change materials.

Peter Grünberg Institut – Funktionale Nanostrukturen an Oberflächen, Forschungszentrum Jülich

S. Tautz

Our research tackles fundamental issues in the quest towards functional nanostructures at surfaces, with a particular emphasis on nanoelectronics. Since our focus is placed on molecular materials, an important aspect of our work covers the structural investigations and spectroscopy of complex molecular adsorbates on metal, semiconductor and insulator surfaces. Based on these interface studies, the growth of thin films and nanostructures is investigated. Here, our work is directed towards hybrid materials, comprising both organic and inorganic components. Charge transport, being the most important function in the context of nanoelectronics, transport experiments on single molecules and nanostructures round off our activities. It is a specific asset of our institute that we combine well-established surface techniques with the development of new experimental methods.

Lehr- und Forschungsgebiet GaN-Bauelementtechnologie, RWTH Aachen University

A. Vescan

GaN Device Technology is performing fundamental and application-oriented research on the deposition and characterization of compound and organic semiconductor materials as well as on electronic and optoelectronic devices. Major research goals are the development of energy-efficient devices for power and RF electronics, displays, solid-state lighting and next-generation photovoltaics.The III-nitride activities include investigation and development of practical technological building blocks for (opto-)electronic devices and also address fundamental issues of materials growth and device physics. In the field of organic semiconductors, we focus on deposition technologies like organic vapor phase deposition (OVPD), device processing and the development of advanced OLED structures. A special focus is on hybrid structures and the specific properties of inorganic-organic heterojunctions for photovoltaics.

Institut für Werkstoffe der Elektrotechnik 2, RWTH Aachen University & Peter Grünberg Institut - Elektronische Materialien, Forschungszentrum Jülich

R. Waser

We focus on the physics and chemistry of electronic oxides and organic molecules, which are promising for potential memory, logic, and sensor functions. Our research aims at a fundamental understanding of nanoelectronic functions based on electrochemical redox processes, memristive phenomena, space charge effects, and ferroelectricity and at the elucidation of their potential for future device applications. For this purpose, our institute provides a broad spectrum of facilities ranging from dedicated material synthesis, atomically controlled film deposition methods, molecular self-assembly routes, and integration technologies, to the characterization of processes, structures, and electronic properties with atomic resolution.

Institute of Integrated Photonics, RWTH Aachen University

J. Witzens

Integration of photonic components and systems in Silicon allows the realization of complex optical systems at the chip scale. At the Institute of Integrated Photonics we are working on the development of Silicon Photonics devices and systems with activities ranging from material science, core device development to system integration. Current activities focus on the development of cost effective, compact and low power electro-optic transceivers based on semiconductor mode-locked lasers, low power and low drive voltage electro-optic modulators, integrated light sources (on-chip comb generation with parametric


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conversion, GeSn based light sources), and misalignment tolerant fiber-to-chip and laser-to-chip couplers, as well as visible wavelength SiN based photonic integrated circuits for life science applications.

I. Physikalisches Institut (IA), RWTH Aachen University

M. Wuttig

The institute’s research activities are focused on the development of novel materials for advanced optoelectronic applications. In particular, materials for optical and electronic data storage have been developed in the last few years. For this class of materials, so-called phase change materials, we have established design rules and an atomistic understanding of essential material properties. This work has enabled novel functionalities of phase change materials in applications as non-volatile memories and is part of the SFB 917 (Nanoswitches). Recently, we could demonstrate that some crystalline phase change materials can possess very high levels of disorder, which gives rise to highly unconventional transport properties. Organic materials are a second focus, where we work on routes to tailor material properties for optoelectronic applications ranging from displays, to solar cells and electronic devices.


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Selected Research Reports


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Structural characterization of ultrathin topological insulator Sb2Te3/Bi2Te3 stacks

G. Mussler1, M. Lanius1, J. Kampmeier1, S. Kölling2, P. Schüffelgen1, M. Luysberg3, P. M. Koenraad2, and D. Grützmacher1 1 Peter Grünberg Institut-9, Forschungszentrum Jülich, Germany 2 Institute for Applied Physics (PSN), Technische Universität Eindhoven, Netherlands 3 Peter Grünberg Institut-5, Forschungszentrum Jülich, Germany We fabricated topological insulating Sb2Te3/Bi2Te3 p-n heterostructures by means of molecular beam epitaxy and characterized the topography of the films by scanning tunneling microscopy. Due to the van der Waals growth mode of the layered Te compounds, the heterostructure is fully relaxed on the Si(111) substrate. Furthermore, scanning transmission electron microscopy measurements unveil the crystalline structure of the p-n interface. Atom probe tomography enable the mapping of the chemical element distribution. We conclude that a diffusion of Sb and Bi during growth causes the formation of ternary compounds. In addition a Sb and Te accumulation at the substrate interface could be detected.

Triggered by the promise of dissipationless surface transport, three-dimensional topological insulators (TI) attracted a lot of attention in applied as well as in basic physics [1]. The current-carrying surface states are topologically protected by time reversal symmetry, i.e. the spin orientation is locked to the direction of motion. This specific property makes topological insulators particularly interesting for spintronics as well as for quantum computation.

One severe problem related to TI is the large intrinsic bulk carrier concentrations. It turns out that Bi2Te3 is intrinsically n-type doped, whereas Sb2Te3 is usually p-type doped. In order to compensate the n- and p-type doping we realized a p-n junction formed by a p-doped topological insulator layer on top of an n-doped one. The underlying idea is that the space charge layer formed at the interface reduces the carrier concentration in the system and by that allows transport in the topologically protected surface states only. Moreover, variation of the thickness of both layers is an elegant way to shift the Fermi energy in a customized fashion. However, in order to fully understand the electronic structure of the Bi2Te3/Sb2Te3 stacks, needed to precisely simulate the shift of the Fermi level in dependence of the top Sb2Te3 layer thickness, a detailed structural analysis is mandatory. This report deals with the structural analysis of the Bi2Te3/Sb2Te3 stacks.

Fig. 1a shows a STM image of an ultrathin heterostructure of 1.5 nm Sb2Te3 on top of 4 nm Bi2Te3. The ultrathin film of Sb2Te3 grows in a layer

by layer mode. These layers are intersected by 0.4 nm step edges (dashed lines in Fig. 1a), consisting of a Sb-Te sub-quintuple layer, which are caused by the Si(111) substrate steps. The resulting roughness of the film is the minimum for films grown on Si(111) and could only be improved by a substrate with a lower miscut.

FIG. 1: STM image of 1.5 nm (a) and 4 nm (b) Sb2Te3 on top of a 4 nm Bi2Te3 layer. Step heights are given in quintuple layers (QL) relative to the plane denoted “0”. c) XRD patterns of heterostructures with increasing Sb2Te3 thickness.

For a film of 4 nm Sb2Te3 on 4 nm Bi2Te3 (Fig. 1b) the growth mechanism changes from a nearly perfect layer by layer to a mound formation on a closed multilayer film. The Sb2Te3 seems to adopt the properties of Bi2Te3 film and shows also substrate induced sub-layers of 0.4 nm in height (marked with black arrows in Fig. 1b), which do not influence the formation of mounds. These substrate steps propagate through the whole TI film and generate stacking defects similar to an antiphase boundary. XRD measurements of several heterostructures with varying Sb2Te3 and constant Bi2Te3 thickness are presented in Fig. 1c. For all heterostructures the characteristic reflections of Bi2Te3 are found. For increasing Sb2Te3 thickness the (0012) reflection of the Sb2Te3 appears. The calculated values for the


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lattice constants of both materials indicate a fully relaxed growth and a high crystalline quality of the films. We conclude that the interface between Bi2Te3 and Sb2Te3 has no detrimental influence on the growth mode of the Sb2Te3 film.

FIG. 2: STEM bright-field (a) and dark-field (b) images of 15 nm Sb2Te3 on 6 nm Bi2Te3. The interface to the Si substrate showing bright contrast in a) is perfectly crystalline. The Bi2Te3 layer is identified by bright atom positions in b). Some of the Bi, Sb, and Te layers are marked with red, blue, and green arrows, respectively.

To reveal the interfacial structure, a 15 nm Sb2Te3 film on 6 nm Bi2Te3 was investigated by high-resolution STEM. Figure 2a displays a bright-field image, where the interface between Si substrate and Bi2Te3 is seen to be of high crystalline perfection, i.e. no structural defects or amorphous layers are revealed. In the corresponding high-angle annular dark-field image (Fig. 2b) contrast scales with the atomic number squared (and with specimen thickness), which causes Si appearing darkest and Bi atomic columns (red arrows) brightest. As Sb and Te exhibit almost the same atomic number, no contrast difference is revealed between Te (green arrows) and Sb (blue arrows) layers. Across the whole image individual quintuple layers can be clearly identified by dark contrast lines at the position of the van der Waals gaps.

To determine the absolute values for the concentration of both layers, we performed atom probe tomography (APT) measurements. Figure 3a represents a 2- dimensional projection of the 3-dimensional ion map of the same Sb2Te3 /Bi2Te3 heterostructure measured by STEM and EDX. The map shows the main isotopes of each element and clusters of SbTe and BiTe are displayed to give a qualitative image of the element distribution. The interdiffusion between the Sb2Te3 and the Bi2Te3 film and the formation of ternary compounds is clearly visible. At the interface to the Si substrate an accumulation of Sb and Te of less than d = 1nm can be identified. A diffusion deeper into the substrate of one of the elements of the heterostructure can be excluded. Figure 4b shows the quantified profile of the heterostructure. In this

representation the signal of clusters of SbTe and BiTe is distributed to the respective elements, counting once for both contained elements, in order to obtain absolute values for the concentration of each element. Because the APT yields no exact information about the thickness of the measured film, the curve was adjusted to the length scale of the STEM measurements. The atom concentration in the bulk of the p-n junction amounts to x = 0.1 and y = 0.6 for the Bi content of the ternary compounds defined in Fig. 3.

FIG. 3: a) 2D projection of the 3D ion map measured by the APT showing the reconstructed positions of the ions and clusters of 15nm Sb2Te3 film on 6nm Bi2Te3. b) Quantified 1D concentration profile of the heterostructure presented in a). Black arrows in both images mark the Sb accumulation at the Si interface.

Furthermore, the width of the interface between both TIs is around d = 4 nm, which is in perfect agreement with the EDX measurement (not shown). The Te content (green curve in Fig. 3a) is constant over the whole heterostructure, indicating diffusion only between Sb and Bi lattice positions. In the region of the first layer at the interface to the Si(111) substrate, the Sb and Te concentrations rise along with the Si concentration, while the Bi signal decreases nearly to zero. The Te excess can be partly explained by the Te-passivation layer at the Si substrate.

The data show that there is indeed a Bi/Sb interdiffusion across the Bi2Te3/Sb2Te3 interface. Hence the n-Bi2Te3/p-Sb2Te3 heterostructure is rather a (Bi,Sb)2Te3/(Bi,Sb)2Te3 junction with a Bi- rich bottom layer and a Sb-rich layer on top. Further detailed information on the structural characterization is found in reference [2].

[1] J. E. Moore, Nature 464, 194 (2010)

[2] M. Lanius, J. Kampmeier, C. Weyrich, S. Kölling, M. Schall, P. Schüffelgen, E. Neumann, M. Luysberg, G. Mussler, P. M. Koenraad, T. Schäpers, and D. Grützmacher, Cryst. Growth Des. 16, 2057 (2016)


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Protected by time-reversal and mirror symmetry: the dual topological insulator Bi1Te1

M. Eschbach1, M. Lanius2, C. Niu3, E. Młyńczak1,4, P. Gospodarič1, J. Kellner5, P. Schüffelgen2, M. Gehlmann1, S. Döring1, E. Neumann6, M. Luysberg7, G. Mussler2, L. Plucinski1, M. Morgenstern5, D. Grützmacher2, G. Bihlmayer3, S. Blügel3, and C. M. Schneider1 1 Peter Grünberg Institut-6, Forschungszentrum Jülich, Germany 2 Peter Grünberg Institut-9, Forschungszentrum Jülich, Germany 3 Peter Grünberg Institut-1 and Institute for Advanced Simulation-1, Forschungszentrum Jülich, Germany 4 Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Krakow,

Poland 5 II. Institute of Physics B, RWTH Aachen University, Germany 6 Helmholtz Nanoelecronic Facility, Forschungszentrum Jülich, Germany 7 Peter Grünberg Institut-5 and Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons,

Forschungszentrum Jülich, Germany

Topological insulators (TIs) and topological crystalline insulators (TCIs) are states of matter where an insulating bulk phase is surrounded by metallic boundaries that can show dissipationless transport. These transport channels are protected against backscattering by time-reversal symmetry in the case of TIs and crystalline (e.g. mirror-) symmetries in case of TCIs. In a Jülich-Aachen collaboration we have shown that new three-dimensional (3D) topological phases can emerge in superlattices containing two-dimensional constituents of these well-known topology classes. We demonstrate that stoichiometric Bi1Te1, which consists of an alternating sequence of two Bi2Te3 quintuple layers and one Bi bilayer, is a dual 3D topological insulator where a weak topological insulator phase and topological crystalline insulator phase appear simultaneously. By density functional theory (DFT), we find 2 indices (0;001) and a non-zero mirror Chern number. We have synthesized Bi1Te1 by molecular beam epitaxy (MBE) and found evidence for its topological crystalline and weak topological character by spin- and angle-resolved photoemission spectroscopy (ARPES). The dual topology opens the possibility to gap the differently protected metallic surface states on different surfaces independently by breaking the respective symmetries, for example, by a magnetic field on one surface and by strain on another surface.

After graphene, a single bilayer (BL) of Bi was one of the first examples of two-dimensional (2D) TIs [1]. Experimentally, it turned out to be difficult to stabilize such layers on weakly interacting substrates, but on the 3D TI Bi2Te3 it was finally possible to stabilize the Bi bilayer [2]. Of course, a 3D TI has topologically protected edge states at its

surface and the deposited Bi layer thus cannot remain insulating in its interior.

FIG 1: (a) Structure of Bi1Te1 consisting of two Bi2Te3 quintuple layers (QL) and one Bi bilayer (BL) as obtained by DFT calculations. Since there is only weak van der Waals bonding between these layers, a rather large separation is found both in the calculation and in experiment. (b) Local STEM image of a Bi1Te1 film showing the individual layers and their spacing. The image also demonstrates the high crystalline quality of the samples (adapted from [4]).

If made sufficiently thin, Bi2Te3 transforms from a 3D to a 2D TI, i.e. instead of having two-dimensional metallic states at the surfaces, there are only one-dimensional states at the edges [3]. A multilayer stack of thin Bi2Te3 and Bi BLs can thus be expected to form what is called a weak topological insulator (WTI), a 3D crystal that carries edge states just on some surfaces, but others (in the stacking direction) remain insulating (dark surfaces). Naturally, a large variety of Bi and Bi2Te3 stacks can be realized, but not all of them retain an insulting character, e.g. Bi4Te3 is known to be metallic. Fig. 1(a) shows a multilayer structure made of two Bi2Te3 quintuple layers (QLs) and one Bi BL that remains insulating in the


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bulk with a 0.2 eV band gap as obtained by DFT calculations [4]. The structure was confirmed by scanning transmission electron microscopy (STEM), shown in Fig. 1(b), as well as X-ray diffraction measurements. Theoretical analysis shows that Bi1Te1 is indeed a WTI with topological invariants (0;001), i.e. the c-axis being perpendicular to the dark surface.

As the growth direction exposes this dark surface of the WTI, one cannot expect to find a topologically protected surface state there. But further analysis shows that the three mirror planes, perpendicular to the surface, allow the crystal to be a TCI with a symmetry protected surface state along the line where the mirror and surface plane intersect. Since the calculated mirror Chern number is -2, one obtains 3 pairs of Dirac-cone like crossings on the (111) surface of the rhombohedral crystal. To confirm this behavior, ARPES measurements have been performed. A particular difficulty is here that three natural surface terminations are possible and the method probes a mixture of these.

FIG 2: Surface states of differently terminated Bi1Te1 surfaces: double quintuple layer (2QL), single QL, bilayer (BL) and a bilayer termination where one of the mirror symmetries (along ΓM’) is broken to show the protection of the surface state crossing by mirror symmetry (QL’, see insets). The spin polarization at the surface is indicated by the size of the symbols, the color denotes the sign of the projection of the spin onto a direction perpendicular to the k-vector. Green circles point to the symmetry protected band crossings along ΓM.

In Fig.2 we show the calculated surface band structures for the individual terminations. In each case, we observe along the line M a band-crossing near the Fermi level. In the 2QL and BL terminated case, there are additional states visible

in the band gap, however – in contrast to the protected states – they do not connect valence- and conduction band and can be removed by non-symmetry breaking perturbations. In ARPES it is also possible to detect the crossing points indicated in green in Fig.2, the spectra suggest a predominantly QL and 2QL terminated surface [4]. The observed “Dirac-cones” are severely distorted and, not unusual for TCIs, their spin-texture also differs from the one observed in 3D TIs. One has to note, however, that the (non-spin resolved) experimental spectra look quite similar to the ones obtained from the 3D TI Bi2Te3 [5]. E.g. in the case of the 2QL terminated surface, two bands with almost linear dispersion extend from the valence- to the conduction band. Without high resolution or spin-sensitive measurements, these states might be mistaken for the topologically protected surface state of a 3D TI. The spin-polarized ARPES measurements, however, performed in our study [4], can clearly discriminate between the two crystals, finding a low spin-polarization of the Bi1Te1 surface states in contrast to the strongly polarized Bi2Te3 states. To observe the edge states of Bi1Te1 that are introduced by its WTI character is even more difficult: a local probe, like scanning tunneling spectroscopy, has to be used to be sensitive to the one-dimensional rims of step-edges on the surface. In how far the different terminations and different step heights can lead to variations remains a subject of further experimental and theoretical study.

Surfaces of TCIs are discussed as platforms for electrically switchable spintronic devices: indeed, a symmetry breaking electric field can open a gap in the surface state crossing of the TCI, like in the configuration QL’ in Fig.2. In addition, the edge states of the WTI are susceptible to magnetic fields that beak the protecting time-reversal symmetry. In principle, this flexibility offers rich possibilities for multi-functional devices, but the present study also shows that a fine control of parameters like surface-termination is essential.

This work has been supported in part by the Virtual Institute for Topological Insulators (VITI) of the HGF and computing time granted on JURECA at the Jülich Supercomputing Centre (JSC).

[1] S. Murakami, Phys. Rev. Lett. 97, 236805 (2006)

[2] T. Hirahara, G. Bihlmayer, Y. Sakamoto, M. Yamada, H. Miyazaki, S-i. Kimura, S. Blügel and S. Hasegawa, Phys. Rev. Lett. 107, 166801 (2011)

[3] G. Bihlmayer, Yu. M. Koroteev, T. V. Menshchikova, E. V. Chulkov and S. Blügel in: Topological Insulators: Fundamentals and Perspectives Eds.: F. Ortmann, S. Roche, and S. O. Valenzuela, Wiley-VCH (2015)

[4] M. Eschbach, M. Lanius, C. Niu, E. Młyńczak, P. Gospodarič, J. Kellner, P. Schüffelgen, M. Gehlmann, S. Döring, E. Neumann, M. Luysberg, G. Mussler, L. Plucinski, M. Morgenstern, D Grützmacher, G. Bihlmayer, S. Blügel, and C. M. Schneider, Nature Comm. 8, 14976 (2017)

[5] A. Herdt, L. Plucinski, G. Bihlmayer, G. Mussler, S. Döring, J. Krumrain, D. Grützmacher, S. Blügel, and C. M. Schneider, Phys. Rev. B 87, 035127 (2013)


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Search for the mysterious SiTe – an examination of the binary Si−Te system using first principles-based methods

S. Steinberg, R. P. Stoffel, and R. Dronskowski

Institute of Inorganic Chemistry, RWTH Aachen University, Germany The A1B1-type tellurides of main-group IV elements are of great interest because of their applications as data and energy-storage materials. While the features of ATe (A = Ge, Sn, Pb) have been determined, there is no report on a solid-state SiTe. Herein, we review preexisting controversy in the literature about the Si−Te system and provide a feasible approach to SiTe.

The A1B1-type tellurides of Ge, Sn and Pb are of great interest due to their uses as data and energy storages [1]. Albeit the structures and the properties of these tetrel-tellurides are known, there is no report on a solid-state SiTe; yet, research on gaseous SiTe revealed the features of SiTe molecules in the vapor phase [2]. Also, there has been a controversy about a solid-state material in the Si−Te system: earlier inspections indicated Si2Te3 as the only phase [3], while others established SiTe2 [4].

Indicators to form a material may be gained from thermochemical data, so research on the reaction quantities of the silicon tellurides bared a negative free formation energy for Si2Te3 [3a]. To identify the factors that promote or complicate the formation of a material at the atomic scale, it is helpful to know the electronic and the vibrational properties for a composition of interest. As the structure model, which shows the lowest total energy and electronic as well as dynamic stability among diverse models, is typically the most favorable to compare to an observed crystal structure, we revisited the controversy between

SiTe2 and Si2Te3 based on evaluations of their total energies, electronic and vibrational properties. Additionally, the total energies and electronic as well as vibrational properties of diverse “SiTe” models were explored to propose a synthesis of SiTe in the solid state. The crystal structures of Si2Te3 and SiTe2 are both composed of hexagonal closest packed layers of Te atoms, but they differ in the amounts and distributions of the Si atoms [3b, 4]. In SiTe2, Si atoms occupy all octahedral voids of every second layer between the Te slabs (Fig. 1c) [4] while, in Si2Te3, disordered Si dumbbells reside in ⅔ of all octahedral voids of every second layer sandwiched by Te atoms (Fig. 1f) [3b]. Since the properties of Si2Te3 depend on the long-range order in its crystal structure [6], the electronic structures were examined for diverse “Si2Te3” models [5]. Two “SiTe” models were derived from the structures of SiTe2 and Si2Te3 by assigning Si atoms and dumbbells, resp., to empty, octahedral voids (Fig. 2), while two “SiTe” models were generated based on the structures of α- and β-GeTe (distorted and ordered NaCl-types, resp.) [7], as a cubic lattice was proposed for SiTe [4].

Examinations of the electronic densities-of-states (DOS) curves for “Si2Te3” (Fig. 1d) reveal that the Fermi level, EF, falls in a band gap suggesting Si2Te3 to be a semiconductor. The different widths of the gaps at EF in other “Si2Te3” models show the influence of the orientation of the Si dumbbells [5].

EF in SiTe2 falls at a maximum in the DOS (Fig. 1a), which points to an electronic instability. Thus, the situations at EF in “Si2Te3” cannot only explain

FIG. 1. (a, d) DOS, (b, e) −pCOHP and (c, f) representations of the crystal structures for SiTe2 and “Si2Te3”-I, respectively. DOS and −pCOHP curves of two other “Si2Te3” models are provided elsewhere [5].


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the red color and the band gap of 1.0 eV for the silicon telluride [4, 8], but also show an electronically more favorable situation in contrast to the features at EF in SiTe2. As the Fermi levels fall in pseudogaps in the DOS of “SiTe”-I to III and a gap in the DOS of “SiTe”-IV, electronically favorable situations are predicted for all “SiTe” models [5].

Bonding analyses for all “SiTe” and “Si2Te3” models and SiTe2 based on the projected crystal orbital Hamilton population (pCOHP) method revealed that bonding interactions mainly reside between the Si−Te and, if Si dumbbells are present, the Si−Si contacts (Fig. 1). In empty layers between Te slabs as observed for SiTe2 and “Si2Te3”, the Te−Te contacts are weakly bonding. This outcome correlates to the fact that Te layers enclosing Si dumbbells in Si2Te3 are easily separated due to the weak nature of the dispersion interactions between the layers [8]. Inspections of the phonon band structures for SiTe2 and “SiTe”-I to -III indicated the presence of negative modes pointing to dynamic instabilities [5]. Such large, imaginary modes are not evident in the phonon band structures of “SiTe”-IV and “Si2Te3”-I [5]; yet, the difference between the total energies of “SiTe”-IV and “SiTe”-III is rather small (Fig. 2). To detect a feasible phase transition from a high- to a low-temperature modification akin to that in GeTe [7], the electronic and vibrational properties were examined for modulated structures derived from the cubic “SiTe”-III (Fig. 2e).

A comparison of the energy-volume, E(V), curves for the original and distorted structures of “SiTe”-III (Fig. 2e) reveals two minima in the E(V) curve of the modulated structure of “SiTe”-III. The minima linked to the larger unit cell volumes are lower in energy than that of the undistorted “SiTe”-III

variant and hint to a feasible transition between “SiTe”-IV and “SiTe”-III.

In summary, the inspections of the electronic and vibrational features for SiTe2 and the “Si2Te3” models evidence that an electronically and dynamically favorable situation is achieved for the latter composition. Among the diverse “SiTe” models, the α-GeTe-type is preferred. The calculated relative enthalpies of “SiTe” and a solid mixture of “Si2Te3”-I and Si as functions of pressure at absolute zero (Fig. 2f) imply that applying a pressure ≥ 7.3 GPa leads to the formation of SiTe. Thus, synthetic efforts are on the way to confirm that finding.

Figs. 1 and 2 adapted from Ref. [5] with permission. © 2016, American Chemical Society. This work was supported by the Deutsche Forschungsgemeinschaft SFB 917, Nanoswitches.

[1] a) D. Lencer, M. Salinga, B. Grabowski, T. Hickel, J. Neugebauer, M. Wuttig, Nature Mater. 7, 972 (2008); b) J. G. Snyder, E. S. Toberer, Nature Mater. 7, 105 (2008)

[2] S. Chattopadhyaya, A. Pramanik, A. Banerjee, K. K. Das, J. Phys. Chem. A 110, 12303 (2006).

[3] a) R. F. Brebick, J. Chem. Phys. 49, 2584 (1968); b) K. Ploog, W. Stetter, A. Nowitzki, E. Schönherr, Mater. Res. Bull. 11, 1147 (1976)

[4] A. Weiss, A. Weiss, Z. Anorg. Allg. Chem. 273, 124 (1953)

[5] S. Steinberg, R. P. Stoffel, R. Dronskowski, Cryst. Growth Des. 16, 6152 (2016).

[6] C. Combs, X. Shen, Y. Puzyrev, L. Pan, S. Pantelides, in APS March Meeting, Baltimore, Maryland, USA, 2016

[7] W. D. Johnston, D. E. Sestrich, J. Inorg. Nucl. Chem. 19, 229 (1961)

[8] S. Keuleyan, M. Wang, F. R. Chung, J. Commons, K. J. Koski, Nano Lett. 15, 2285 (2015)

FIG. 2. (a, b, c, d) images of the structures for the “SiTe” models I to IV. (e): E(V) curves of “SiTe”-III and a modulated structure of “SiTe”-III. (f): relative enthalpies of “SiTe” and a mixture of Si and “Si2Te3”-I as functions of pressure.


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Ballistic transport in InAs nanowire quantum point contacts

S. Heedt1, W. Prost2, A. Manolescu3, G. A. Nemnes4, J. Schubert1, D. Grützmacher1, and Th. Schäpers1 1 Peter Grünberg Institut-9, Forschungszentrum Jülich, Germany 2 Solid State Electronics Department, University of Duisburg-Essen, Duisburg, Germany 3 School of Science and Engineering, Reykjavik University, IS-101 Reykjavik, Iceland 4 Faculty of Physics, MDEO Research Center, University of Bucharest, 077125 Magurele-Ilfov, Romania Ballistic transport is observed in InAs nanowire based quantum point contact devices. In these structures a constriction is formed inside the nanowire by biasing a gate finger crossing the wire. At low temperatures quantized conductance is measured, i.e. conductance steps in units of 2e2/h are found when the gate voltage is varied. These measurements demonstrate that the transport takes place in the ballistic regime with a finite number of quantized channels in the constriction. When a magnetic field is applied, sub-steps are resolved, which can be attributed to an additional energy splitting due to the Zeeman effect. From transconductance measurements the Zeeman energy splitting and the g-factor are determined. It is found that the g-factor is smaller than the value for bulk InAs, which is attributed to confinement effects. The investigation of quantum point contacts connected in series showed that the overall conductance is governed by the point contacts with the smaller conductance. These measurements demonstrate that the backscattering length exceeds the distance between the quantum point contacts.

Semiconductor nanowires are not only very promising as building blocks for future nano-scaled devices such as gate-all-around field-effect transistors, they are also very interesting objects for studying fundamental quantum phenomena. As a prominent example, semiconductor nanowires are considered as components of devices utilized to realize Majorana fermions [1], which are the basis for topological quantum computing. In this respect, it is important to obtain a single electronic mode in the wire, which can be confirmed by studying quantized conductance in the ballistic transport regime. Here, nanowires based on InAs are very attractive, because of the low effective electron mass and the high mobility, which ensure large quantization energies and a large elastic mean free path. Both parameters are relevant to obtain quantized conductance in the ballistic transport. Previously, ballistic electron transport was mainly investigated by using high mobility two-dimensional electron gases in semiconductor heterostructures, where a constriction is formed by means of a split-gate. While the constriction is narrowed by increasing the gate voltage the conductance is reduced in steps of 2e2/h, with the

electron charge e and Planck’s constant h. Here, we report on magnetotransport measurements on InAs nanowire devices, where the quantum point contact constriction is formed by biasing a narrow gate finger crossing the nanowire.

The InAs nanowires were grown by means of gold-catalyzed metal-organic vapor phase epitaxy [2]. The nanowires were mechanically transferred to a pre-pattered thermally oxidized Si substrate. A 100-nm-thick high-k dielectric layer (LaLuO3) was deposited as gate dielectric by pulsed laser deposition. The magnetoconductance measure-ments were performed on samples with seven closely-spaced top-gate electrodes, i.e. gates I to VII. Each gate covers a width of 180 nm and the pitch is 30 nm (see Fig. 1). The overall carrier concentration can be controlled by a back gate. The transport measurements were performed in a dilution refrigerator at a temperature of 100 mK with magnetic fields applied perpendicularly to the nanowire axis.

FIG. 1: Scanning electron micrograph of an InAs nanowire which is partially covered with 100 nm of the high-k dielectric LaLuO3. The seven top-gate fingers each have a width of 180 nm. The pitch is 30 nm.

As shown in Fig. 2 (a), clear conductance steps in units of 2e2/h due to ballistic transport are observed as a function of gate voltage [3]. The presence of conductance steps can be attributed to transport through one-dimensional subbands. By applying a magnetic field the step width increases, which originates from the additional contribution due to Landau quantization. For magnetic fields larger than about 3 T, sub-steps due to the Zeeman effect are resolved. By


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performing transconductance maps as a function of source-drain bias as well as gate voltage the energy level separations and the Zeeman splitting could be determined. Typical energy splittings due to Landau quantization were found to be in the order of 10 meV. The energy splitting owing to the Zeeman effect were employed to obtain the g-factor for each subband. As can be seen in Fig. 2 (b), the g-factor is smaller than the bulk value of 14.9, because of confinement-induced quenching of the orbital motion. For the last level the observed g-factor enhancement can be related to Coulomb exchange interaction.

FIG. 2: (a) Quantized conductance of a quantum point contact formed by biasing a gate finger. The measurements were taken at 100 mK for the magnetic field values in the range from 0 to 5 T. The sample was biased at a voltage of 4 mV. (b) g-factor as a function of subband index for five different quantum point contacts. The weighted average values are also shown (green line).

In the ballistic transport regime the resistance of two point contacts in series is non-additive. This is illustrated in Fig. 3, where the conductance is plotted as a function of the voltage at gate III with the voltage on gate V varied as a parameter [4]. The distance between the point contacts is 240 nm. A constant magnetic field of 10 T is applied. One finds that each quantum point contact can individually control the conductance, while the other one is set to zero gate voltage. When both quantum point contacts are biased simultaneously, the overall conductance is governed by the quantum point contact with the lowest conductance. The non-additivity of the quantum point contact resistances in series can be understood in the framework of edge channel transport [5]. At large magnetic fields well-separated edge channels are formed at the sidewalls of the nanowire. The transport at opposite sidewalls is in counter directions. Due to the spatial separation, no scattering occurs. The transport is adiabatic, i.e. the electrons flowing in these edge channels remain in the subbands they occupied in the quantum point contact. The observation that the conductance is not reduced is thus due to the fact that the electrons are neither backscattered to the opposite side nor scattered into adjacent channels on the same side. The total conductance is governed by the point contact comprising the lowest number of transport channels. In case that the potential barrier formed by the point contact is relatively high, i.e. close to

pinch-off, the edge channels merge at the barriers. A closed loop state is formed. By measuring the conductance as a function of a perpendicular magnetic field, regular conductance oscillations are observed [4]. These oscillations can be explained in the framework of Aharonov-Bohm type interference where the magnetic edge channels enclose a magnetic flux. The oscillations are periodic with the magnetic flux quantum h/e. Indeed, it was found that for point contacts at even larger distance a smaller oscillation period is observed. The experimental results are confirmed by a theoretical model based on edge channel transport [4].

FIG. 3: (a) Quantized conductance plateaus at 10 T and at a source-drain bias voltage of 5 mV as a function of the voltage at gate III with the voltage at gate V as a parameter. Both quantum point contacts can individually control the number of transmitted subbands. The quantum point contact with the lowest transmission determines the overall conductance. The red and yellow lines visualize the conductance steps at fixed voltages on gate V.

In conclusion, quantized conductance was observed in quantum point contacts formed in InAs nanowires. Measurements in a magnetic field allowed to extract the g-factor. It was found that the g-factor is reduced due to confinement effects. At finite magnetic field backscattering is suppressed, so that two quantum point contacts are connected adiabatically. As a result, the overall conductance is governed by the point contact with the lowest conductance.

[1] V. Mourik, K. Zuo, S. M. Frolov, S. Plissard, E.P.A. M. Bakkers, and L. P. Kouwenhoven, Science 336, 1003 (2012)

[2] Q.-T. Do, K. Blekker, I. Regolin, W. Prost, and F. J. Tegude, IEEE Electron Device Letters 28, 682 (2007)

[3] S. Heedt, W. Prost, J. Schubert, D. Grützmacher, and Th. Schäpers, Nano Letters 16, 3116 (2016)

[4] S. Heedt, A. Manolescu, G.A. Nemnes, W. Prost, J. Schubert, D. Grützmacher, and Th. Schäpers, Nano Letters 16, 4569 (2016)

[5] L. P. Kouwenhoven, B. J. van Wees, W. Kool, C. J. P. M. Harmans, A. A. M. Staring, and C. T. Foxon, Phys. Rev. B 40, 8083 (1989)


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Lazarevicite-type short-range ordering in ternary III-V nanowires

M. Schnedler1, I. Lefebvre2, T. Xu2,3, V. Portz1, G. Patriarche4, J.-P. Nys2, S. R. Plissard2,5, P. Caroff2,6, M. Berthe2, H. Eisele7, R. E. Dunin-Borkowski1, Ph. Ebert1, and B. Grandidier2 1 Peter Grünberg Institut-5, Forschungszentrum Jülich, Germany 2 Département ISEN, Institut d'Electronique, de Microélectronique et de Nanotechnologie (IEMN), CNRS,

Lille Cedex, France 3 Key Laboratory of Advanced Display and System Application, Shanghai University, Shanghai, People's

Republic of China 4 Laboratoire de Photonique et de Nanostructures (LPN), CNRS, Université Paris-Saclay, France 5 Laboratoire d'Analyse et d'Architecture des Systèmes (LAAS), CNRS, Université de Toulouse, France 6 Department of Electronic Materials Engineering, The Australian National University, Canberra, Australia 7 Institut für Festkörperphysik, Technische Universität Berlin, Germany Stabilizing ordering instead of randomness in alloy semiconductor materials is a powerful means to change their physical properties. We reveal the existence of an unrecognized ordering in ternary III-V materials. The lazarevicite short-range order (SRO), found in the shell of InAs1−xSbx nanowires (NW), is driven by the strong Sb-Sb repulsion along 110 atomic chains during their incorporation

on unreconstructed 110 sidewalls. Its spontaneous formation under group-III-rich growth conditions offers the prospect to broaden the limited classes of ordered structures occurring in III-V semiconductor alloys.

[111]-oriented zinc-blende (ZB) InAs1−xSbx NW segments were grown by Au droplet-assisted gas source molecular beam epitaxy (MBE) on top of wurtzite (WZ) InAs NWs, which were first nucleated on InP stems on InP(111)B substrates [1]. The growth temperature was set at 410°C. Figure 1(a) illustrates the overall structure schematically for a InAs0.90Sb0.10 NW. The bottom WZ InAs segment has a sixfold symmetry and consists of m-plane 1010 sidewall facets separated by small 1120 facets. The top InAs1−xSbx NW segment crystallizes in the ZB structure and shows six equivalent nonpolar 110 sidewall surfaces. In this segment, a sawtooth faceting occurs at the edges between the neighboring 110 sidewalls, which is connected to pseudoperiodic twin boundaries and consists of either alternating 111 and 111 or 001 and 001 surfaces.

Figure 1(b) shows an atomically resolved STM image of the sidewall facet of an InAs0.90Sb0.10 NW segment. It exhibits 5-10 nm wide twinned ZB domains, separated by twin boundaries (dashed vertical lines), consistent with the structure illustrated in Fig. 1(a). The anion atomic positions appearing brighter in the STM images are the signature of Sb atoms on anion lattice sites. The atomically resolved view of the individual chemical

FIG. 1: (a) Schematic of a [111]-grown InAs0.9Sb0.1/InAs nanowire. (b) Atomically resolved filled state STM image of the sidewall surface measured at 77 K (−3 V, 10 pA, constant current image). The image shows the filled dangling bond states above the surface anions. The bright atomically localized contrast features arise from Sb atoms incorporated on anion sites. Twin boundaries are marked by dashed vertical lines. The inset [marked I in (b)]: high-resolution STM image (−2 V, 700 pA) of one SbAs atom in the surface layer. The location of the atomic zigzag chain of alternating anion and cations is indicated by a ball model. (c) High-resolution STM image (−2 V, 700 pA) of area II in (b). Sb atoms in the first layer (Sb1) and third layer (Sb3) are visible. Local lazarevicite- [2] and CuPt-type ordered areas are labeled (i) and (ii), respectively. (d) and (e) illustrate the respective atomic models.

species enables a chemical mapping and hence the investigation of ordering in a ternary NW. Using this chemical map, we derived the two-dimensional pair correlation function (PCF) for Sb atoms [Fig. 2(a)]. The PCF exhibits large values above 1 along


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the [001] and [001] directions, indicating the frequent occurrence of Sb pairs across the atomic chains. In contrast, nearest- and second-nearest-neighbor Sb pairs within the atomic chains in the ±[110] directions occur rarely, as indicated by values significantly smaller than 1. Hence, the ternary alloy of the NW is not statistically distributed but rather exhibits a SRO, defined by an ordering vector ( ) in the [001] direction and an anti-correlation vector ( ) in the [110] direction.

FIG. 2: (a) Two-dimensional pair correlation function c(x,y) of Sb atoms in InAs0.9Sb0.1 NWs measured on the 110 sidewall facet. Values above (below) 1 indicate a higher (lower) than statistically expected occurrence of Sb pairs (scale on the right). (b) Calculated two-dimensional pair interaction energy of Sb pairs. (c) Lazarevicite crystal structure with indications of the ordering ( =[001]) and anticorrelation ( =[110]) vectors on a 110 plane. The cubic unit cell is indicated on the right side.

Density functional theory calculations [Fig. 2(b)] show an increased Sb-Sb pair interaction energy along the [110] direction, which indicates a strong repulsion within the atomic zigzag chains due to the larger size of Sb atoms as compared with As. In [001] and in [112] direction, the strain can be relaxed better, lowering the pair interaction energy.

and , are compatible with a lazarevicite-type SRO [2] [Fig. 2(c)]. In the STM images, the lazarevicite ordering shows up as Sb alignments along the [001] direction [Fig. 1(d) and (i) in Fig. 1(c)]. Note, besides the lazarevicite SRO, CuPt SRO is observed in the STM image [Fig. 1(e) and (ii) in Fig. 1(c)], however less frequently. Since both SROs exhibit nearly identical low pair interaction energies, the question arises why the lazarevicite SRO is dominant in InAs1−xSbx NWs?

To answer this question, we turn to a NW grow-model. First, scanning transmission electron microscopy images [3] revealed that SRO takes place during lateral overgrowth on the 110 sidewalls of the NW: The growth of the NWs by MBE involves not only the direct supply of precursors into the Au droplet, but also

impingement (and dissociation) of In atoms and As2/Sb2 molecules at the sidewalls and the substrate. These adatoms diffuse from the substrate toward the Au droplet [Fig. 3(a)] [4].

FIG. 3: (a) Schematic of the growth of InAs1−xSbx NWs. The shell growth on the 110 sidewalls of a [111]-oriented nanowire occurs through step flow motion of 112 -oriented steps. (b) and (c) Atomic models of the Sb

incorporation at the step edge on the sidewall surface. Sb cannot be incorporated in the atomic zigzag chains [pos. 2 in (b)] if another Sb rests on the neighboring anion lattice site of the same chain (pos. 1). Hence, the new Sb atom is deviated into the neighboring chain (e.g., pos. 3), leading to a [001] ordering vector.

Considering the growth temperature, the growth rate (0.5 ML/s) and the V-III flux ratio of 2 to 3, the lateral overgrowth proceeds in the step flow mode, in analogy to homoepitaxy on GaAs(110) [5]. The islands nucleate preferentially at the bottom of the NWs and grow by step flows in the [111] direction [Fig. 3(a)]. The steps are parallel to the 112 direction [6]. Hence we turn to the Sb incorporation at 112 -oriented steps. The 110 surfaces of ZB III-V semiconductors exhibit a relaxed 1×1 bulklike structure under stoichiometric conditions. Furthermore, the edge of the 112 -oriented steps on the III-V 110 cleavage surfaces exhibit a ×1 bulklike structure, too [7]. We exemplify the situation where one Sb atom was incorporated already in the previous [112]-oriented atomic row [red atom marked 1 in Fig. 3(b)]. The strong repulsion of Sb-Sb pairs along the atomic zigzag chain in the [110] direction blocks a new Sb atom to be incorporated at lattice position 2. Hence, the Sb atom must sidestep towards one of the neighboring lattice positions (3 or 4). If incorporated at position 3, lazarevicite-type [001] ordering forms [Fig. 3(c)]. CuPt SRO occurs, if [001]-oriented step facets (i.e., kinks of 112 steps) are present or when several Sb atoms are incorporated simultaneously in neighboring atomic chains. Due to the low V/III ratios, the arrival of two or more Sb atoms at the same time is, however, unlikely. Therefore, the lazarevicite SRO is preferred.

[1] T. Xu et al., Nanotechnology 23, 095702 (2012)

[2] C. B. Sclar and M. Drovenik, Bull. Geol. Soc. Am. 71, 1970 (1960)

[3] M. Schnedler et al., Phys. Rev. B 94, 195306 (2016)

[4] V. G. Dubrovskii et al., Nano Lett. 15, 5580 (2015)

[5] P. Tejedor et al., Surf. Sci. Lett. 424, L309 (1999)

[6] P. Tejedor et al., Phys. Rev. B 59, 2341 (1999)

[7] M. Heinrich et al., Phys. Rev. B 53, 10894 (1996)

a) b)

c)


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Complementary Si nanowire GAA TFETs inverter with suppressed ambipolarity

K. Narimani ,G. V. Luong, A. T. Tiedemann, P. Bernardy, S. Trellenkamp, S.Mantl, and Q.T.Zhao

Peter Grünberg Institute-9 (IT), Forschungszentrum Jülich, Germany In this paper, we fabricated complementary tunneling field effect transistors (CTFET) based on strained Si with gate all around nanowire (GAA NW). The main focus is to suppress the inherent ambipolar behaviour of the TFETs with a gate-drain underlap by using a SiO2 spacer at the drain side. Detailed device characterization and the demonstration of a CTFET inverter show that the ambipolarity is successfully eliminated for both p-and n-devices. The voltage transfer characteristics of the fabricated CTFET inverter indicate maximum separation of the high/low level with steep transition for supply voltage down to 0.4V and a high noise margin.

In terms of low power electronics, transistor concepts enabling steep slopes are fundamental in order to maintain high Ion/Ioff ratios within a small voltage window. Due to the fact that switching ability of conventional MOSFETs at room temperature is limited by 60mV/dec, these devices either suffer from high Ioff-current or experience significant performance degradation when the supply voltage Vdd is scaled down. Among all steep slope concepts, Tunneling field effect transistors (TFETs) is the most promising energy-efficient switch since the first experimental proof of inverse subthreshold slope SS<60mV/dec [1]. The reason for this is that current injection in TFETs rely on band-to-band tunneling from the source to the channel neglecting the thermally broadened Fermi distribution of carriers which causes the SS limitation in MOSFETs [2]. Extensive research had been carried out in the past years with the goal to improve the Ion-current in TFETs [3]. TFETs based on all-Si do not seem competitive when the physical properties are considered. However, the huge advantage from the immense CMOS technology compensate this allowing advanced design concepts and easier processing of p- and n-type devices at the same time, as required for complementary TFETs. The employment of thin, strained Si body, e.g. NW architecture, in conjunction with high-k oxides provides optimized electrostatics to enhance tunneling currents. In the past, single p- and n-TFETs had been demonstrated as well as first functional TFET inverters all based on strained Si NWs. So far, all-Si TFETs appears primary to have very high Ion/Ioff ratio and the lowest Ioff-current [4]. But the average SS of these devices are still higher than desired

which is mainly attributed to residual defects formed by ion implantation. Furthermore, the latest results of sSi NW-GAA TFET suffers from not negligible Ioff -currents caused by charge carrier tunneling from the channel to the drain referred as ambipolar behavior. Hence, TFET becomes conductive for positive and negative gate potential. Suppressing the ambipolarity in TFETs is mandatory to allow full functionality of CMOS logic as reported in [5]. In this work, experimentally present sSi NW-GAA TFET with the suppression of the parasitic tunneling current of TFETs by introducing a gate-drain overlap. For demonstration purpose complementary TFET inverters were applied in order to present the improved inverter transfer characteristics without the presence of ambipolar behavior of TFETs.

Fig. 1 shows the fabrication process of the strained Si GAA NW TFETs inverter on biaxial tensely (ε=0.8%) strained 15nm SOI on 145nm BOX. Firstly, transistor mesa is defined by e-beam and etched in RIE (Fig.2 (a)). Then 3nm HfO2 and 60nm TiN is deposited and patterned to form the gate of the transistors (Fig.2(b)). Afterwards to create a drain-gate underlap, a SiO2 spacer is formed on the drain side of the device (Fig. 2(c)). 2nm Ni is deposited by PVD and annealed to form NiSi2 as source-drain contacts (Fig.1(c)). Finally, tilted implantation of boron and phosphorous into the silicide NiSi2 followed by activation at 550°C for 10 seconds defines source and drain doping.

FIG. 1: (a) Schematic layout of the complementary sSi GAA NW TFET. The process flow for the corresponding inverter. (b) The cross section along a single NW through both n-and p-TFET (c) Transmission electron microscopy (TEM) image along a NW.


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FIG. 2: (a) Scanning electron microscopy (SEM) image showing the patterned and suspended sSi NW. (b) After the gate process the sSi NW is clearly wrapped around by the 60nm TiN. (c) SiO2 passivation on the drain side to create a gate-drain underlap to prevent ambipolarity.

The Id(Vgs) transfer characteristics of the p- and n-TFETs with 45x5cm² sSi GAA NWs fabricated with the process above are presented in Fig.3. Each device contains 50 NWs and the gate length is 350nm. In order to emphasize the ambipolarity suppression, similar TFETs were taken into account which had been fabricated with the same process but without the attempt to suppress the ambipolar behaviour (dashed lines). When biased generously into the OFF-state, both p-and n-devices with introduced gate-drain underlap by using SiO2 passivation at the drain side (solid lines) show a significant reduction of the parasitic gate-drain tunneling.

FIG. 3: Id-Vgs characteristics of single (a) p- and (b) n-TFETs with 45x5nm² sSi GAA NWs (solid lines). The transfer curves of similar sSi GAA TFETs but without gate-drain underlap has been plotted for comparison (dashed lines).

In the insets of Fig.3 the SS is plotted as a function of Id at Vds =|0.1V|. We extracted the average SS value from Id = 10-6 to 10-3 µA/µm at room temperature. The p-TFET with the phosphorus pocket at the source yields SS = 110mV/dec. The output characteristics are displayed in Fig.4 showing the desired current saturation for transistors.

FIG. 4: Id-Vds characteristics of single (a) p- and (b) n-TFETs with 45x5nm² sSi GAA NWs.

As expected from the transfer characteristics, the p-TFET shows higher Ion at all gate overdrive

voltages. The typical exponential onset in the linear region can be observed. Complementary TFET inverter based on strained Si GAA NW were fabricated according to the schematic shown in Fig.1a). The source of the n-TFET is connected to the ground potential while the source of the p-TFET is attached to Vdd. Both drain terminals of the devices are connected together forming the output terminal of the inverter Vout. A pair of electrically matched n-and p-TFETs with suppressed ambipolar behavior had been chosen as depicted in Fig. 3. The resulting inverter voltage transfer characteristics is demonstrated in Fig. 5 starting from Vdd=1V down to 0.4V.

FIG. 5: Voltage transfer characteristics (VTC) of the inverter at supply voltages Vdd from 1V to 0.4V. The voltage gain of the inverter at the corresponding voltages as a function of the input voltage Vin.

In comparison to our inverter shown in the past, this optimized inverter without ambipolarity exhibits excellent noise margins. The high-level reaches the maximum Vdd voltage even for very low input voltage Vin which proves that the n-TFET is not conductive at this state. Same holds for the low-level of the inverter which can be pulled down entirely to the GND level due to the suppressed ambipolarity of the p-device. Based on that, both logical levels (high/low) have the maximum separation of Vdd. The abrupt junction between these levels are given by the voltage gain which indicates how well Vout is amplified for a small change in Vin. At Vdd =1V the gain amounts to 38 and a gain of 11 is achieved for Vdd=0.4V. It is also worth to mention, that the threshold voltage of the inverter is not located at Vdd/2. This results from the different work function of the utilized p- and n-TFET as visible in Fig.4(a). In conclusion, a solution for this would be to apply different metal with matched work functions to align the transfer curve of the complementary devices at the point of origin.

[1] J. Appenzeller, Y. M. Lin, J. Knoch, Z. Chen, and P. Avouris, IEEE Trans. Electron Devices. 52 (12), 2568 (2005)

[2] A. M. Ionescu and H. Riel, Nature 479 (7373), 329 (2011)

[3] H. Lu and A. Seabaugh, IEEE J. Electron Devices Soc. 2 (4), 44 (2014)

[4] L. Knoll, Q.-T. Zhao, A. Nichau, S. Trellenkamp, S. Richter, A. Schäfer, D. Esseni, L. Selmi, K. K. Bourdelle, and S. Mantl, IEEE Electron Device Lett. 34 (6), 813 (2013)

[5] L. Knoll, Q. T. Zhao, S. Habicht, C. Urban, B. Ghyselen, and S. Mantl, IEEE Electron Device Lett. 31 (4), 350 (2010)


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Reconfigurable multi-gate transistors

M. R. Müller1,2, T. Grap1, F. Riederer1, T. Finge1, K. Kallis2, A. Seabaugh4, J. Appenzeller3, and J. Knoch1 1 Institute of Semiconductor Electronics, RWTH Aachen University, Germany 2 Chair for Intelligent Microsystems, TU Dortmund University, Germany 3 Birck Nanotechnology Center, Purdue University, USA 4 Department of Electrical Engineering, University of Notre Dame, USA We present a study on multi-gate field-effect transistors that allow adjusting the potential landscape in semiconducting nanostructures on the nanoscale. To this end, a damascene-like process is employed that allows fabricating substrates with a varying number of buried gate structures that are contacted individually and exhibit lengths ranging from hundreds of nanometers down to the 5nm regime with inter-gate distances down to the 5nm regime. Applying appropriate voltages to the various gates allows, e.g., the realizing of reconfigurable transistors. WSe2 transistors as n-FET, p-FET and with the functionality as band-to-band tunnel transistors have been realized. In addition, InAs nanowire multi-gate field-effect transistors are presented.

An attractive approach to increase the functionality of highly integrated circuits is to add functionality to the devices themselves. Reconfigurable transistors have therefore attracted a great deal of attention (see e.g. Ref. [1)]. Reconfigurable field effect transistors (FETs) usually consist of at least two gate electrodes, namely one that is the actual gate and the other gate electrode - the program or polarity gate - that allows to reconfigure the transistor. However, adding a third gate electrode enables conventional n-/p-type devices and also to configure the transistor as band-to-band tunneling FETs [2]. To this end, we fabricated buried tri-gate substrates [3] employing a local-oxidation process together with a damascene-process. 4" silicon-on-insulator (SOI) wafers with a top-layer of 340nm (100) Si and a 400nm buried oxide (BOX) were degenerately doped by P-implantation (75keV, 1015cm-2, 7.5° tilt). Next, 200nm Si3N4 was depo-sited with PECVD. Subsequently, dopant activation was carried out at 900°C for 20min. Optical lithography and dry etching (CHF3/O2 plasma) was then used to pattern the nitride layer and after the removal of the photoresist (PR), tetramethyl-ammonium hydroxide (TMAH), 25 wt.% at 80°C was used to anisotropically etch through the SOI layer. In the following thermal oxidation step, the TMAH etch flanks were locally oxidized to realize a gate-sidegate (GSG) insulation since the nitride layer on top of the SOI prevents diffusion of oxygen (cf. Fig. 1 (a)). After the oxidation, the Si3N4 is removed by CHF3/O2 plasma etching and hot H3PO4. A thin SiO2 (<10 nm) is grown on the silicon surface to provide additional insulation and as a stopping layer for the following damascene process: After sputter-deposition of aluminum onto the surface, chemical-mechanical planarization

(CMP) was employed to remove the Al overburden yielding a planar substrate surface. Finally, the gate dielectric was realized with atomic layer deposition (ALD) of 7nm Al2O3. The tri-gate structures are completed by re-moving the oxides in the contact regions and patter-ning of Ti/Au contacts using a lift-off procedure [3].

FIG. 1: (a) Electron micrograph of a cross section of the buried tri-gate structures with a ~90nm GSG insulation. (b) AFM image with WSe2 flake on top of a tri-gate substrate.

Figure 2 shows Id-Vgs curves in the case of an n-type (a), p-type (b) and a TFET (c) configuration [3]. In (a) and (b) the current is ultimately limited by the injection through the source-side Schottky-barrier as is clearly observable by the current saturation occurring at larger gate voltage. Nevertheless, applying appropriate side-gate voltages allows the realization of n- as well as p-type regions in the source and drain areas proving that in (c) a TFET has been realized. Such a reconfiguration from conventional to TFET is highly attractive since it ultimately it allows high-performance circuits that can be switched dynamically into the TFET-mode whenever ultralow power consumption of the circuit is required. Although in the present case, the switching behavior in the TFET configuration is still inferior to a conventional transistor, a rough estimate of the electrostatics shows that further


40

improvements of the device can be obtained if the design of the tri-gate substrates is modified, in particular, if a thinner GSG insulation is realized. To do so and, in addition, to being able to manipulate the potential within the device such that, e.g., the injection of carriers at the source/drain contacts can be controlled independently from the potential in the source/drain extensions we fabricated buried multi-gate substrates, again based on a damascene process, illustrated in Fig. 3 (a)-(f).

FIG. 2: Transfer characteristics of a ~3 nm-thick WSe2-flake, electrostatically doped to function as (a) an n-FET (Vsidegates>0), (b) p-FET (Vsidegates<0) and (c) a TFET. Note that the gate leakage current is always less than 10-12 A; for clarity, it is not shown in (a) and (b) [2].

After etching a donut-shaped mold into a silicon substrate with a steep etch flank in the center and a very shallow etch flank at the edges, a multi-layer consisting of alternating layers of TiN and SiO2 was deposited with ALD and PECVD. Subsequently, a planarization was carried out and a 30nm Al2O3 gate dielectric was realized with ALD. Next, electron beam lithography (EBL) is used to open contact holes in the area where due to the shallow etch flank of the Si mold, a substantial broadening of the TiN layers occurred. Finally, contact fingers are patterned using EBL and lit-off. In the case shown in Fig. 3 the gates have a length of 15nm with a 25nm insulation between adjacent gates. However, the process also enables the fabrication of gate structures down to 5nm with a 5nm inter-gate insulation (not shown here).

To demonstrate the functionality of the multi-gate substrates, InAs nanowires with a diameter of 65nm were dispersed onto the substrates and contacted with EBL and lift-off: electron micrographs of a fabricated device are shown in Fig. 3 (f) and (h). Applying voltages at various different gates allows indeed to realize field-effect transistors as displayed by the Id-Vgs curves shown in the lower panel of Fig. 3. In the present case, however, the effective gate oxide thickness in combination with the rather large diameter of the

nanowire yields an effective natural length scale for potential variations approximately given by

⁄ ~65nm which is substantially larger than the gate length. Scaling down the nanowire diameter or replacing it with, e.g., a carbon nanotube will allow exploiting the full capacity of the multi-gate substrates.

FIG. 3: (a)-(f) Schematic illustration of the fabrication process of multi-gate substrates. (g) SEM-image of an InAs-NW multi-gate device and (h) the InAs-NW with the multi-gate area underneath. Lower panel: Drain current as a function of three different gates showing that the potential can be manipulated by the individual gate electrodes.

In conclusion, we presented multi-gate field-effect transistors with gate lengths down to the 15nm regime that allow adjusting potential landscapes within the transistor and thus facilitate reconfigurability. WSe2 FETs acting as n-FET, p-FET and TFET were experimentally demonstrated.

[1] A. Heinzig, T. Mikolajick, J. Trommer, D. Grimm, and W. Weber, Nano Lett. 13, 4176 (2013)

[2] J. Knoch, Semiconduct. Semimet. 94, 273 (2016)

[3] M. Müller, R. Salazar, S. Fathipour, H. Xu, K. Kallis, U. Künzelmann, A. Seabaugh, J. Appenzeller and J. Knoch, Nanoscale Res. Lett. 11(1), 512 (2016)

[4] T. Grap, F. Riederer, C. Gupta and J. Knoch, European Solid-State Dev. Res. Conf. 2017


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Phase diagram of Eu magnetic ordering in Sn-flux-grown Eu(Fe1-xCox)2As2 single crystals

W. T. Jin1,2, Y. Xiao1, Z. Bukowski3, Y. Su2, S. Nandi4, A. P. Sazonov2,5, M. Meven2,5, O. Zaharko6, S. Demirdis2, K. Nemkovski2, K. Schmalzl7, L. M. Tran3, Z. Guguchia8, E. Feng2, Z. Fu2, and Th. Brückel1,2 1 Jülich Centre for Neutron Science JCNS-2, Forschungszentrum Jülich, Germany 2 Jülich Centre for Neutron Science JCNS at MLZ, Garching, Germany 3 Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Wroclaw, Poland 4 Department of Physics, Indian Institute of Technology, Kanpur, India 5 Institut für Kristallographie, RWTH Aachen University, Germany 6 Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institut, Villigen, Switzerland 7Jülich Centre for Neutron Science JCNS at ILL, Grenoble, France 8 Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institut, Villigen, Switzerland Iron-based superconductors discovered in 2008 have provided new opportunities to study the interplay between superconductivity (SC) and magnetism, as the SC in these new materials was found to emerge based on the suppression of static long-range ordered antiferromagnetism. EuFe2As2 is a unique family within the iron pnictides as it contains two magnetic sublattices. Superconductivity can be achieved in this system by suppressing the spin-density-wave (SDW) ordering of Fe by chemical substitution. Nevertheless, for electron-doped Eu(Fe1-xCox)2As2 system, the reports about its physical properties remain quite controversial. Here we have performed comprehensive and systematic studies on single crystals of Eu(Fe1−xCox)2As2 with different Co-doping levels grown from Sn flux. We have microscopically investigated the evolution of the ground-state Eu2+ magnetic order with Co doping by single-crystal neutron diffraction measurements, and have established the phase diagram of Sn-flux-grown Eu(Fe1−xCox)2As2 single crystals based on both macroscopic and microscopic measurements.

EuFe2As2 is a special member of the “122” AFe2As2 family, since the A site is occupied by an S-state (orbital moment L = 0) Eu2+ rare-earth ion. In a purely ionic picture, it has a 4f7 electronic configuration and a total electron spin S = 7/2, corresponding to a theoretical effective magnetic moment of 7.94 μB. This compound exhibits a spin-density-wave (SDW) ordering of the itinerant Fe moments concomitant with a tetragonal-to-orthorhombic structural phase transition below 190 K. In addition, the localized Eu2+ spins order below 19 K in an A-type antiferromagnetic (AFM) structure (ferromagnetic layers stacking antiferromagnetically along the c direction). Superconductivity can be achieved in this system by suppressing the SDW ordering of Fe in the form

of chemical substitution or applying external pressure. In contrast to the EuFe2(As1−xPx)2 system whose phase diagram was already thoroughly investigated, a specific phase diagram of Eu(Fe1−xCox)2As2 describing how the magnetic order of the Eu2+ moments develops with Co doping and how it is linked with the occurrence of SC has not been established yet.

In order to establish a specific phase diagram of Eu(Fe1-xCox)2As2 describing its physical properties, we have performed complimentary macroscopic (resistivity, magnetization, specific heat) and neutron diffraction measurements on a series of Sn-flux grown single crystals with different Co-doping levels. The single-crystal neutron diffraction measurements were performed on the hot-neutron four-circle diffractometer HEiDi and the diffuse scattering cold-neutron spectrometer DNS at the Heinz Maier-Leibnitz Zentrum (Garching, Germany), the thermal-neutron four-circle diffractometer TriCS at Paul Scherrer Institute (Villigen, Switzerland), and thermal-neutron two-axis diffractometer D23 at Institut Laue Langevin (Grenoble, France).

Fig. 1 shows the temperature dependencies of the normalized in-plane resistivity (ρab) of Eu(Fe1-xCox)2As single crystals with x = 0.014(2), 0.027(2), 0.053(2), 0.075(2), 0.100(4), and 0.180(5). For the parent compound EuFe2As2 (x = 0), an upturn around 190 K was observed in the ρ(T ) curve, due to the opening of a gap at the Fermi surface associated with the SDW order and structural transition. Upon Co doping, it is clear that the structural phase transition (Ts) gets gradually suppressed. For x = 0.100(4), the resistivity shows a drastic drop below the superconducting transition temperature TSC = 10.1(4) K (see the inset of Fig. 1) and almost reaches zero. For x = 0.18, the resistivity drops sharply below TSC = 8.3(3) K and finally a zero-resistance state is achieved.


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FIG. 1: The temperature dependencies of the normalized in-plane resistivity (ρab) of Eu(Fe1-xCox)2As2 single crystals.

Fig. 2 illustrates how the ground-state magnetic structure of Eu(Fe1-xCox)2As2 evolves with the Co concentration, determined by neutron diffraction measurements [1]. For x ≤ 0.053, the Eu2+

moments keep the alignment within the ab plane in the A-type AFM order. Upon further increase of the Co doping level, the Eu2+ spins start to rotate toward the c axis within the ac plane, exhibiting the A-type canted AFM structure with a net FM moment component along the c direction. At base temperature, the canting angle of the Eu2+ spins out of the ab plane increases with Co doping, from 23.8(6)° for x = 0.075(2), to ～ 65° for x = 0.100(4), finally to 90° for x = 0.180(5) showing a pure FM order [2].

FIG. 2: The ground-state magnetic structure of Eu(Fe1-

xCox)2As2 with x ≤ 0.053 (a), x = 0.075(2) (b), x = 0.100(4) (c), and x = 0.180(5) (d), as determined by single-crystal neutron diffraction.

Combining the results of both macroscopic and neutron diffraction measurements, we have established the phase diagram of Sn-flux-grown Eu(Fe1-xCox)2As2 single crystals. As shown in Fig. 3, both the structural phase transition (TS, determined from resistivity measurements) and the SDW transition of Fe (TSDW, determined from neutron diffraction) get continuously suppressed by Co doping. On the other hand, the magnetic ground state of the Eu2+ moments is found to depend strongly on the Co doping level. The ordering temperature of the Eu sublattice, TEu, declines linearly at first, and then reverses upwards after reaching a minimum value of 16.5(2) K around x = 0.100(4).

FIG. 3: The phase diagram of Sn-flux-grown Eu(Fe1-

xCox)2As2 single crystals. The dotted lines are linear fittings to TS and TSDW.

The change of the magnetic ground state and the ordering temperature of the Eu sublattice probably arises from the combined effects of the doping-induced modification of the indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the Eu2+ moments, which is mediated by the conduction d electrons on the (Fe,Co)As layers, as well as the change of the strength of the direct interaction between the Eu2+ and Fe2+ moments. In addition, for Eu(Fe1-xCox)2As2 single crystals with 0.10 ≤ x ≤ 0.18, strong ferromagnetism from the Eu sublattice is well developed in the superconducting state, where a spontaneous vortex state is proposed to account for the compromise between the two competing phenomena.

[1] W. T. Jin, Y. Xiao, Z. Bukowski, Y. Su, S. Nandi, A. P. Sazonov, M. Meven, O. Zaharko, S. Demirdis, K. Nemkovski, K. Schmalzl, Lan Maria Tran, Z. Guguchia, E. Feng, Z. Fu, and Th. Brückel, Phys. Rev. B 94, 184513 (2016)

[2] W. T. Jin, S. Nandi, Y. Xiao, Y. Su, O. Zaharko, Z. Guguchia, Z. Bukowski, S. Price, W. H. Jiao, G. H. Cao, and Th. Brückel, Phys. Rev. B 88, 214516 (2013)


43

Study of free charge carrier distributions in electrically contacted InAs nanowires with infrared s-SNOM

R. Ukropec1, F. Haas1, L. Jung2, B. Hauer2, H. Lüth1, D. Grützmacher1, T. Taubner2, H. Hardtdegen1, and T. Schäpers1 1 Peter Grünberg Institut-9, Forschungszentrum Jülich GmbH, Germany 2 I. Physikalisches Institut IA, RWTH Aachen University, Germany

Due to the high surface to volume ratio of nanowires (NWs), surface states play a major role in their overall electrical characteristics. Standard electrical measurements unfortunately only provide an insight into the NW properties as a whole or in segments between contacts. However, with scattering type scanning near-field optical microscopy (s- SNOM) is possible to extract the local free charge carrier concentration and mobility at very high resolution (≈ 30 nm) from infrared spectra [1]. Here, highly doped InAs NWs were deposited onto a back-gated SiO2 covered Si-n++ substrate and electrically contacted to a sample holder developed specifically for the use with an s-SNOM. With s-SNOM imaging we observed charge carrier concentrations in the order of n ≈ 5·1018 cm−3 and n ≈ 16·1018 cm−3 for two differently doped InAs NWs, which are consistent with the incorporated doping [2]. Variations in the spatial charge carrier concentrations were observed along the NW. Infrared s-SNOM was then employed for investigating the changes in conductivity affected by chemical passivation treatments or by backgate electrostatic effects.

Semiconductor nanowires are considered to be very promising elements for future nano- and optoelectronic device applications. This is mainly due to the bottom-up approach employed for their fabrication, which simplifies the fabrication process enormously. For all types of nanowires there is one big issue, which needs to be addressed, which is related to the large surface-to-volume ratio. In low-band gap materials e.g. InAs or InSb, the carriers are accumulated at the surface due to the Fermi level pinning within the conduction band. Especially for bulk nanowires consisting of these materials it is known that the environment and the history of fabrication and characterization steps have a large influence on the electron transport properties: For example, for InAs nanowires a significant unintentional increase of electron concentration was observed after electron irradiation. Here, we want to modify the surface properties of InAs nanowires by sulfide passivation and subsequently investigate these samples by scattering-type scanning near-field optical

microscopy (s-SNOM) and electrical transport measurements (Fig.1).

FIG. 1: Top: Sketch of s-SNOM tip optically probing an InAs nanowire exhibiting local variations. Bottom: Light microscope image of n s-SNOM-tip attached to a cantilever above various contacted InAS nanowires and scheme of electrical contacting.

In s-SNOM, the very high spatial resolution in the order of a few 10 nanometers is realized by the localization of light at the end of a metalized tip. Its operation with mid-infrared light allows to directly address free charge carriers in doped semiconductors and even individual nanowires. InAs nanowires are fabricated by selective-area metal-organic vapor phase epitaxy (SA-MOVPE) with two different doping factors (DF 500 and DF 1000) by changing the precursor gas ratios. Subsequently, we contacted individual InAs nanowires by nanolithography and transferred them onto a specially designed sample holder which enables electrical gating during near-field optical characterization. By subsequently imaging the same nanowire while changing the s-SNOM illumination wavelength, maps of the infrared amplitude and optical phase are acquired (Fig.2).


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By extracting line profiles of infrared amplitude and phase at different wavenumbers, the local charge carrier density and its variations can be extracted by fitting with a model describing the s-SNOM contrasts for samples with Drude-like behavior.

FIG. 2: Infrared s-SNOM images of a single contacted InAs nanowire in amplitude (top) and phase (bottom) taken at different wavenumbers of the illumination laser show systematic and reproducible variations along the nanowire.

A change of the charge carrier density along the nanowire was observed for nanowires with two different doping factors (with only DF 500 plotted in Fig. 3). Up to now, however, the physical origin of these charge carrier variations (bulk or surface states) has not yet been addressed.

FIG. 3: Distribution of the local charge carrier density along a single InAs nanowire (Doping factor DF500) extracted from spectroscopic s-SNOM measurements by fitting of a Drude-term into the s-SNOM model.

For a further investigation we also imaged a doped nanowire (again DF 500) under different bias voltages (-7V and +20V) at different infrared wavenumbers. The bias voltage was limited to this rather small range because of leakage currents. The corresponding s-SNOM images did show rather uniform signals, which varied only slightly with wavenumber. Especially, their spectral response did not allow for a fitting with the Drude-model mentioned above. We assume that in our nanowires, either the charge carrier concentration was so high that a small bias voltage lead to only negligible changes of the free charge carrier density or that our laser tuning range was too

narrow to match the Drude resonance. In a recent publication [3], InAs nanowires with a controlled gradient in charge carrier density (ranging from n ≈ 1·1016 cm−3 to n ≈ 5·1018 cm−3) were imaged with an infrared s-SNOM at a fixed wavenumber. By applying large bias voltages (up to +/-100V), a shift of a bright area matching the Drude resonance condition in s-SNOM was observed. However, a spectroscopic study was not performed in ref. [3].

FIG.4: The surface passivation of InAs with ammonium sulfide is expected to remove the native oxide and thus alter the accumulation layer.

In our project, we also investigated nanowires before and after passivation with ammonium polysulfide. Since chemical passivation removes the native oxide, it is in principle a modification of surface states. For example, on the surface of InAs the native oxide contributes to the formation of an accumulation layer. Therefore, any removal of the native oxide can alter this accumulation layer, leading to an altered density of states and charge carrier concentration. However, the passivation did not show an overall significant change in the s-SNOM signal from the nanowire itself. Features and variations in signal seen before passivation at one end of the nanowire appear unchanged after etching. This is not unexpected when one considers that although the passivation increases the band bending at the surface of the nanowire, the high doping concentration is still the dominating source for carriers. Thus, any changes introduced by passivation would be effectively screened by the high levels of the already present carrier concentration. For the passivated nanowires, the characterization of the measured data using the Drude and finite dipole model was not conclusive as the spectroscopy was clearly out of the optical resonance for the majority of wavenumbers.

In summary, we have presented a first proof-of principle study investigating the local free charge carrier properties of doped semiconductor nanostructures under an applied bias and after surface passivation with s-SNOM spectroscopy.

This Project was financed by JARA-FIT Seed Funds as part of the Excellence Initiative II of the Deutsche Forschungsgemeinschaft (DFG).

[1] B. Hauer, T. Saltzmann, U. Simon, and T. Taubner, Nano Letters 15, 2787 (2015)

[2] S. Wirths, K. Weis, A. Winden, K. Sladek, C. Volk, S. Alagha, T. E. Weirich, M. von der Ahe, H. Hardtdegen, H. Lüth, N. Demarina, D. Grützmacher, and T. Schäpers, Journal of Applied Physics 110, 053709 (2011)

[3] A. Arcangeli et al., Nano Letters 16, 5688 (2016)


45

GeFe3N0.6: an itinerant nitride with a frustrated magnetic ground state

T. Scholz and R. Dronskowski

Institute of Inorganic Chemistry, RWTH Aachen University, Germany Frustrated magnetism arises when the arrangement of magnetic atoms and their interaction are geometrically incompatible. These conditions are met in many materials, the classic example being a crystal with triangular structural motifs and antiferromagnetic interactions. Upon studying the series of compounds GexFe4–xNy (0 x 1), we recently found a frustrated magnetic ground state for the Ge-rich GeFe3N0.6. Here, we present composition and structure of this series and the drastic weakening of ferromagnetic interactions while going from -Fe4N to the canonical spin glass GeFe3N0.6 with RKKY indirect-exchange interactions.

In recent years, material scientists and solid-state chemists have been working, with increasing efforts, on the topic of transition-metal based ternary nitrides and carbides. Despite the very simple antiperovskite-like structure of those compounds, they feature remarkable physical properties such as superconductivity, giant magnetoresistance, negative thermal expansion, and also spin-glass behavior, to only name a few. The spin-glass behavior was found in various chromium-, manganese-, cobalt-, and nickel-based ternary nitrides, and it is somewhat surprising that similar properties have not been reported for ternary iron nitrides so far.

The first ternary iron nitride spin glass, Sn0.9Fe3.1N, appeared in 2016, and it features a glassy transition temperature Tg of roughly 13 K [1,2]. All previously published nitrides MxFe4–xN with M being a transition-metal element are ferromagnetic, as is the parent compound -Fe4N, too. The binary nitride has been known for long and widely studied for its exceptional magnetic properties (i.e., a large saturation magnetization and a low coercivity), its high mechanical hardness, and impressive corrosion resistance. Now, yet another canonical spin glass of the nitride family of phases, GeFe3N0.6 (precisely: Ge0.97Fe3.03N0.56), has been characterized [3]. For the Ge-rich compound, dynamic (AC) magnetization measurements evidenced a spin-glass-like transition at a Tg of 36.68(5) K.

To better understand the transition from the ferromagnet -Fe4N to the spin glass GeFe3N0.6, the entire series of compounds GexFe4–xNy (0 x 1) has been synthesized. Synthetically, nitrides are quite demanding because a direct use of the inert N2 molecule with its firm triple bond is seldomly rewarding. Instead, one may use a classical NH3/H2 ammonolytic reaction of Fe–Ge

precursor alloys as applied by Stadelmaier and Fraker early in 1962 [4]. Also, an improved two-step ammonolytic reaction may be utilized that combines the formation of an intermetallic alloy and the nitridation in a single reaction [5]. The latter approach drastically improves the reaction efficiency and the quality of the nitride.

The structure of GexFe4–xNy was derived from powder X-ray diffraction (Mo radiation). XRD reveals a cubic antiperovskite-like structure like in -Fe4N for almost all compounds of the series. A tetragonally distorted structure (Fig. 1) was only observed for the highest germanium concentrations (x ≥ 0.97). The tetragonal structure is closely related to the cubic structure since the iron octahedra surrounding a nitrogen atom are twisted against each other, and the cubic motif is elongated along the c-axis. First-principles (DFT) total-energy calculations yield that this tetragonal structure is preferred by a significant 28 kJ mol–1 over cubic GeFe3N. Furthermore, DFT calculations were used to determine where iron atoms in antiperovskite -Fe4N (at the cube’s corners or faces) are replaced by germanium atoms. As later confirmed from experiment, the calculations predicted that the substitution only takes place at

FIG. 1: Crystal structure of GeFe3N0.6 with iron octahedra that are twisted against each other. The nitrogen position is only occupied by 60%.


46

the larger corner position. That scenario is favored by a huge 100 kJ mol–1 over all other possible atomic arrangements.

Besides the tetragonal structure, GexFe4–xNy stands out among the family of ternary iron nitrides for another reason: it is the only nitride with a limited nitrogen concentration (y 1), and this fact was noticed early on [4]. The nitrogen concentration strongly depends on the Fe–Ge composition and on the nitridation temperature, and it varies between 34 and 88% for the nitrides depicted in the phase diagram in Fig. 2. Interestingly, the nitrogen occupation decreases with increasing germanium content for the cubic nitrides along a curve from -Fe4N to intermetallic Fe3Ge, and then it drastically increases again for the tetragonal Ge-rich nitrides.

FIG. 2: Excerpt from the iron–germanium–nitrogen phase diagram with compounds GexFe4–xNy synthesized at two different nitriding temperatures.

Even though the nitrogen concentration behaves non-linearly, Rietveld refinements of the powder XRD pattern reveal a linear, Vegard-type decrease of the cubic lattice parameter upon substituting iron by germanium atoms. A calculated average Ge–Ge distance for the tetragonal Ge-rich nitrides perfectly fits this Vegard line and equals an overall unit cell contraction of 3% upon full substitution. Since germanium’s metallic radius is larger than that of iron (but the covalent radius is smaller), the importance of covalent bonding in this nitride is evident.

Finally, let’s get back to the magnetic properties. The AC susceptibility confirms the spin-glass-like transition of GeFe3N0.6 at a glassy transition temperature Tg of 36.68(5) K (Fig. 3a). Further, a fit to a critical power law gives a relaxation time of the individual particle moment * of 10–13.8(2) s and a dynamic critical exponent zv of 7.2(1) that characterizes the compound as a canonical spin glass with RKKY indirect-exchange interactions. Considering finally the entire solid solution GexFe4–xNy, the atomic substitution affects the magnetic properties such that the magnetic transition temperature decreases continuously from a high Curie point of 767(15) K for -Fe4N to a low Tg for GeFe3N0.6. Furthermore, the molar

magnetization approaches linearly a mean of zero, thereby reflecting the lack of long-range magnetic order and the highly degenerate ground state in the spin glass (Fig. 3b). While ferromagnetic spin arrangements dominate in -Fe4N, antiferromagnetic interactions prevail in GeFe3N0.6. Returning to the opening sentences, these antiferromagnetic interactions are incompatible with the iron arrangements in corner- and edge-sharing triangles of the octahedra – a frustrated situation.

FIG. 3: a: Proof of the spin-glass behavior of Sn0.9Fe3.1N: Temperature dependence of the real AC susceptibility component χ'm. The peak temperature Tm is a function of frequency. The inset shows the frequency dependence of Tm with the best power-law fit. b: The molar magnetization of GexFe4–xNy decreases with increasing germanium concentration (dots) and is compared to several theoretical models.

Further details on the comprehensive experimental and theoretical study on the entire solid solution have been published [3]. Figs. 2 and 3 adapted with permission from The Royal Society of Chemistry.

This work was supported by Deutsche Forschungsgemeinschaft and Graduierten-förderung of RWTH Aachen University (scholarship to T.S.).

[1] T. Scholz and R. Dronskowski, Inorg. Chem. 54, 8800 (2015)

[2] T. Scholz and R. Dronskowski, AIP Adv. 6, 055107 (2016)

[3] T. Scholz and R. Dronskowski, J. Mater. Chem. C 5, 166 (2017)

[4] H.H. Stadelmaier and A.C. Fraker, Z. Metallkde. 53, 48 (1962)

[5] A. Houben, V. Šepelák, K.-D. Becker, and R. Dronskowski, Chem. Mater. 21, 784 (2009)


47

Suppressing relaxation in superconducting qubits by quasiparticle pumping

G. Catelani1, S. Gustavsson2, F. Yan2, J. Bylander3, A. Kamal2, J. Birenbaum4, D. Hover4, D. Rosenberg4, G. Samach4, A. Sears4, S. Weber4, J. Yoder4, J. Clarke5, A. Kerman4, F. Yoshihara6, Y. Nakamura6, T. Orlando2, and W. Oliver2,4 1 JARA-FIT Institute for Quantum Information, Forschungszentrum Jülich, Germany 2 Research Laboratory of Electronics, MIT, Cambridge, MA, USA 3 Microtechnology and Nanoscience, Chalmers University, Gothenburg, Sweden 4 MIT Lincoln Laboratory, Lexington, MA, USA 5 Department of Physics, University of California, Berkeley, CA, USA 6 Institute of Physical and Chemical Research (RIKEN), Wako, Japan Dynamical error suppression techniques are commonly used to improve coherence in quantum systems. They reduce dephasing errors by applying control pulses designed to reverse erroneous coherent evolution driven by environmental noise. However, such methods cannot correct for irreversible processes such as energy relaxation. In this work, we investigate a complementary, stochastic approach to reducing errors: instead of deterministically reversing the unwanted qubit evolution, we use control pulses to shape the noise environment dynamically. In the context of superconducting qubits, we implement a pumping sequence to reduce the number of unpaired electrons (quasiparticles) in close proximity to the device. We report a 70% reduction in the quasiparticle density, resulting in a threefold enhancement in qubit relaxation times.

Since Hahn's invention of the spin-echo in 1950, coherent control techniques have been crucial tools for reducing errors, improving control fidelity, performing noise spectroscopy and generally extending coherence in both natural and artificial spin systems. All of these methods are similar: they correct for dephasing errors by reversing unintended phase accumulations due to a noisy environment through the application of a sequence of control pulses, thereby improving the dephasing time T2. However, such coherent control techniques cannot correct for irreversible processes that reduce the relaxation time T1, where energy is lost to the environment. Improving T1 requires reducing the coupling between the spin system and its noisy environment, reducing the noise in the environment itself, or implementing full quantum error correction.

We demonstrate a pumping sequence that dynamically reduces the noise in the environment and improves T1 of a superconducting qubit through an irreversible pumping process. The sequence contains the same type of control pulses common to all dynamical-decoupling sequences,

but instead of coherently and deterministically controlling the qubit time evolution, the sequence is designed to shape the noise stochastically via inelastic energy exchange with the environment. Similar methods have been used to extend T2 of spin qubits by dynamic nuclear polarization, and irreversible control techniques are commonly used to prepare systems into well-defined quantum states through optical pumping and sideband cooling, but outside of quantum error correction, to our knowledge no dynamic enhancement of T1 has been previously reported.

We implement the pumping sequence in a superconducting flux qubit, with the aim of reducing the population of unpaired electrons or quasiparticles in close vicinity to the device. As a superconducting circuit is cooled well below its critical temperature, the quasiparticle density via BCS theory is expected to be exponentially suppressed, but a number of experimental groups have reported higher-than-expected values in a wide variety of systems. Although the reasons for the enhanced quasiparticle population or the mechanism behind quasiparticle generation are not fully understood, their presence has a number of adverse effects on the qubit performance, causing relaxation, dephasing, excess excited-state population, temporal variations in qubit parameters, and is predicted to be a major obstacle for realizing Majorana qubits in semiconductor nanowires. Our results provide an in situ technique for removing quasiparticles, especially in conjunction with recent experiments showing that normal metal islands can act as quasiparticles traps [1], thus keeping the quasiparticles away from the Josephson junctions where they may contribute to qubit relaxation.

We characterize and quantify the quasiparticle population by measuring qubit relaxation. Generally, the relaxation rate is given by a sum of contributions from many different decay channels. Quasiparticles contribute to the relaxation in a process whereby the qubit releases its energy to a quasiparticle tunneling across one of the


48

Josephson junctions (Fig. 1A). Because of the small number of quasiparticles typically present in the device, fluctuations in the quasiparticle population lead to large temporal variations in the qubit decay rate. As a consequence, if the number of quasiparticles changes between trials while one repeats an experiment to determine the average qubit polarization, the time-domain decay no longer follows a single exponential, but rather takes the form

p t e⟨ ⟩ / e / . 1

Here, ⟨nqp⟩ is the average quasiparticle population in the qubit region during the experiment, T1qp is the relaxation time induced by one quasiparticle and T1R is the residual relaxation time from other decay channels such as flux noise, Purcell decay or dielectric losses. Because only quasiparticles are responsible for the non-exponential decay, Eq. (1) provides a direct method for separating out quasiparticle contributions from other relaxation channels. Figure 1B shows the measured relaxation of a flux qubit together with a fit to Eq. (1). The decay is clearly non-exponential, exhibiting a fast initial decay due to quasiparticle fluctuations, followed by a slower, constant decay due to residual relaxation channels.

FIG. 1: Non-exponential decay in a superconducting flux qubit. (A) Schematic drawing of the device, consisting of a flux qubit (lower loop) coupled to a dc SQUID for qubit readout (outer loop). The red crosses mark the positions of Josephson junctions. Qubit relaxation is induced by quasiparticles tunneling across the qubit junctions, as illustrated by the turquoise circle. (B) The qubit decay is clearly non-exponential. The solid line shows a fit to the decay function expected from quasiparticle tunneling, Eq. (1).

Interestingly, the same mechanism that leads to qubit relaxation also provides an opportunity for reducing the quasiparticle population. When the qubit relaxes through a quasiparticle tunneling event, the quasiparticle both tunnels to a different island and acquires an energy ħω from the qubit (ω/2π is the qubit frequency). The increase in energy leads to a higher quasiparticle velocity (at constant mean free path), so that a quasiparticle can move more quickly away from the regions close to the qubit junctions where it may cause qubit relaxation. The situation is depicted in Fig. 1A, where the quasiparticle tunneling out from the section of the qubit loop containing the junctions may diffuse away towards the ground electrode.

We make use of this mechanism by applying a pulse sequence consisting of several qubit π-pulses separated by a fixed period (Fig. 2A). The first π-pulse excites the qubit and, during the

subsequent waiting time, it has some probability of relaxing to the ground state. Because T1qp T1R, this most likely occurs through a quasiparticle tunneling event, which transfers a quasiparticle across a junction, increases the quasiparticle energy and thereby enhances its diffusion rate. The process is stochastic and may transfer quasiparticle in any direction, but by repeating the sequence we expect to pump quasiparticles away from the qubit junctions.

FIG. 2: Dynamic improvement in qubit decay time. (A) Pulse sequence for pumping quasiparticles. The last π-pulse acts as a probe pulse to measure the qubit polarization. (B) Normalized population vs. read-out delay for increasing number of pulses N. The decay time steadily increases from / 8 to 26 after 40 pump pulses. The solid lines are fits to Eq. (1). (C) Average quasiparticle number ⟨ ⟩ and (D) decay time per quasiparticle , extracted from the fits shown in panel B. The measured qubit decay using up to 40 pumping pulses demonstrates a more than threefold enhancement in qubit decay time compared to the bare decay, where the decay time is defined as the time T1/e it takes for the signal to decay by a factor of 1/e. The solid lines in Fig. 2B are fits to Eq. (1) and Figs. 2C,D show the resulting fitting parameters ⟨nqp⟩ and T1qp as a function of the number of pumping pulses. The average quasiparticle population drops from ⟨nqp⟩ 2.2 to about 0.5 after 40 pulses, and then saturates at this level. At the same time, the decay time associated with one quasiparticle drops from T1qp 20μs to about 7μs. The reduction of T1qp is somewhat unexpected, as one might generally expect the decay time per quasiparticle to remain constant as the quasiparticles are pumped away. However, as the number of π-pulses increases, the quasiparticles remaining near the junctions generally have higher energy and hence cause qubit excitation as well as qubit relaxation; since 1/T1qp is the sum of decay and excitation rates, this conceptually explains in part the suppression of T1qp. A fuller account of this work can be found in [2].

[1] R.-P. Riwar, A. Hosseinkhani, L. D. Burkhart, Y. Y. Gao, R. J. Schoelkopf, L. I. Glazman, and G. Catelani, Phys. Rev. B 94, 104516 (2016)

[2] S. Gustavsson et al., Science 354, 1573 (2016)


49

Orbital magnetism as a fingerprint of topological magnetic structures

M. dos Santos Dias, J. Bouaziz, M. Bouhassoune, S. Blügel, and S. Lounis

Peter Grünberg Institut–1 and Institute for Advanced Simulation–1, Forschungszentrum Jülich, Germany Magnetic nanovortices, so-called “skyrmions”, count among the most promising candidates for the future of information technology. Processors and storage media making use of these tiny structures could one day lead to the further miniaturization of IT devices and improve their energy efficiency significantly. Identifying suitable host materials is still experimentally challenging. One way is to access the topological charge, the hallmark of a skyrmion, which manifests itself in the topological Hall effect. We have discovered that also the orbital magnetism of a skyrmion-hosting sample should contain fingerprints of this quantity. We undertook a step-by-step investigation of this new physical effect, starting from the smallest structures that can host it, magnetic trimers. Combining first-principles calculations and a simplified electronic structure model the analysis could be extended to skyrmionic structures of realistic size, and a new experimental protocol was devised, utilizing soft x-ray spectroscopy.

Magnetic skyrmions have attracted enormous attention in recent years, not only from the fundamental physics side, but also due to their role as candidate single bits for magnetic information storage and processing. A magnetic skyrmion is a non-trivial three-dimensional twist in a ferromagnetic background. To create or destroy such a twist costs a significant amount of energy, from which the skyrmion inherits its stability. The non-trivial nature of the spin structure is quantified by its integer topological charge, which ensures

that the skyrmion is also stable against deformations. These properties conspire to endow magnetic skyrmions with a particle-like nature, and help explain why they can be moved with much lower current densities than their one-dimensional counterparts, magnetic domain walls.

Skyrmions in ultrathin films, such as the PdFe/Ir(111) bilayer, can be as small as a few nanometers in diameter, potentially enabling a large bit areal density. However, such small length scales make their experimental characterization challenging. Here spin-polarized scanning tunneling microscopy and magnetic force microscopy have proven successful. From the theory side, nanometer-sized skyrmions are at the edge of what state-of-the-art density functional theory (DFT) calculations can describe. Insight into the electronic structure is nevertheless invaluable: recent work has shown that an electrical readout of a skyrmion bit is feasible, using the tunneling spin-mixing magnetoresistance [1].

The interplay between the electronic degrees of freedom and the magnetic structure of the skyrmion is at the heart of future spintronics applications, but is also fertile ground for fundamental condensed matter physics. The complex magnetic structure generates so-called emergent electromagnetic fields that influence the electron motion, leading to the topological Hall effect. The very efficient coupling between an electric current and a skyrmion can be explained using similar arguments.

FIG. 1: Magnetic structures of a trimer and a skyrmion. The trimer can be: (a) collinear ferromagnetic; (b) noncoplanar; (c) coplanar antiferromagnetic. (d) Unit cell of a skyrmion lattice with 961 magnetic sites. The cones represent the spin direction at each site, and are color-coded by their polar angle.

(a)

(b)

(c)

θ (º)(d)


50

In [2] we describe a new kind of orbital magnetism arising from a noncollinear (and noncoplanar) magnetic structure, as found for a skyrmion. We consider the underlying electronic structure of magnetic trimers and skyrmions, using DFT as implemented in the Jülich KKR codes, and a simplified tight-binding model, to be able to analyze skyrmions up to 8 nm in diameter. We provide a complete characterization of this chirality-driven orbital magnetism, contrasting it with the conventional spin-orbit driven one more familiar in condensed matter systems. We show that the chirality-driven orbital moment is entwined with the emergent magnetic field, and inherits the topological character of the magnetic structure. We also propose a new experimental protocol for detection of topological magnetic structures.

The new contribution to the orbital magnetic moment was uncovered in DFT calculations for homoatomic trimers made with Cr, Mn, Fe and Co, assembled on the Cu(111) surface. The orbital moment generated by the spin-orbit interaction was found to be essentially local to each atom. In contrast, the new contribution depends on the global magnetic structure: it can only arise if the scalar chirality S1 . (S2 S3) is finite. Referring to Fig. 1, (a) and (c) do not host chirality-driven orbital moments, but (b) does. The magnitude of the chirality-driven orbital moment is roughly proportional to the magnitude of the scalar chirality of the magnetic structure, and also depends on whether there are magnetic d-orbitals near the Fermi energy. The most counter-intuitive aspect is that the direction of the chirality-driven orbital moment is not dependent on the real-space orientation of the spin moments: if the magnetic structure is rotated in a way that preserves the sign and value of the scalar chirality, the chirality-driven orbital moment is unaffected. Its orientation in real-space is governed by the character of the d-orbitals that give rise to it, and so by the local symmetry of each atom.

Fig. 1(d) shows the unit cell of a hexagonal skyrmion lattice, comprising 961 magnetic sites. The experimental skyrmion profile is used, and a tight-binding model including the x2-y2 and the xy d-orbitals is parametrized using the DFT calculations for the ferromagnetic phase of

PdFe/Ir(111). The magnetic structure of the skyrmion can be thought of being composed of elementary triangles, as the atomic lattice is also hexagonal. Then a connection to the magnetic trimers is apparent: wherever a finite local net scalar chirality is present in the magnetic structure, we should expect the chirality-driven orbital moments to arise. This has been confirmed by our calculations. The local properties of these orbital moments are similar to those of the trimers.

However, there is also a new global property: the skyrmion has a finite topological charge, that can be seen as a quantization of the net flux of the emergent magnetic field generated by the spin structure. As the emergent magnetic field is closely connected to the local scalar chirality, this quantization is also imprinted on the orbital moments. This is highlighted by considering skyrmionic structures with different topological charges, and by not including the spin-orbit interaction in the calculations. As seen in Fig. 2, above a critical size the net orbital moment is independent of the radius of the skyrmionic structure. Furthermore, the net moment is proportional to the topological charge Nsk of each structure, another evidence for the connection to the quantization of the emergent magnetic field.

To conclude, we note that it is possible to disentangle the contribution to the orbital moment arising from the noncollinear magnetic structure from the contribution stemming from the spin-orbit interaction [2]. It exploits the x-ray magnetic circular dichroism sum rules, that provide the net spin and orbital magnetic moment of the sample separately, and the approximate proportionality between the orbital moment generated by the spin-orbit interaction and the net spin moment.

Work funded by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (ERC consolidator grant 681405 — DYNASORE).

[1] D.M. Crum, M. Bouhassoune, J. Bouaziz, B. Schweflinghaus, S. Blügel, and S. Lounis, Nat. Commun. 6, 8541 (2015)

[2] M. dos Santos Dias, J. Bouaziz, M. Bouhassoune, S. Blügel, and S. Lounis, Nat. Commun. 7, 13613 (2016)

FIG. 2: Quantization of the chirality-driven net orbital magnetic moment in skyrmionic structures. The Néel-typeskyrmion has Nsk = -1, and is found in thin films such as PdFe/Ir(111). The achiral skyrmion has Nsk = +1, and has so fanot been experimentally detected. The other structures are obtained from the former two by winding the spin structuretwice around the center, instead of only once.


51

Dynamic stability of topological spin structures governed by Landau-Lifshitz-Gilbert equations

O. Chugreeva, L. Döring, and C. Melcher Lehrstuhl I für Mathematik, RWTH Aachen University, Germany The Landau-Lifshitz-Gilbert equation (LLG) is the fundamental evolution equation in ferromagnetism. The topological nature of magnetic pattern formation and dynamics governed by micromagnetic energies and the combination of gyroscopic precession and dissipative damping has attracted the attention of physicists and mathematicians [1]. Traditional mathematical question of blow-up and singularity formation found their relevance in nucleation and annihilation processes of topological defects and solitons proposed as information carrier. Their dynamic response to spin-transfer or spin-orbit torques is therefore at the core of future spintronic applications. Here we shall report on mathematical progress in the context of topological spin structures in dimension two and three.

2D: Dynamic of chiral skyrmions near the conformal limit [2]. We have addressed the dynamic stability of chiral skyrmions driven by a spin-transfer torque. This is ultimately a question of regularity for 2D LLG, for which finite time blow-up, typically accompanied by topological changes, has to be expected if energy accumulates to the critical threshold. In the presence of an in-plane spin-velocity ∈ we analyzed

eff( ) [ ( ) ]t t m v m m m v m h (1)

where and are positive constants and ⁄ is the effective field. In the Galilean

invariant case traveling wave solutions are obtained by transporting equilibria 0 along . In the conformal case of a pure Heisenberg ferromagnet traveling wave solutions are obtained for arbitrary and by transporting conformal or anti-conformal equilibria of unit degree along ∈ determined by the free Thiele equation

( ) c v c v

We are interested in the almost conformal regime of chiral skyrmions featured in the large field asymptotics. It is natural to perform a Galilean

boost ( ) ( )t t t m x m x c where ( ) c v c vleading to

eff( ) [ ( ) ]t z t z m m m h (2)

with effective coupling parameter 2

2( )

1

v

, where 0 is now the intensity of the spin current, and with the Cauchy-Riemann operator

1 2

1

2z m m m m (3)

revealing the conformal character of (1). We observe that any anticonformal (i.e. 0), which is also an equilibrium for the energy, is a static solution for the boosted dynamic equation, i.e. a traveling wave profile for (1). To exploit this attractive mathematical structure further, we investigated a simple micromagnetic model admitting (explicit) anticonformal chiral skyrmions

2

2 23

1( ) ( ) (1 )

2RE m

m m m m (4)

The chosen scaling guarantees a finite core size when 0 is a small parameter. We proved that, for 0 1, the energy exhibits the lower bound

4 1 for all admissible with skyrmion number 1, where

2

1( ) ( )

4 x yRQ

m m m m (5)

and that this bound is attained by the modified stereographic map

2

0 2 2

2 1( )

1 1

x xm x

x x (6)

Moreover, we obtained the following dynamic stability result for

Theorem 1.

1. The field minimizes the energy in its homotopy class independently of 0 1. It is a static solution of (2) and therefore a steady state solution of (1).

2. Suppose ≪ is an admissible family of initial data with 1 and such that for a constant c 0 independent of

0( ) 1Q m and 0( ) 4E c m

3. Then there exists a unique family ≪ of local smooth solutions of (2) with a uniform survival time


52

2 232 (1 )

cT

4. If → strongly in energy as → 0, then → strongly in energy for every .

The theorem can be seen as a first step towards the question whether chiral skyrmions driven by spin-transfer torques remain stable at arbitrary velocities or whether there is a critical velocity at which they will collapse. It guaranties a form of uniform dynamic stability and strong convergence to a global solution in the almost conformal regime ≪ 1.

FIG. 1: Bloch-type skyrmion (courtesy of O. Sander)

3D: Stochastic Landau-Lifshitz-Gilbert equations with higher order exchange interactions [3]. Stochastic versions of LLG in the style of Langevin are a classical issue, starting with the work of Brown. The noise is integrated into the equation by means of a the noisy precession induced by a vector field and a standard Brownian motion

eff eff( ) ( ) td dt dB m m m h m h m h (7)

to be read in the Stratonovich sense. Investigations mainly focussed on the stochastic dynamics of uniformly magnetized small particles examined on the level of stochastic ordinary differential equations (SDE) (i.e. is a vector field), where mainly classical mathematical theory applies. Stochastic LLG is now experiencing a revival and new mathematical challenges in the context of thermal stability of novel magnetic nanoscale objects such as vortices and skyrmions in 2D or strings-like solitons in 3D, leading to the stochastic partial (i.e. is a differential operator, e.g., ∆ ) accompanied by notorious mathematical difficulties related to blow-up and non-uniqueness. The appropriate notion of solvability based on

finite-dimensional approximation schemes (i.e. by a sequence of SDE systems of increasing dimension) is weak solvability. From the analytic point of view the solution is weak in the sense that differential operations are understood not in a classical but in a generalized distributional sense to incorporate possible singular behaviour. From the probabilistic point of view the solution is weak in the sense that the solution and the driving Brownian motion are constructed simultaneously on some unknown probability space and are thus interconnected. We propose a regularized version of (7) with ∆ to restore strong solvability in dimensions two and three. The regularization stems from an advanced exchange energy, that still exhibits 3 invariance in real- and magnetization space. We consider the exchange energy functional

2 2 2 21( )

2E dx m m m (8)

where a new positive parameter balances the higher order contribution. In contrast to conventional Heisenberg exchange ( 0), the new energy controls the modulus of continuity. General version of the regularized functional arise from the classical discrete Heisenberg model in a continuum limit beyond nearest neighbor interactions (work in progress with F. Rybakov, N. Kiselev, A. Borisov, L. Döring, and S. Blügel). Second order terms were also proposed to stabilize skyrmionic configurations in frustrated magnets in 2D. With resulting effective field

2 2eff h m m we obtained in [3] the following

result strong solvability result which ensures stability of topological solitons in dimensions two and three:

Theorem 2.

For every finite time horizon and every probability space, there exists a stochastically strong finite energy solution of (7) with (8), which is pathwise unique and almost surely continuous in space and time. It therefore preserves the topology of the initial data.

We would like to thank S. Blügel and Y. Mokrousov for inspiring discussions and greatly acknowledge financial support by JARA-FIT Seed Funds.

[1] C. Melcher and J.D.M. Rademacher, J Nonlinear Sci., accepted (2017)

[2] L. Döring and C. Melcher, Calc. Var. 56, 60 (2017)

[3] O. Chugreeva and C. Melcher, arXiv:1705.10184 (2017)


53

Topological orbital ferromagnetism driven by chiral spin textures

J.-P. Hanke, F. Freimuth, S. Blügel, and Y. Mokrousov

Peter Grünberg Institut-1 & Institute for Advanced Simulation-1, Forschungszentrum Jülich, Germany The emergence of orbital magnetism in solids is typically ascribed to the spin-orbit interaction lifting partially the orbital moment quenching. However, this interpretation fails to explain unconventional orbital magnetism that stems from the non-trivial topology of the underlying chiral spin texture rather than the spin-orbit interaction. Here we report on two representative examples of a new class of magnetic materials to which we refer as topological orbital ferromagnets. The obtained ferromagnetic ordering in these spin-compensated systems is purely due to orbital ferromagnetism driven by the spin structure without any reference to spin-orbit coupling. Using first-principles theory, we reveal the intimate interplay of the orbital ferromagnetism with the complex spin topology in these materials, which can ultimately be utilized to detect and characterize magnetic structures such as skyrmions.

Spin and orbital degrees of freedom of electrons give rise to two essential contributions to the magnetism in solids, both of which are usually distinguished through magnetic circular dichroism. While firm knowledge of spin magnetism has been acquired due to extensive research over the past several decades, exploration of the concept of the orbital magnetization (OM) in condensed matter is still at a rather premature stage. Even an accurate theoretical description of orbital magnetism has been missing until the recent advent of a rigorous Berry phase theory. Since the OM affects a plethora of elementary properties like spin-dependent transport, orbital magnetoelectric coupling, and Dzyaloshinskii-Moriya interaction, a deeper understanding of orbital magnetism in solids is in general of outstanding relevance.

Spontaneous orbital magnetism in ferromagnets is conventionally explained as a key manifestation of the spin-orbit interaction lifting the orbital degeneracy. While such an interpretation applies to most condensed-matter systems, it fails to describe orbital magnetism in crystals exhibiting a finite topological OM (TOM) prominent even in the absence of spin-orbit coupling. In these systems, an emergent magnetic field rooting in the non-coplanarity of neighboring spins replaces the spin-orbit interaction as the main mechanism lifting the orbital moment quenching by coupling to the orbital degrees of freedom. The latter non-coplanarity between three neighboring spins is usually quantified by the so-called scalar spin chirality, which plays also a fundamental role in the physics of skyrmions. Moreover, the non-vanishing scalar

spin chirality replaces the spin-orbit interaction in giving rise to the anomalous Hall effect, also referred to as topological Hall effect (THE) in this context, which is intimately related to the TOM both from microscopic and symmetry considerations.

FIG. 1: Top: (a) Orbital magnetization and (b) anomalous Hall conductivity as function of the position of the Fermi level, with and without taking into account the spin-orbit interaction (SOI). In contrast to the Berry phase theory, the simple atom-centered approximation (ACA) does not capture important features of the orbital magnetization. Bottom: Non-coplanar spin texture in the magnetic layer of Mn/Cu(111). Taken from [1].

Revealing a diverse spectrum of spin structures in real space, non-collinear antiferromagnets provide an intriguing and rich playground to study unconventional magnetic properties and transport phenomena. In particular, Mn/Cu(111) films and disordered FexMn1-x alloys are known to exhibit the non-coplanar 3Q structure, rendering these systems ideal candidates to study topological contributions to the OM and the accompanying anomalous Hall effect, as well as to estimate the efficiency of the scalar spin chirality as alternative degeneracy-breaking mechanism [1, 2].

Theoretically, we described the electronic structure of the above systems using density functional theory, where we employed additionally the virtual crystal approximation to treat the effect of disorder in the FexMn1-x alloys. In order to identify uniquely TOM and THE driven by the chiral 3Q spin texture, we compared calculations of the respective Berry


54

phase expressions with and without taking into account spin-orbit coupling. Owing to the cubic symmetry of the FexMn1-x alloys, we had to apply strain along the [111] crystallographic direction in order to obtain a finite TOM in this case. Contrary to the computationally demanding Berry phase approach, a simpler yet routinely employed atom-centered approximation proved insufficient in describing the TOM in the considered materials.

FIG. 2: (a) Orbital magnetization and anomalous Hall conductivity as function of the angle characterizing the spin texture (see insets) of FexMn1-x with x=0. Red and blue circles denote the results without and with taking into account the spin-orbit interaction. (b) Distribution of orbital moment and Berry curvature in momentum space for the 3Q spin structure. Taken from [2].

Amounting to about -1.5 B per unit cell, the sizable OM in the Mn/Cu(111) film is of topological origin since it stems from the emergent magnetic field associated with the non-coplanar spin texture, see Fig. 1. Including the effect of spin-orbit coupling does not at all alter the obtained orbital magnetism. An analogous conclusion can be drawn for the strained FexMn1-x alloys, rendering these spin-compensated systems thus prototypical topological orbital ferromagnets. In the case of the FexMn1-x alloys, we investigated also the influence of composition ratio and strain on the TOM, which changes sign with increasing Fe concentration as well as with respect to the application of compressive or tensile strain. These findings promote the possibility to efficiently control both magnitude and sign of the TOM by means of proper electronic-structure engineering.

In the case of FexMn1-x alloys, we also scrutinized explicitly the effect of the antiferromagnetic spin distribution in real space on TOM and THE by varying details of the spin texture, and thereby tuning effectively the scalar spin chirality, Fig. 2a. Reaching its maximum of about 0.1 B per unit cell for the 3Q structure, the TOM follows in its dependence on the texture details excellently the expected behavior according to the scalar spin chirality. Apparently, there is no TOM if the spin structure is coplanar in case of which the chirality

vanishes. Figure 2b displays the microscopic origin of topological orbital magnetism and THE in momentum space, revealing close correlations between both phenomena.

Our results clearly demonstrate that the considered materials are representative examples of topological orbital ferromagnets, for which an emergent magnetic field rooting in the scalar spin chirality completely replaces the spin-orbit coupling in lifting the orbital degeneracy. The nonzero charge and orbital currents are the result of the chiral spin texture leading to purely topological contributions to orbital magnetism and anomalous Hall effect. Analogously, we further expect non-coplanar magnets to exhibit a topological spin Hall effect stemming solely from the non-trivial topology of the spin texture, without any reference to the spin-orbit interaction.

The emergence of a ferromagnetic ordering of a large TOM as we predict in these zero-spin-magnetization magnets opens a path to exciting physics since orbital moments couple to external magnetic fields, optical perturbations, and orbital currents. Thus, effective spin Hamiltonians used to describe the phase diagrams of topological orbital ferromagnets in an external magnetic field require an amendment by the orbital Zeeman energy. The latter interaction of the TOM with external magnetic fields can also be utilized to modify the chirality of the texture owing to the intimate interplay between magnetic spin structure and TOM. Moreover, this close correlation can enable the unique identification of real-space spin textures based on the associated emergent magnetic fields that manifest in topological orbital ferromagnetism.

In a wider context, as compared to the spin of electrons, the orbital degrees of freedom offer higher flexibility regarding their internal structure and the size of orbital moments, rendering them versatile operational building blocks in the field of orbitronics. In this respect, topological orbital ferromagnets as a new class of materials occupy a special place since their non-trivial orbital magnetism is a direct consequence of the complex spin arrangement. This means that the properties of topological orbital ferromagnets can be directly tuned by altering the latter spin distribution, e.g., via electric-field induced spin torques or by modifying the strength of spin-spin interactions.

This work was supported by the FET-Open project MAGicSky (grant agreement number 665095) of the European Unions Horizon 2020 program and the priority program SPP 1538 of the Deutsche Forschungsgemeinschaft.

[1] J.-P. Hanke, F. Freimuth, A. K. Nandy, H. Zhang, S. Blügel and Y. Mokrousov, Phys. Rev. B 94, 121114(R) (2016).

[2] J.-P. Hanke, F. Freimuth, S. Blügel and Y. Mokrousov, Sci. Rep. 7, 41078 (2017).


55

Self-energy effect on interaction-driven phase transitions in mono- and bilayer graphene

C. Honerkamp

Institute for Theoretical Solid State Physics, RWTH Aachen University Monolayer graphene exhibits a semi-metallic ground state that is believed to be stable with respect to symmetry-breaking due to electronic interactions because of the vanishing density of states at the Fermi level. In Bernal-stacked graphene bilayers and related few-layer systems, the low-energy density of states is higher and interactions may cause phase transitions that might have been observed in suspended graphene samples[1]. Here we ask the question if the energy scales for such transitions, that come out rather high in simpler theoretical analyses, are renormalized considerably by self-energy effects. By using a self-consistent random-phase approximation (RPA) for Hubbard models on honeycomb lattices we find that the potential transition temperatures for antiferromagnetic ordering are strongly renormalized downwards by self-energy corrections compared to bare RPA or mean-field scales by almost one order of magnitude. Hence, even for order-of-magnitude quantitative theory estimates, neglecting self-energies may not be allowed.

The Hubbard model on single- and bilayer Hubbard model is possibly the simplest model to understand why monolayer graphene remains semi-metallic whereas bilayer graphene can be expected a gapped ground state, as suggested by some recent experiments on suspended samples [1]. Furthermore, the Hubbard model at half band filling can also be solved using Quantum-Monte-Carlo (QMC), yielding rather controlled threshold interaction strengths and gap values for the ordered ground state [2,3]. QMC however is hard to extend to doped situations or to more long-ranged hoppings and interactions (although some interaction profiles are tractable [4]). Here, functional renormalization group (fRG) techniques are viable alternatives and have been applied to few-layer graphene as well [5]. These fRG approaches however usually neglect self-energies. Hence it is important to understand how drastic this approximation is. Such an estimate can be provided within the random-phase approximation (RPA), which can also be interpreted as another approximation done to the fRG. The interaction ground state ordering (in the case of the Hubbard model of antiferromagnetic type) can easily be inferred from a divergence of the respective RPA susceptibility, yielding information on the required interaction strength or temperatures. Furthermore, the approach is technically so simple that self-

energy corrections can be included in a reasonable way. Then, one can monitor how the threshold interaction strengths or ordering temperatures are renormalized by the self-energies. Of course, at nonzero temperature in a truly two-dimensional system of infinite extension, the Mermin-Wagner theorem makes long-range order of an O(3) order parameter impossible. In realistic situations, one can however expect that the question of the ordering scale still makes sense, either because inhomogeneities, finite-size effects or spin anisotropies cut off the collective-mode physics in the infrared or on long scales, or because the electronic energy spectrum gets altered at this scale in a sense of a pseudo-gap due to magnetic short-range fluctuations regardless of whether the long-range ordering actually takes place or not.

FIG. 1: Data obtained by self-consistent RPA for the monolayer honeycomb Hubbard model, 1=0. Left plot: Growth of the AF eigenvalue of the matrix U(q=0) in the denominator of renormalized-RPA spin susceptibility with U toward the critical value 1, for different temperatures T=0.010, … , T=0.08 0 from left to right. We stop the U-increase in the calculation when this eigenvalue exceeds 0.99. The curves for larger T appear to bend over before they cross 1. This is an effect of increasing self-energy feedback. Right plot: Corresponding data for the self-consistent RPA Matsubara self-energy Im 11 (K,i=T), in band 1 at the Dirac point K for the same set of temperatures T=0.010, …, T=0.080 from bottom to top.

We hence computed the RPA transition temperatures for mono- and (Bernal-stacked) bilayer honeycomb Hubbard models as function of the interaction strength U, based on the basic tight-binding parametrization of the bare Hamiltonian in terms of in-plane hopping 0 ~2.5eV and inter-plane hopping 0[6]. We first compute the RPA spin susceptibility with bare Green’s function, i.e. without self-energies. This leads to rather high values for the antiferromagnetic ordering temperatures that easily exceed the experimentally observed gap scales of a few meV or <0.010 (see solid lines in Fig. 2). Then we determine the (band-diagonal) electron self-energy on the imaginary


56

(Matsubara) axis using the RPA effective interaction at the band crossing points K, K’ and in their vicinity. This shows that the self-energy value at K or K’ can be taken as good estimate for the whole low-energy region. The main aspect of the self-energy is the frequency dependence of the imaginary part, which gets steep around zero frequency near the RPA instability. If the self-energy at K/K’ is fed back into the electron Green’s function, its imaginary part acts like a temperature increase that slows down the divergence of the RPA spin susceptibility when the interaction is increased or T is lowered. Hence, including the self-energy shifts the phase transition to lower T or higher U. In the left plot of Fig. 1 we show the self-energy-impeded growth of the antiferromagnetic RPA eigenvalue, which loses in slope when U is increased and one tries to drive the system toward the phase transition. On the right side of Fig. 1 we plot the corresponding self-energies at the lowest Matsubara frequency that grow toward the transition, albeit without becoming very steep. Quantitatively, the downward-shifting of the RPA transition temperature is a large effect of almost one order of magnitude, as can be seen in Fig. 2.

FIG. 2: Critical AF ordering temperatures in bare RPA (solid lines) and self-consistent RPA (bullets) for the monolayer 1=0 (left plot) and AB-stacked bilayer 1=0.20.(right plot). The bullets mark the parameters values for which the AF eigenvalue reaches 0.99 when U is increased.

This result shows on one hand that these renormalizations are important physical ingredients that affect the low-temperature behavior of the system quantitatively. On the other hand, additional effects like quantum fluctuations of the magnetic order parameter that are still unaccounted for in RPA theory may play an additional role in ultimately determining the critical value. In true graphene, the interaction profile is more long-ranged. Then, QMC calculations can only address a part of the parameter space and other, more perturbative approaches like the functional renormalization group are helpful to analyze possible ordering tendencies. For those approaches, our study provides estimates how much self-energy effects will change their results.

For both mono- and bilayer we found quite strong downward-renormalizations of the critical temperatures for ordering. This is important in view of the small experimental temperatures scales of a few K for the opening of the interaction-induced gaps. In contrast with this, in bare mean-field or bare RPA theory (we are not aware of any QMC results for finite T here), the critical scales for AF

ordering easily get large. In fact, our results show that already at temperatures more than one order of magnitude below the bare mean-field Tc, the divergence of the RPA spin susceptibility can be killed by self-energy effects. If the gap opening is related to this divergence, it will occur at much reduced temperatures as well. The fluctuation regime above will exhibit quasiparticle scattering rates of the order of the temperature. It may be interesting to compare these findings with finite-T QMC. We attribute the strength of the temperature effects to the fact that the self-energy grows fast with temperature, most likely through the thermal filling of the density-of-states minimum around the Fermi level. It would be interesting to see if this is a more general feature of semi-metallic systems.

All in all, our results help to reduce the 'scale discrepancy' between the bare mean-field or fRG scales and the experimental or (if available beyond the critical U in the monolayer) 'exact' theoretical QMC scales. Our study refers to the situation without external magnetic field, but similar self-energy effects that strongly reduce the temperature scale below which symmetry breaking occurs should also be checked for the case with external magnetic field. This might play a role for the observability of quantum Hall ferromagnetism.

[1] F. Freitag, J.Trbovic,M.Weiss and F. Freitag, J. Trbovic, M. Weiss and C. Schönenberger, Phys. Rev. Lett. 108, 076602 (2012); W. Bao, J. Velasco Jr., F. Zhang, L. Jing, B. Standley, D. Smirnov, M. Bockrath, A. MacDonald and C. N. Lau, Proc. Nat. Acad. Sci. 109, 10802 (2012); J. Velasco Jr., L. Jing, W. Bao, Y. Lee, V. Aji, M. Bockrath, C.N. Lau, C. Varma, R. Stillwell , D. Smirnov, Fan Zhang, and A. MacDonald, Nature Nanotechnology 7, 156 (2012); A. Veligura, H. J. van Elferen, N. Tombros, J. C. Maan, U. Zeitler, and B. J. van Wees, Phys. Rev. B 85, 155412 (2012); W. Bao, L. Jing, J. Velasco Jr., Y. Lee, G. Liu, D. Tran, B. Standley, M. Aykol, S. B. Cronin, D. Smirnov, M. Koshino, E. McCann, Bockrath, and C.N. Lau, Nature Physics 7, 948 (2011)

[2] Z. Y. Meng, T. C. Lang, S. Wessel, F. F. Assaad, and A. Muramatsu, Nature 464, 847 (2010); S. Sorella, Y. Otsuka and S. Yunoki, Scientific Reports 2, 992 (2012)

[3] T. C. Lang, Z. Y. Meng, M. M. Scherer, S. Uebelacker, F. F. Assaad, A. Muramatsu, C. Honerkamp, and S. Wessel, Phys. Rev. Lett. 109, 126402 (2012)

[4] M. V. Ulybyshev, P. V. Buividovich, M. I. Katsnelson, M. I. Polikarpov, Phys. Rev. Lett.111, 056801, (2013); H. Tang, E. Laksono, J. N. B. Rodrigues, P. Sengupta, F. F. Assaad, S. Adam, Phys. Rev. Lett.115, 186602 (2015)

[5] M. M. Scherer, S. Uebelacker, and C. Honerkamp, Phys. Rev. B 85, 235408 (2012); M. M. Scherer, S. Uebelacker, D. D. Scherer, and C. Honerkamp, Phys. Rev. B 86, 155415 (2012); D. Sanchez de la Pena, J. Lichtenstein, C. Honerkamp, arXiv:1606.01124

[6] E. McCann and M. Koshino, Rep. Prog. Phys. 76, 05603 (2013)


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Competing pairing channels in the doped honeycomb lattice Hubbard model

X. Y. Xu1, S. Wessel2, and Z. Y. Meng1 1 Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing, China 2 Institute for Theoretical Solid State Physics, RWTH Aachen University, Germany Proposals for superconductivity emerging from correlated electrons in the doped Hubbard model on the honeycomb lattice range from chiral d + id singlet to p + ip triplet pairing, depending on the considered range of doping and interaction strength, as well as the approach used to analyze the pairing instabilities. We considered these scenarios using large-scale dynamic cluster approximation (DCA) calculations to examine the evolution in the leading pairing symmetry from weak to intermediate coupling strength. These calculations focus on doping levels around the van Hove singularity (VHS) and are performed using DCA simulations with an interaction-expansion continuous-time quantum Monte Carlo cluster solver. While at weak coupling the d + id singlet pairing dominates close to the VHS filling, an enhanced tendency towards p-wave triplet pairing upon further increasing the interaction strength is observed.

Many aspects of the fascinating physics of the low- energy Dirac electrons in graphene can be explored based on noninteracting tight-binding models on the honeycomb lattice, in particular close to charge neutrality, where the effects of the electronic interactions are delayed to the strong-coupling regime due to a vanishing density of states (DOS) at low energies. At finite doping, however, the presence of even weak interactions among the electrons is predicted by several studies to lead to new collective behavior. Of particular recent interest are interaction-driven instabilities towards unconventional superconductivity in doped honeycomb systems. Early studies concluded that superconductivity might not be stable with respect to charge or spin order for the basic Hubbard model on the honeycomb lattice, and a possible quantum liquid state has been suggested recently for the van Hove singularity (VHS) filling. In most of the recent theoretical studies, however, a general tendency towards some flavor of superconductivity upon doping the honeycomb lattice is indeed observed (see Ref. [1] for a review). However, various proposals on the nature of the emerging superconducting state and the stability range of competing pairing channels still lead to a mosaic of different scenarios. Several mean-field theory and

renormalization-group (RG) calculations predict chiral d + id singlet superconductivity to emerge in the weak- coupling region upon doping towards or onto the VHS, which corresponds to electronic densities of n = 3/4 and 5/4 for the Hubbard or related models with explicit spin-exchange terms or extended interactions. Variational Monte Carlo simulations also showed a chiral d-wave solution over a wide range of doping. The d-wave pairing state in this scenario is related to enhanced antiferromagnetic fluctuations near half filling as well as the VHS-increased DOS. On the other hand, a recent study using the variational cluster approximation (VCA) and cellular dynamical mean field theory (CDMFT) performed for larger values of the local repulsion found a stable p-wave triplet pairing state for a weak nearest-neighbor repulsion, with possibly a coexisting Kekule pattern. A possible p + ip pairing state was also reported at low filling from determinantal quantum Monte Carlo studies; however, the sign problem poses restrictions on the accessible system sizes, interaction strengths, and temperature ranges. In addition, Grassmann tensor renormalization calculations have been performed, and a d + id state was reported for the t − J model, while for infinite local repulsion a p + ip superconducting state, coexisting with ferromagnetic order, has been proposed for the Hubbard model at low doping. Hence, despite active pursuits, such deviations among the various proposals and employed methods show that a consistent picture of possible superconductivity even in the basic Hubbard model on the honeycomb lattice is still lacking, apparently due to competition among several possible low-energy states upon varying the doping or interaction strength. It thus appears promising and necessary to examine this problem from the perspective of a method that allows us to tune these parameters over a wide range while accounting for the growing local electronic correlations beyond the weak-coupling regime. We employed such an approach by providing results from large-scale dynamic cluster approximation (DCA) [2] calculations, with a focus on pairing susceptibilities to probe for uniform superconducting instabilities. Upon systematically increasing the cluster size, we find that a consistent picture starts to emerge for the leading pairing channels on the honeycomb lattice Hubbard model from small- to medium- sized local


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interactions: while at weak coupling, chiral d + id singlet pairing dominates close to VHS filling, when the interaction becomes stronger, a tendency towards p-wave triplet pairing develops. Our calculations were performed within a Hubbard model description, using an interaction-expansion continuous-time quantum Monte Carlo (CT-INT) cluster solver, keeping up to Nc=12 unit cells (24 cluster sites). For details about the considered model calculations and the DCA computational framework for calculating the pairing susceptibilities we refer to Ref. [3]. Below, we summarize some of our main findings.

We calculated both the temperature and the filling dependence of the effective pairing susceptibility χeff for different values of the local Hubbard repulsion U and cluster sizes Nc. In the following, we present explicitly our results for d- and p-wave pairing, which we observe to be the most dominant channels. In Fig. 1(a), the temperature dependence of the effective pairing susceptibilities at U = 2t and the VHS filling are shown for the Nc = 3 cluster, where t denotes the nearest neighbor hopping strength. Here, we identify d + id as the dominant pairing channel. However, for U = 6t [Fig. 1(b)], the p + ip triplet channel increases and also becomes positive and furthermore exhibits a tendency to diverge. In order to monitor this behavior as a function of doping, the dependence of χeff on the density n for different interaction strengths at βt = 20 is shown in Fig. 1(c). The dome-shaped behavior in the d + id singlet pairing channel χeff indicates an optimal doping between n = 0.75 and 0.85. At U = 6t, p + ip also exhibits a dome-shaped χeff maximum, even though the amplitude is still lower than for d + id.

FIG. 1: (a) Effective pairing susceptibilities for Nc = 3, U = 2t at the VHS density n = 3/4. (b) Effective pairing susceptibilities for Nc = 3, U = 6t at the VHS density. (c) Density dependence of the effective pairing susceptibilities at a temperature of T/t = 0.05.

Within the DCA approach, one can monitor the systematic behavior upon increasing Nc in order to draw conclusions about the thermodynamic limit. It is in particular important to consider clusters that capture the low-energy fluctuations. We thus also employed Nc = 4 and Nc = 12 clusters within the DCA/CT-INT framework. The Nc = 12 cluster has cluster momenta that include the Γ point, the two K points, the three M points, and six other momenta.

It thus provides a more detailed structure of the pairing symmetry than the Nc = 3 and 4 clusters.

FIG. 2: (a) Effective pairing susceptibility for Nc = 12, U = t at the VHS density n = 3/4. (b) Effective pairing susceptibility for Nc = 12, U = 2t at the VHS density. Note that the data in (a) and (b) share labels. (c) Density dependence of the effective pairing susceptibility at a Temperature of T/t = 0.025.

Unfortunately, the minus-sign problem becomes much more severe for Nc = 12, and we cannot access large values of U for Nc = 12. Nevertheless, we can draw interesting observations from the accessible parameter range: In Fig. 2(a), for U = t, the dominant pairing channel is still d + id near the VHS filling, while in Fig. 2(b), we find for U = 2t that the p-wave starts to increase at low temperatures. The p-wave channels (in particular py) tend to increase more rapidly upon cooling than the d-wave channels. This trend indicates that the p-wave channels compete strongly with d-wave pairing at this interaction strength, such that in the intermediate interaction range, there is an enhanced tendency for p-wave triplet pairing to eventually dominate over d-wave singlet pairing in the thermodynamic limit. To analyze this trend in more detail, the density dependence of the leading effective pairing susceptibilities is shown in Fig. 2(c). The optimal doping range for d + id pairing is consistent with the Nc = 3 results. In addition, the enhancement of the p-wave channels upon increasing the interaction strength is quite pronounced on the Nc = 12 cluster. Hence, although the minus-sign problem renders us unable to make a definitive statement about whether p-wave triplet pairing will eventually replace d-wave singlet pairing, the available Nc = 12 data up to U = 2t suggest such a scenario.

For the future, it would be interesting to allow also for inhomogeneous pairing states within the DCA calculations in light of several recent proposals of superconductivity coexisting with Kekule patterns. On a more general note, the effect of Hund’s coupling and spin-orbit coupling could be included in the DCA calculations as well.

[1] A. M. Black-Schaffer and C. Honerkamp, J. Phys. Condens. Matter 26, 423201 (2014)

[2] T. Maier, M. Jarrell, T. Pruschke, and M. H. Hettler, Rev. Mod. Phys. 77, 1027 (2005)

[3] X.Y.Xu, S. Wessel, and Z. Y. Meng, Phys. Rev. B 94, 115105 (2016)


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Defects in graphene nanostructures

F. Hajiheidari1, A. Pidatella1,2, W. Zhang3, C. Stampfer4,5, C. Honerkamp1, and R. Mazzarello1 1 Institute for Theoretical Solid State Physics, RWTH Aachen University, Germany 2 Institute for Solid State Physics, Technische Universität Dresden, Germany 3 Center for Advancing Materials Performance from the Nanoscale, Xi’an Jiaotong University, China 4 2nd Institute of Physics, RWTH Aachen University, Germany 5 Peter Grünberg Institute-9, Forschungszentrum Jülich, Germany Graphene [1] is a fascinating two-dimensional system with peculiar properties and many potential applications. More specifically, graphene quantum dots are promising candidates for solid state qubits [2] due to the small spin-orbit coupling and the possibility of creating carbon devices without nuclear spins, whereas zigzag graphene nanoribbons (ZGNRs) could be employed in spintronic devices - such as spin filters - exploiting their one-dimensional magnetic edge states [3]. The presence of defects, however, can drastically decrease the spin-coherence and spin-relaxation times of graphene dots. Adatoms have been shown to enhance the spin-orbit coupling, furthermore edges and defects such as vacancies and hydrogen adatoms lead to the formation of localized electronic states, which are expected to interact strongly with the qubit spin in the quantum dot. As far as ZGNRs are concerned, it has been argued that edge magnetism may be unstable against chemical passivation, edge reconstruction, thermal fluctuations and the chemical interaction with a substrate.

This project is divided into two parts: in the first one, we have investigated various prototypical defect states in graphene (vacancies, adatoms and edges). In the second one, we have considered ZGNRs deposited on the topological insulator Sb2Te3 to assess the stability of edge magnetism and the effects of the substrate on the magnetic coupling between the edge states, as well as the back action of edge magnetism on the surface states of Sb2Te3. For both projects, we have employed ab initio methods based on density functional theory.

First we discuss the simulations of defect states in graphene. We have studied the structural, electronic and magnetic properties of several impurity types on graphene, including H, B, N and O adatoms [4]. H adatoms have been intensively investigated and their properties are well understood [5]. The presence of an H adatom results in the removal of the C atom beneath from the network of orbitals, leading to an imbalance between the two sublattices and the formation of a localized state (see Figs. 1(a),(c)). Our results are in good agreement with previous work. B and N atoms have been considered because, in the experiments carried out in C. S.'s group, graphene is typically deposited on hexagonal boron nitride

(on which graphene shows a high carrier mobility) [6]. We find that the most favorable adsorption for B impurities is the bridge site and there is net charge transfer from the B adatom to graphene. As a result, the bonding is ionic and the defect is non-magnetic. The doping effects due to B impurities are potentially harmful for quantum computing applications, in that they shift the Fermi level of the dot in an unwanted way.

FIG. 1: Spin polarization density for an H adatom at on-top site (a) and a N adatom at bridge site (b). Red and blue color correspond to excess majority-spin and minority-spin densities, respectively. (c)-(d) Diagram of the bond orbitals in a plane through the H adatom (c) and the N adatom (d) and the nearest-neighbour carbon atom(s). The red orbital in (d) is perpendicular to said plane.

N adatoms exhibit interesting magnetic properties, which, however, make them detrimental for applications as well. In fact, these defects possess a magnetic moment of approximately 1 Bohr magneton. This property can be understood by a simple bonding analysis. The N adatom displays a sp2 hybridization. Two electrons of the N atom participate in the covalent bonding with the nearest neighbour C atoms (which have a “quasi” sp3 hybridization) and two electrons fill the third sp2 orbital. The remaining valence electron occupies the pz orbital perpendicular to the sp2 plane. This electron is unpaired and thus yields a net 1/2 spin (see Figs. 1(b),(d)). A fractional value for the magnetic moment of N was reported in [7]. We have found that this is a spurious effect due to an insufficient sampling of the Brillouin zone. It turns out that O adatoms on graphene are instead non-magnetic. The hybridization mechanisms are similar to the case of N (namely, sp2-like), however


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the additional electron saturates the pz orbital, yielding no net spin. Doping effects are also small. Hence, O defects are expected to be less harmful than the other 3 defects. Nonetheless, O adatoms will probably lead to an enhancement of spin-orbit coupling (not included in our simulations). We have also investigated the interaction between magnetic N adatoms. Our calculations indicate that the coupling is ferromagnetic, irrespective of the distance between the impurities.

An interesting follow-up project would be the investigation of the decoherence effects in graphene spin qubits due to the interaction of the qubits with the localized states induced by the H and N defects. This would require the development of effective Hubbard-like or Heisenberg-like Hamiltonians to model said interaction and the use of many-body methods to solve such Hamiltonians.

In the second part of the project, we have considered both hydrogen-terminated [8] and unpassivated [9] ZGNRs on Sb2Te3. We have shown that the chemical interaction between passivated ZGNRs and the substrate is weak. As a result, the ZGNR-surface distance is large, of the order of 3.4 Angstrom, doping effects are almost negligible, and the magnetic properties of the ZGNR are mostly preserved. Nevertheless, the presence of the substrate affects significantly the magnitude of the exchange coupling constants between the edges [8]. Remarkably, edge magnetism is preserved for unpassivated edges too, in spite of the strong chemical interaction between the edge C atoms and the substrate. In this case, however, the effective spin-orbit coupling induced by the substrate strongly affects the magnetic interaction between the edge states. In particular, it leads to a Dzyaloshinskii-Moriya interaction (DMI), which twists the two antiferromagnetically (AFM)-coupled edge states of the ZGNR (see Fig. 2) [9]. To our knowledge, this is the first report of a chiral magnetic structure formed by these one-dimensional edge states. We have mapped the system onto a classical spin Hamiltonian containing a) an isotropic exchange interaction between the spins on opposite edges with positive exchange constant J favouring the AFM coupling; b) the antisymmetric DMI, whose strength depends on the so-called Dzyaloshinskii vector D; c) the magnetic anisotropy energy for the two spins, which depends on the anisotropy energy tensor K. The quantities J, D and K have been estimated from a set of self-consistent simulations with different directions of the edge magnetizations [9]. Finally, we have studied the effects induced by the presence of the ZGNR on the surface states of the topological insulator. The first important effect is a pronounced reduction in the dispersion of the surface state due to the interaction with the orbitals of the edge C atoms. More quantitatively, the Fermi velocity vF of the state is reduced by an order of magnitude as compared to the clean surface. The second effect stems from the finite net magnetization of the nanoribbon, which is due to the tilting of the spins. This magnetization shifts the Dirac point of the

surface state and can even induce a gap depending on its direction [9].

FIG. 2: (a) Side and (b) top view of an unpassivated GNR on Sb2Te3(111) after relaxation. Sb, Te and C atoms are rendered with yellow, green and black spheres, respectively. The red arrows render the spin structure formed by the edge states in the lowest-energy configuration obtained. The directions of the edge magnetic moments deviate slightly from the y axis due to the Dzyaloshinskii-Moriya interaction. The yz and xz planes are also depicted: the first one is an exact mirror plane, whereas the second mirror symmetry is broken by the substrate.

From the point of view of applications, the finite magnetization of the ZGNRs on Sb2Te3 could be exploited in graphene-based spin filters. In fact, there has recently been considerable experimental and theoretical work aimed at stabilizing the ferromagnetic state in ZGNRs. In some respects, our system is simpler to realize than alternative approaches, which either require edge modification (a complex chemical process) or the application of a very strong electric field.

Project financed by JARA-FIT Seed Funds as part of the Excellence Initiative II of the Deutsche Forschungsgemeinschaft (DFG).

K. S. Novoselov et al., Science 306, 666 (2004) [1]

B. Trauzettel et al., Nature Phys. 3, 192 (2007) [2]

Y. Son et al., Nature 444, 347 (2006) [3]

A. Pidatella and R. Mazzarello in "Correlations in [4]Condensed Matter under Extreme Conditions" edited by G. G. N. Angilella and A. La Magna, Springer (2017)

O. V. Yazyev and L. Helm, Phys. Rev. B 75, [5]125408 (2007)

S. Engels et al., Appl. Phys. Lett. 103, 073113 [6](2013)

Y. Ma et al., Phys. Rev. B 72, 205416 (2005) [7]

W. Zhang, F. Hajiheidari, Y. Li, and R. Mazzarello, [8]Sci. Rep. 6, 29009 (2016)

W. Zhang, F. Hajiheidari, and R. Mazzarello, [9]"Chiral Magnetic Interactions in Graphene Nanoribbons on Topological Insulator Substrates", submitted to Phys. Rev. Lett.


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High quality CVD graphene devices fabricated by a dry transfer method

L. Banszerus1, M. Schmitz1, St. Engels1,2, J. Dauber1,2, M. Oellers1, F. Haupt3, K. Watanabe4, T. Taniguchi4, B. Beschoten1, and C. Stampfer1,2 1 2nd Institute of Physics, RWTH Aachen University, Germany. 2 Peter Grünberg Institute-9, Forschungszentrum Jülich, Germany. 3 JARA-Institute for Quantum Information, RWTH Aachen University, Germany. 4 National Institute for Materials Science, Tsukuba, Japan.

Because of its very high room-temperature mobility, graphene promises a wide range of possible applications ranging from ultra-sensitive Hall sensors to high frequency electronics and optoelectronics. In order to realize these devices on an industrial scale, it is necessary to produce large area, high quality graphene. Chemical vapor deposition (CVD) is a promising technology to grow graphene on copper foil on arbitrary scales. However, for a long time the electronic properties of CVD grown graphene was inferior to that of graphene obtained from mechanical exfoliation of graphite. Here we show that the electronic quality of CVD graphene depends critically on the transfer technique employed. We furthermore introduce a dry transfer technique that allows to remove the graphene from its growth substrate without wet chemical contaminations, resulting in charge carrier mobilities of up to 350,000 cm2/(Vs) [1].

State-of-the-art transfer methods for CVD grown graphene include the deposition of a polymer directly on the graphene and the wet chemical etching of the copper foil. These wet chemical steps deteriorate the electronic properties of the graphene. In order to prevent this, we use a contamination free transfer process based on van-der-Waals interactions between graphene and hexagonal boron nitride (hBN). At first, individual graphene crystals of several hundred microns in size are grown on the inside of an enclosure folded from a 25 µm thick copper foil using a low pressure CVD process (see Fig. 1a) [2]. After several days under ambient conditions a cuprous oxide layer forms on the graphene copper interface and weakens the adhesion of the graphene to the substrate (see Fig. 1c). Consequently, a polymer stack consisting of poly(vinyl alcohol) (PVA) and poly(methyl methacrylate) (PMMA) is prepared and a flake of hBN is exfoliated onto the polymer stack. Now, the stack is placed on a stamp of poly(dimethylsiloxane) (PDMS) and aligned with a graphene crystal on the copper. While heating the stack to 125°C the hBN is brought into contact with the graphene (see Fig. 1b). After cooling down, the copper and the polymer are separated again and the graphene is transferred onto the hBN. The copper foil can then be reused in consecutive growth cycles without a decrease in quality of the obtained graphene. The graphene/hBN stack can

consequently be placed on arbitrary substrates e.g. another hBN flake on a Silicon chip with a 300nm SiO2 layer. In order to probe the electronic properties of the resulting heterostructure, the stack is etched into a Hall bar shape using reactive ion etching and contacted using Cr/Au contacts [3].

FIG. 1: a) Graphene crystals grow on the inside of a copper enclosure. b) Schematic of the transfer process. The graphene is lifted of the copper foil using the van-der-Waals interaction with a hBN flake on a polymer stamp and can be placed on arbitrary substrates avoiding wet chemical contaminations. c) Optical image of a graphene flake on copper.

A microscope image of a typical device is shown in the inset of Fig. 2a. The recorded four-terminal resistance of the device as function of the applied back gate voltage is shown in Fig. 2a at 1.6 K (blue) and 300 K (black). From the conductivity and the charge carrier density, the charge carrier mobility can be obtained by using the Drude formula σ neμ.The device shows charge carrier mobilities of 145,000 cm2/(Vs) at 1.6 K and remains well above 50,000 cm2/(Vs) at room temperature. These remarkably high charge carrier mobilities are well comparable to those of high quality devices obtained from exfoliated graphene encapsulated in hBN.

High mobilities are a prerequisite to observe the quantum Hall effect in two dimensional electronic systems. Figure 2b) shows the differential transverse conductivity of the Hall bar as function of the applied back gate voltage and the external perpendicular magnetic field. The measurements show the expected anomalous Quantum Hall effect for graphene with Landau levels corresponding to the filling factors of ±2, ±6, ±10 and so forth.


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FIG. 2: a) Four-terminal resistance as function of back gate voltage at 1.6 K (blue) and 300 K (black).inset: optical image of the Hall bar device. b) Differential transverse conductivity as function of back gate voltage and perpendicular magnetic field. c) Charge carrier mobility and residual charge carrier density fluctuations n* for CVD samples assembled using the dry and wet transfer technique.

For magnetic fields exceeding 5 T a lifting of the spin and valley degeneracy of the Landau levels can be observed, indicating a high electronic quality of the graphene.

In order to get a statistical measure of the electronic quality of dry transferred CVD graphene, we fabricated a number of individual devices. Figure 2c displays the charge carrier mobilities and charge carrier density fluctuations at the charge neutrality point, a measure for the charge homogeneity of the sample, for devices built from CVD graphene using the dry transfer technique (squares) after a first (blue) and second growth cycle (red) and for devices built using the common wet transfer technique. The data clearly demonstrates the superiority of the dry transferred samples in terms of electronic properties yielding charge carrier mobilities of up to 350,000 cm2/(Vs) with an average close to 100,000 cm2/(Vs).

In summary, we demonstrate that the electronic properties of synthetic graphene are intrinsically identical to those of graphene obtained from natural graphite. In fact, the transfer process is the critical step that can lead to a deterioration of the electronic properties. We also demonstrate a transfer method to prevent contaminations of the graphene that results in electronic properties which are for the first time equivalent to those of high quality exfoliated graphene. These results lead the way towards a scalable high quality graphene technology which could support many of the possible applications of graphene.

In a more recent work, we are able to demonstrate that the remarkably high electronic quality of our dry transferred CVD graphene can result in a ballistic mean free path of charge carriers exceeding 28 µm in square shaped devices [4]. In these devices, the mean free path is limited by the sample size which allows for entirely new device concepts such as Veselago lenses and Dirac Fermion optics on a micrometer scale.

[1] L. Banszerus, M. Schmitz, S. Engels, J. Dauber, M. Oellers, F. Haupt, K. Watanabe, T. Taniguchi, B. Beschoten, and C. Stampfer, Science Advances, 1, e1500222 (2015)

[2] X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, O. Jung, E. Tutuc, S. K. Banerjee, L. Colombo, and R. S. Ruoff, Science 324, 1312 (2009)

[3] L. Wang, I. Meric, P. Y. Huang, Q. Gao, Y. Gao, H. Tran, T. Taniguchi, K. Watanabe, L. M. Campos, D. A. Muller, J. Guo, P. Kim, J. Hone, K. L. Shepard, and C. R. Dean, Science 342, 614 (2013)

[4] L. Banszerus, M. Schmitz, S. Engels, M. Goldsche, K. Watanabe, T. Taniguchi, B. Beschoten, and C. Stampfer, Nano Letters 16, 1387 (2016)


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Flexible graphene devices for extracellular measurements

D. Kireev, S. Seyock, D. Sarik, M. Ernst, V. Maybeck, B. Wolfrum, and A. Offenhäusser

Peter Grünberg Institut-8, Forschungszentrum Jülich, Germany Recording extracellular potentials from electrogenic cells, especially neurons is one of the main targets of modern bioelectronics. In this sense, graphene is a promising material that possesses such features as high conductivity, biocompatibility and physical robustness. Graphene-based electrode arrays (GMEAs) and graphene field effect transistors (GFETs) are the two main devices explored in this work. The pure two-dimensional structure of graphene opens new device operation routines that are perfect for bioelectronic applications. The advantages of graphene as part of such devices are numerous: from a general flexibility and biocompatibility to the unique electronic properties of graphene.

Performing extracellular measurements and being able to record electrical cell signals over a long period of time is important for understanding many physiological processes, such as degeneration of neuronal tissue. Degeneration, or changes in signaling occur in many diseases, such as Alzheimer’s and Parkinson's. Microelectrode arrays (MEAs) as well as field effect transistor arrays (FETs) have been shown to be able to perform such measurements even over the course of several months. However, the main disadvantage of the classical devices are their inflexibility and instability in vivo. Therefore it was important to find new ways to fabricate such devices as a step towards stable and non-invasive in vivo measurements. In this work we report on fabrication of flexible and robust graphene-based transistors [1] and microelectrode arrays [2] on a biocompatible polyimide (PI) substrate. In both cases, the chips were fabricated using a patented [3] high throughput graphene transfer method that extends the production capability of a single graphene sheet by a factor of 34.

The graphene field effect transistors (GFETs), used were fabricated on flexible polyimide-on-steel (PIonS) substrates that could be controllably bent. The steel substrate, supporting thin a PI layer fits specifically into PGI-8's mechanically controllable break-junction set-up (see Fig. 1a). Such composition allows us to bend the devices and test their stability. Bending tests and ex vivo measurements proved the reliability of the devices during and after mechanical deformation. Moreover, the devices exhibited extremely large transconductance values, up to 11 mS·V-1, and mobility over 1750 cm2·V-1·s-1. This is the largest value of transconductance (sensitivity) reported to date for solution gated GFETs. With this we prove

that PI is one of the best substrates for interfacing with graphene.

As an intermediate step towards in vivo measurements, we performed ex vivo recordings of embryonic rat heart tissue on the PIonS GFETs. The heart tissue was carefully placed directly in the middle of the chip (Fig. 1b), source and drain were connected manually via conducting magnets, a Ag/AgCl pellet electrode was placed right on top of the tissue, and a large enough drop of electrolyte applied in order to transfer the gate potential, but small enough to not lift the tissue up from the surface.

FIG. 1: (a) A picture of the bent PionS-based GFET. S and D mark the connection via conducting magnets. A PDMS ring works as the reservoir for the electrolyte solution. (b) A photo of the ex vivo experiment PionS-based GFET, with heart tissue directly on top of the chip. (c) The heart tissue measurement timetrace. (d) The averaged action potentials (n=34, in gray) from one recorded channel.

The timeseries recording (see Fig. 1c), of almost two minutes long, shows very remarkable peaks. The signal-to-nose ratio of the measurements is estimated around 10.5±0.5. An average peak’s FWHM (Fig. 1d) is 6-7 ms, which corresponds to the membrane potential changes and not to the mechanical movement. Moreover, releasing the underlying steel substrate would result in fully flexible GFETs, which could be easily implementable for in vivo applications where the large transconductance is particularly necessary. However, the noise of graphene-based devices is still comparably large (100-200 µV peak-to-peak), and is a target for further improvements. Based on our previous knowledge, multielectrode arrays are very suitable for the in vitro and in vivo experiments, since they usually have very low


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noise. Therefore, flexible GMEAs were fabricated via sacrificial technology. A released chip is approximately 13 µm thick with a PI base and passivation around the metal feedlines and graphene electrodes. The passivating polyimide layer is developed in a way that only graphene and PI parts are exposed to the liquid. The final GMEAs have circular recording apertures 20 µm in diameter. Once the devices are fabricated, the sacrificial layer is etched, releasing the flexible chips (see Figure 2d), which can be further connected to a carrier for in vitro studies [2].

The flexible GMEAs were also used for ex vivo heart tissue measurements, and almost 80% of the devices were able to record the electrical activity as shown in the spatial diagram (see Figure 2a). The shape of the recorded potentials (see an average of APs in Figure 2b and 2c) clearly resembles the P, Q, R, S, and T regions of an electrocardiogram.

FIG. 2: (a) The spatial resolution map of heart tissue recordings from a flexible GMEA. The electrode pitch is 200 µm. (b)-(c) The zoom-in into one action potential of 2 seconds and 200 ms long are given for a clear observation of P, Q, R, S and T regions. (d) One flexible chip, which was crumpled (e), then bonded and encapsulated (f). (g) A DIC picture of HL-1 cells grown on top of a GMEA surface. (h) Time trace recordings of HL-1 cells from eleven channels on one GMEA chip showing a time delay in recording of different electrodes that reflects spatial propagation. (i) The variety of different HL-1 action potential shapes recorded with the GMEA due to differences in cell-chip coupling.

Interestingly, prior to further cell culture (of cardiac-like HL-1 cells) and measurements, one of the flex GMEA chips was severely crumpled (see Fig. 2e and ref. [2] for a video). Regardless such a harsh treatment, multiple subsequent in vitro uses successfully detected extracellular signals. Low noise and high signal-to-noise ratio recordings were both maintained. The HL-1 cells were plated on top of the GMEAs and incubated until confluent and contracting. While growing, HL-1s cells form a continuous layer of electrically active cells, connected via gap junctions and then begin to fire APs and contract. In Figure 2g, an optical image of

such a continuous cellular layer is shown. The corresponding timetrace recordings are shown in Figure 2h, where electrical activity is shown from eleven electrodes. Moreover, there is a visible shift between the occurrences of the APs at different channels, which proves that the electrical signal is propagating through the cellular layer. The AP’s amplitude, width and shape can be different from channel to channel, but stays persistent in one channel. The main reason for different AP shapes is cell-chip coupling, which, in the case of HL-1 cells, is a more relevant parameter, compared to the above-reported heart tissue signals. The main waveforms of the APs are represented in Figure 3i, and the results are in accordance to the previously published works.

The typical background noise of the recordings is 20±6 µV. The average heart tissue spike amplitudes recorded were 1±0.2mV, and the average HL-1 cells’ action potential amplitudes are in the range of 300 µV. Therefore the overall SNR is 65±15 for ex vivo recordings and 20±10 for in vitro recordings.

Our results [2] indicate that the use of graphene’s extraordinary properties for fabrication of electrode arrays on a biocompatible polyimide substrate results in good cell-interface properties and is promising for further applications. The devices exhibit extraordinary stability and even after severe mechanical deformations were used for in vitro and ex vivo extracellular recordings multiple times, providing low noise and high signal-to-noise ratio recordings. Due to the transparency of our devices, the concept can be extended for optogenetic experiments. Furthermore, the fabrication technique, explored in the manuscript can be adjusted for the design of in vivo devices as bioimplants. These bioimplants are designed to spatially match physiological structures and to not require mounting onto a PCB prior to interfacing with the amplifier.

Simplicity of fabrication, handling, and measurements, combined with mechanical stability and flexibility, provides high expectations for further in vivo implementation of the GFETs and GMEAs.

The authors thank I. Zadorozhnyi, S. Vitusevich, J. Lewen, F. Brings, T. Qiu, of PGI-8 for help with experiments, and J. Garrido, B. Blaschke, and M. Lottner of TU München and X. Xie and T. Wu of SIMIT, for graphene.

[1] D. Kireev et al., IEEE Trans. Nanotechnol. 17, 140 (2016)

[2] D. Kireev, S. Seyock, M. Ernst, V. Maybeck, B. Wolfrum, and A. Offenhäusser, Biosensors 7, 1 (2016)

[3] D. Kireev, D. Sarik, B. Wolfrum, and A. Offenhäusser, "Verfahren zum Transfer von graphen-stuecken auf ein Substrat", DE 102015016143.1, filed 21 Dec. 2015.


65

Dual-SQUIDs with graphoepitaxial step-edge Josephson junctions

M. I. Faley1, V. Yu. Slobodchikov2, Yu. V. Maslennikov2, V. P. Koshelets2, and R. E. Dunin-Borkowski1,3 1 Peter Grünberg Institut-5, Forschungszentrum Jülich, Germany 2 The Kotel’nikov Institute of Radio Engineering & Electronics RAS, Moscow, Russia 3 Ernst Ruska-Centrum für Mikroskopie und Spektroskopie mit Elektronen, Forschungszentrum Jülich,

Germany The microstructural and noise properties of serially connected high-Tc direct current superconducting quantum interference devices (high-Tc DC SQUIDs) with step-edge Josephson junctions are studied. It is shown that the implementation of novel graphoepitaxial step-edge Josephson junctions on buffered MgO substrates helps to improve the reproducibility of conventional step-edge high-Tc Josephson junctions as a result of the self-arranged growth of two identical [100]-tilted 45° grain boundaries over a wide range of step heights. The use of such Josephson junctions in two serially connected SQUIDs that are directly coupled to a common pick-up loop in a dual-SQUID configuration with current biasing of the individual SQUIDs results in a doubling of voltage swings and an improvement in the magnetic field resolution of the sensors [1].

The unrivaled sensitivity of direct current superconducting quantum interference devices (DC SQUIDs) to magnetic flux makes them indispensable for applications such as biomagnetic measurements, geomagnetic surveys and non-destructive evaluations (see [2] and references therein). SQUIDs that are based on high-Tc superconductors such as YBa2Cu3O7-x (YBCO) are able to provide magnetic field resolutions of approximately 4 fT/Hz at 1 kHz and 77 K using a thin-film superconducting flux transformer, which concentrates magnetic flux from a larger area into the SQUID loop [3]. SQUIDs that are directly connected to single turn pick-up loops can be mounted closer (3 mm) to room temperature objects [4]. There is an urgent need for SQUID magnetometers, for applications in biomagnetic measurements and non-destructive evaluation, that can be placed in a similar manner to a directly coupled magnetometer but provide better magnetic field resolution.

A serial connection of two SQUIDs that are directly coupled to a common pick-up loop can enhance the peak-to-peak value of the output voltage (voltage swing) of the sensor and provide a reduction in its intrinsic noise by a factor of 1/2. In this paper, we report the preparation and characterization of high-Tc dual-SQUID magnetometers made with graphoepitaxial step-edge Josephson junctions [3, 5], as well as the modification of the readout electronic circuit, which

can be used for simultaneous operation of both SQUIDs in such sensors.

Samples were prepared on single crystal MgO (100) substrates of size 10 mm × 10 mm × 1 mm, whose edges were oriented along [100] and [010]. The surface of the substrate was Ar ion beam etched using a photolithography mask to prepare steps on its surface with a height in the range of 0.5 - 1 µm and with a slope angle of the step edge of ~45o. A second ion beam milling step was used to clean the surface to remove residuals of resputtered material, such as fences (which appeared at the top corners of the steps during the first ion beam milling procedure), as well as to create a texture in the form of linear trenches along the [100] and [010] directions of the MgO substrate on the surface (see Fig. 1).

FIG. 1: SEM image of a step edge with a surface texture made by a second ion milling procedure. The bottom corner of the step is smoothed due to the simultaneous effects of the angular dependence of the ion beam milling and the partial redeposition of etched material.

The steps were covered by an epitaxial double buffer layer made from a seed layer and a blocking layer. The texture was used to achieve graphoepitaxial growth of an approximately 10-nm-thick non-superconducting YBCO seed film, resulting in an in-plane orientation of the a-axis or b-axis of the YBCO seed film normal to both grain boundaries of the step edge junction. An epitaxial 30-nm-thick blocking layer of SrTiO3 (STO) was deposited above the YBCO seed layer to avoid contamination of the subsequent YBCO film by impurities from the MgO substrate. Two [100]-tilted 45o grain boundaries were formed at the top and bottom corners of the step.


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The magnetometers were realized in the form of two serially connected DC SQUIDs, which were galvanically (directly) connected by a common pick-up loop (“dual-SQUID magnetometer”; see Fig. 2). Each SQUID had an ~140-µm-long SQUID loop with an inductance LS of ~100 pH. Together with a 7 mm × 8 mm pick-up loop of inductance Lpu ~4 nH, the field-to-flux coefficients of the SQUID magnetometer and the dual-SQUID magnetometer were ~5 nT/0, corresponding to an effective area of 0.4 mm2. Peak-to-peak voltage swings of each of the SQUIDs were ~40 µV at 77 K.

FIG. 2: SEM image of the inner part of a dual-SQUID with two DC SQUIDs each having two 2 µm wide step-edge Josephson junctions and a 140 µm long SQUID loop with an inductance of approximately 100 pH.

Measurements of intrinsic noise of the magnetometers were performed with the sensors immersed in liquid nitrogen in a magnetic shield constructed from 3 cylinders of µ-metal and one superconducting shield made from a 100-µm-thick YBCO film deposited on the inner and outer surfaces of the YSZ tube. The spectral densities, recalculated to the input noise of the output signals of the system with the SQUID magnetometers and the dual-SQUID magnetometer, are shown in Fig. 3. A decrease in white noise from that of the single SQUID magnetometer of ~50 fT/Hz to that of the dual-SQUID magnetometer of ~35 fT/Hz at frequencies above 100 Hz and 77 K was observed.

FIG. 3: Noise spectra S1/2(f) of one of the SQUIDs and of the dual-SQUID magnetometer measured with the same pick-up loop, recorded with ac biasing. The top curve corresponds to the noise measured with an individual SQUID connected to the pick-up loop. The bottom curve represents the noise of the dual-SQUID magnetometer.

The graphoepitaxial Josephson junctions are sufficiently reproducible that they demonstrate good superconducting properties and they are also relatively cheap. The disadvantage of this technology is that such junctions are sophisticated and, at least during optimization of their technological parameters, there is a need for high resolution electron microscopy to control the orientation of the YBCO on the step-edge surface.

We have observed that a small spread in the parameters of individual SQUIDs can be corrected by the injection of an additional current into the pick-up loop. In this case, not only a doubling of the voltage swings, but also a reduction in white noise can be achieved. For a larger difference of the bias currents of individual SQUIDs, the voltage swings of the dual-SQUID magnetometer can be increased to a value equal to the sum of the voltage swings of the individual SQUIDs. However, the reduction in noise is less pronounced, if at all. The latter effect is likely to be a result of the non-optimal operation of the ac-bias SQUID electronics and can probably be corrected in future experiments. The low intrinsic noise of the dual-SQUIDs and the corresponding electronic circuit means that they are useful for biomagnetic measurements, geomagnetic surveys and non-destructive evaluations.

In conclusion, the microstructural and noise properties of graphoepitaxial step-edge Josephson junctions and high-Tc dual-SQUIDs have been studied. Novel graphoepitaxial step-edge Josephson junctions on buffered MgO substrates have sufficient reproducibility and superconducting parameters for the preparation of dual-SQUID magnetometers with high voltage swing values of ~80 µV and low white noise values of ~35 fT/Hz at 77 K. The dual-SQUIDs that are studied here and the proposed electronic circuit can be used for biomagnetic measurements, geomagnetic surveys and non-destructive evaluations.

[1] M.I.Faley, V.Yu.Slobodchikov, Yu.V.Maslennikov, V.P.Koshelets, and R.E.Dunin-Borkowski, IEEE Transactions on Applied Superconductivity 26 (3), 1600404 (2016)

[2] Applied Superconductivity: Handbook on Devices and Applications, Volume 2, Ed. by Paul Seidel (Weinheim: WILEY-VCH Verlag GmbH&Co. KGaA), ISBN 978-3-527-41209-9 (2015)

[3] M. I. Faley, U. Poppe, R. E. Dunin-Borkowski, M.Schiek, F. Boers, H. ChocholACS, J. Dammers, E. Eich, N. J. Shah, A. Ermakov, V. Yu. Slobodchikov, Yu. V. Maslennikov, and V. P. Koshelets, IEEE Transactions on Applied Superconductivity 23 (3), 1600705 (2013)

[4] F. Öisjöen, J. F. Schneiderman, G. A. Figueras, M.L.Chukharkin, A. Kalabukhov, A. Hedström, M.Elam, and D. Winkler, Applied Physics Letters 100, 132601 (2012)

[5] M. I. Faley, “Reproducible step-edge Josephson junction”, Patent US20150069331 (published March 12, 2015) and Patent EP 2834860 B1 (granted 30.12.2015)


67

GeSn microdisk lasers on Si

D. Stange1, N. von den Driesch1, S. Wirths2, C. Schulte-Braucks1, B. Marzban3, S. Mantl1, J. Witzens3, D. Grützmacher1, and D. Buca1 1 Peter Grünberg Institute-9 (IT), Forschungszentrum Jülich, Germany 2 IBM Research – Zurich, Rueschlikon, Switzerland 3 Institute of Integrated Photonics, RWTH Aachen University, Germany Microdisk lasers are advantageously used for on-chip integration of light sources due to their compactness and their potential for high quality factors. Partially compressively strained GeSn grown on Ge is used for fabrication of group IV microdisk lasers. The structures are undercut, additionally resulting in strain relaxation at the rim of the GeSn disk. This strain relaxation is analyzed, revealing lower bandgaps at the edges. Optical pumping of the structures leads to whispering gallery mode (WGM) lasing. This layout allows not only the lasing of direct bandgap Ge0.875Sn0125, but also of Ge0.915Sn0.085 with a conduction band L- to -valley energy offset of 0 eV for a completely relaxed lattice. Here, the strain distribution is investigated by Raman mapping and COMSOL simulations enabling the calculation of the band structure along the disk diameter. Microdisk laser characteristics are demonstrated for both GeSn alloy configurations and the limited maximal lasing temperature is investigated.

Integration of optical interconnects in silicon microelectronics is an outstanding objective to reduce power consumption and overcome bandwidth limitations. GeSn is a prime candidate for epitaxial integration into CMOS technology, since it is a group IV material and can offer a direct bandgap in certain configurations.

The directness of the bandgap depends on strain and on the Sn concentration of the alloy. A higher amount of Sn reduces the conduction band energy faster at the -point than at the L-point. This leads to a transition from an indirect bandgap to a direct bandgap semiconductor at around 9 at.% Sn [1]. GeSn is grown compressively strained on a Ge buffer, which always acts against the effect of Sn alloying. By decreasing the compressive strain, the semiconductor can become more direct again (meaning the energy offset EL- can be increased). The direct bandgap in GeSn was proven in 2014 for an alloy with 12.5 at.% Sn incorporation. The design of undercut GeSn microdisks offers the possibility to investigate strain relaxation effects and the effect of lasing in a single device.

GeSn alloys with 12.5 at.% and 8.5 at.% Sn are grown on Ge buffered Si wafer via reactive gas source epitaxy in a reduced-pressure CVD reactor, with thicknesses of 560 nm and 800 nm, respectively [2]. Microdisk mesas with diameters between 8 µm and 80 µm are defined by electron beam lithography and reactive ion etching with

Cl2/Ar plasma. Isotropic CF4 plasma in a barrel reactor allows etching of Ge selectively to GeSn and is applied to create an undercut of 3.5 µm below the disk. To reduce non-radiative surface recombination, the disks are passivated by 10 nm Al2O3 by atomic layer deposition afterwards. A scanning electron micrograph of a processed structure can be seen in Fig. 1.

FIG. 1: SEM of an 8 µm diameter GeSn microdisk with Ge pillar on Si.

The initial strain in the samples is determined by X-Ray diffraction spectroscopy to be -0.40% for Ge0.875Sn0.125 and -0.27% for Ge0.915Sn0.085. By removing the Ge buffer below the alloy, the GeSn is released and able to fully relax to the rim of the disk. However, the middle part of the disk remains anchored to the Ge post stays compressively strained. This effect is proven by Raman mapping of a 20 µm diameter disk. With higher degrees of relaxation, the signal of the Ge-Ge vibrations in the GeSn surrounding is shifted to lower wavenumbers, as can be seen in Fig. 2a. To model the strain field inside the disk COMSOL simulations are performed, which confirm a complete strain relaxation at the edge of the disk, for more details see Ref. [3]. This strain field was used to calculate the corresponding band structure, which changes with strain in the material. The simulated band structure is depicted in Fig. 2b over a disk diameter of 20 µm. Indeed the strain creates an energy potential with of ~30 meV at the rim of the disk. Labeling of valence energy bands are not valid at the edge of the disk since the biaxial strain symmetry of the lattice is broken in this region. The built-in energy potential helps moving the excited carriers exactly to the location where WGM modes are formed and supports lasing action in the disk resonator. The bandgap difference between center and edge of the disk can be seen by photoluminescence (PL) with a 532 nm pump beam in Fig. 2c, where a redshift from the center region compared to the edge region is observed.


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FIG. 2: Strain and band structure analysis. a) Raman mapping of a 20 µm disk. b) 8 band k∙p calculation of the band structure along the disk diameter. c) PL of a 70 µm disk at the edge and center confirms local narrowing of the bandgap [3].

Optical pumping of the microdisk resonators was done by a pulsed Nd:YAG laser with 5 ns pulse length and a 17 kHz repetition rate. Due to different Sn concentrations, the output wavelength varies between 2000 nm and 2500 nm, see Fig. 3a. Both alloys show typical lasing characteristics such as narrowing of the linewidth and the characteristic shape of the light-in light-out (LL) curve, compare Figs. 3b and 3c.

FIG. 3: a) Lasing spectra of two differently composed disks are shown for different temperatures. Light in-Light out curves for Ge0.875Sn0.125 (b) and Ge0.915Sn0.085 (c) [3].

Temperature dependent spectra at constant excitation power density in Fig. 3a reveal a lower maximum lasing temperature for lower Sn concentration disks. The maximum value is strongly influenced by the energy difference ∆E since more carriers are able to scatter into the L-valley, resulting in increased non-radiative recombination, if this energy offset is smaller. An outstanding finding regarding lasing of the Ge0.915Sn0.085 disk resides in the fact that the aforementioned energy offset at the relaxed rim of

the disk is zero. Therefore, the material does not need a large energy offset to enable lasing at cryogenic temperatures. Of course, a vanishing energy offset limits the performance, as can also be seen in the LL curves by comparing Figs. 3b and 3c: The thermal roll-over occurs at much lower pump powers for Ge0.915Sn0.085 than for the high Sn content disk.

In conclusion, analyzing undercut disks in GeSn enables investigation of strain relaxation and light emitting characteristics. To increase laser performances, further steps will be the investigation of double hetero- and multi-well-structures. Those can confine carriers spatially inside the GeSn, while quantum effects and lower active material volume might decrease lasing thresholds. The most appropriate barrier material will be SiGeSn ternaries, which has recently shown promising properties, starting to be investigated [4]. It has already been tested for lower Sn concentration LEDs, revealing good prospects in terms of efficiency [5].

This work was done in collaboration with R. Geiger, T. Zabel and Hans Sigg, Laboratory for Micro- and Nanotechnology, Paul Scherrer Institute, Switzerland; J-M- Hartmann, CEA-LETI, France, and Z. Ikonic, Institute of Microwaves and Photonics, University of Leeds, UK. This research received funding from the BMBF project UltraLowPow (16ES0060 K) and from the Deutsche Forschungsgemeinschaft (DFG) for project “SiGeSn Laser for Silicon Photonics”.

[1] S. Wirths, R. Geiger, N. von den Driesch, G. Mussler, T. Stoica, S. Mantl, Z. Ikonic, M. Luysberg, S. Chiussi, J. M. Hartmann, H. Sigg, J. Faist, D. Buca, and D. Grützmacher, Nat. Photonics 9, 88 (2015)

[2] N. von den Driesch, D. Stange, S. Wirths, G. Mussler, B. Holländer, Z. Ikonic, J. M. Hartmann, T. Stoica, S. Mantl, D. Grützmacher, and D. Buca, Chem. Mater. 27 (13), 4693 (2015)

[3] D. Stange, S. Wirths, R. Geiger, C. Schulte-Braucks, B. Marzban, N. von den Driesch, G. Mussler, T. Zabel, T. Stoica, J.-M. Hartmann, S. Mantl, Z. Ikonic, D. Grützmacher, H. Sigg, J. Witzens, and D. Buca, ACS Photonics 3 (7), 1279 (2016)

[4] N. von den Driesch, D. Stange, S. Wirths, D. Rainko, I. Povstugar, A. Savenko, U. Breuer, R. Geiger, H. Sigg, Z. Ikonic, J. Hartmann, D. Grützmacher, S. Mantl, and D. Buca, Small, 1–9 (2017)

[5] D. Stange, N. von den Driesch, D. Rainko, S. Roesgaard, I. Povstugar, J. M. Hartmann, T. Stoica, Z. Ikonic, S. Mantl, D. Grützmacher, and D. Buca, Optica 4 (2), 185 (2017)


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Adiabatic demagnetization refrigeration for ultra-high vacuum: Magnetothermal investigation of Gd(HCOO)3 and YbPt2In

P. Borgens1, C. Besson2, B. Klobes3, P. Kögerler2, R. Hermann3,4, and R. Temirov1 1 Peter Grünberg Institut-3, Forschungszentrum Jülich, Germany 2 Peter Grünberg Institut-6, Forschungszentrum Jülich, Germany 3 Peter Grünberg Institut-4, Forschungszentrum Jülich, Germany 4 Oak Ridge National Laboratory, Oak Ridge, USA Adiabatic demagnetization refrigeration (ADR) is the only technique that allows cooling to sub-Kelvin temperatures without the use of the rare helium isotope He3. Unfortunately, the materials used for magnetic refrigeration are not suitable for use in ultra-high vacuum (UHV) conditions. Here we report on production and investigation of UHV-compatible materials for magnetic cooling including a Gadolinium-based 2D-framework material and a ternary alloy of Ytterbium, Platinum and Indium.

The basis of ADR is the magnetocaloric effect. A typical ADR cycle (see Figure 1) consists of isothermal magnetization of a paramagnetic material by application of an external magnetic field B, while expelling the heat of magnetization to a heat sink (typically, a liquid helium reservoir at 1-4K). Afterwards the thermal connection to the sink is broken and the paramagnetic material is demagnetized under adiabatic conditions. This causes an increase of the entropy related to spin degrees of freedom of the paramagnet. At adiabatic conditions the increasing entropy drives the temperature of the paramagnet down. Demagnetization can be effected in two different ways: B-field may be reduced from the maximum to zero quickly which allows reaching the lowest temperature; alternatively, the field may be ramped quickly to a defined temperature value and then reduced slowly which allows the system to be kept at a fixed temperature for an extended time. The hold time is then defined by properties of the paramagnetic material and the heat leak into the system. From Figure 1 it becomes apparent that the cooling capacity per cycle per 1Kg of paramagnetic material is defined by the amount of magnetic entropy S available at a fixed temperature and magnetic field. Therefore in order to provide good ADR cooling temperatures, the material has to fulfill certain requirements: it should have a high density of electron spins weakly interacting with each other.

The materials typically used in ADR for reaching temperatures of less than 500 mK are

paramagnetic salts that include transition metal ions and water molecules in their crystal structure.

FIG. 1: Schematic drawing of an ideal ADR cycle. Isothermal magnetization to 1 at 0.7 is followed by controlled adiabatic demagnetization at the final temperature of 0.1 . The total heat that can be absorbed at is represented by the green shaded area ( 0.83 ), and therefore the hold time of such an ADR cycle for a heat leak of 10 would be~23 .

The water provides necessary spatial separation between electronic spins residing on metal ions thus reducing their mutual interactions. The presence of water, however, makes these salts very sensitive to temperature conditions. The most heavily used salt Ferric Ammonium Alum (FAA, NH4Fe(SO4)2·12 H2O) loses its crystal structure integrity already at 40C which makes its application in UHV conditions very difficult.

Therefore, our goal was to find other types of paramagnetic materials that are less sensitive to environmental conditions but nevertheless possess sufficient magnetic entropy at millikelvin temperatures. We characterized the specific heat of two compounds, Gadolinium Formate (Gd(HCOO)3) [1] and YbPt2In alloy [2], down to 50mK temperatures and different magnetic fields.

Gadolinium Formate has been synthesized according to the experimental details in [1]. Heat


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capacity measurement is shown in Figure 2a. By integrating the heat capacity according to

the entropy has been obtained (see

Fig. 2b). Note that due to the problems with the experimental setup measurements in the range 0.8-1.2K were not possible for this compound.

FIG. 2: Heat capacity CV (a) and magnetic entropy S (b) of .

can be seen from Figure 2b, Gadolinium Formate exhibits large zero-field entropy S at 0.5 . However, the synthesis of large amounts of crystalline Gadolinium Formate turned out to be difficult due to its poor solubility. It has therefore been decided to study YbPt2In alloy which, according to the recent results of Gruner et al. [2], exhibits large magnetocaloric effect and unusually low magnetic interaction at low temperatures. The heat capacity of YbPt2In sample provided by T. Gruner was measured in a temperature range of 0.05K to 4K and magnetic fields of 0T, 1T and 4T, respectively (see Figure 3a). The cooling power and hold times of both compounds are compared at different working temperatures Tf to the reference material FAA in Table 1.

TABLE 1: Bi and Si are the magnetic field and the entropy at starting temperature 1.2 , Sf the entropy at zero field and working temperature Tf. Q is the total heat that can be cooled in one ADR cycle. The hold time is given assuming a 10 heat leak and a total cooling material volume of~500 .

FIG. 3: Heat capacity (a) and magnetic entropy (b) of YbPt2In. In (c) the zero-field entropies of different compounds are compared. YbPt2In shows significantly higher entropy per volume starting from 0.05K.

YbPt2In shows considerably higher Q and hold time compared to the other two compounds. As an alloy YbPt2In has high thermal stability necessary for operation at UHV conditions. Moreover its thermal conductivity should be higher than the conductivity of traditionally used inorganic salts. Those properties make YbPt2In alloy a promising candidate for design of an ADR pill for use in UHV conditions.

Our further plans include YbPt2In thermal conductivity and direct magnetocaloric effect measurements at millikelvin temperatures. Afterwards a working prototype of the ADR pill on the basis of YbPt2In alloy will be designed.

Project financed by JARA-FIT Seed Funds as part of the Excellence Initiative II of the Deutsche Forschungsgemeinschaft (DFG)

[1] G. Lorusso, J. W. Sharples, E. Palacios, O. Roubeau, E. K. Brechin, R. Sessoli, A. Rossin, F. Tuna, E. J. L. McInnes, D. Collison, and M. Evangelisti; Adv. Mater. 25, 4653 (2013)

[2] T. Gruner, D. Jang, A. Steppke, M. Brando, F. Ritter, C. Krellner, C. Geibel; Journal of Physics: Condensed Matter 26, 485002 (2014)

Bi

[T]

Si

[J/cm3K]

Tf

[K]

Sf

[J/cm3K]

Q

[mJ/cm3]

hold time

[d]

FAA 2 0.0120 0.1 0.0431 3.11 1.8

0.3 0.0516 11.88 6.9

(1) 7 <0.0001 0.1 <0.0001 0 0

0.3 0.0319 9,62 5.5

(2) 4 0.0835 0.1 0.1340 5.03 2.9

0.3 0.3903 92.01 53.3


71

Electrical properties of gold nanowire arrays made by microstructured hydrogel templates

M. Noyong1, P. Wünnemann2, K. Kreuels2, R. Brüx2, P. Gordiichuk3, P. van Rijn3,4,5, F. A. Plamper6, U. Simon1, and A. Böker7 1 Institute of Inorganic Chemistry, RWTH Aachen University, Germany 2 Lehrstuhl für Makromolekulare Materialien und Oberflächen, RWTH Aachen University, Germany 3 Zernike Institute for Advanced Materials, University of Groningen, The Netherlands 4 Department of Biomedical Engineering-FB40, University of Groningen, The Netherlands 5 W.J. Kolff Institute for Biomedical Engineering and Materials Science-FB41, University of Groningen,

The Netherlands 6 Institute of Physical Chemistry, RWTH Aachen University, Germany 7 Fraunhofer-Institut für Angewandte Polymerforschung IAP & Lehrstuhl für Polymermaterialien und

Polymertechnologien, Universität Potsdam, Germany Microstructured hydrogels can be utilized in a new, facile template-guided imprinting process to obtain conductive nanowire arrays on a large scale [1]. The poly(N-vinylimidazole)-based hydrogel is directly synthesized inside the grooves of wrinkled polydimethylsiloxane (PDMS). Subsequently, tetrachloroaurate(III) ions from aqueous solution are coordinated at the embossed areas, reduced by air plasma until conductive wires are formed. The several steps and structures were analyzed with scanning force microscopy (SFM), and energy-dispersive X-ray spectroscopy (EDX). The electrical conductivity was determined in situ in the scanning electron microscope (SEM), showing highly electrically conductive gold wires.

Electrically conducting metal nanowires are of great interest in many fields of research, e.g. optoelectronics or sensors. With dimensions of the nanowires in the range of several micrometer lengths and diameters of a few nanometers, the generation of such 1D structures is still a challenging task. Highly complex and sophisticated techniques as electron beam lithography (EBL) are capable of generating defined nanostructures, but due to the serial production procedure not suitable for large scale production requiring a high throughput. Template assisted techniques utilizing masks or imprint techniques allow fast and reproducible formation of structures, however, the achievable integration density is limited. Nevertheless, each procedure, the advanced top-down as well as the bottom-up approach, provides special capabilities which can be applied favorably upon combination. In this work, we present a highly flexible and cost-effective method, which brings the advantages of the top-down guidance together with the bottom-up assembly. Thus, the fabrication of 1D nanowire arrays on a large area is enabled in a two-step process, which is illustrated in Fig. 1.

FIG. 1: Fabrication process of wrinkled responsive nanostructures: first, the transfer from wrinkled PDMS to a responsive hydrogel nanopattern via hydrogel imprinting; second, the incorporation and reduction of gold ions in the responsive network as well as the removal of the gel to gain Au nanowire arrays. (Printed with permission of Wiley-VCH from [1])

In detail, a wrinkled polydimethylsiloxane (PDMS) is generated by oxygen plasma treatment in the stretched state of the PDMS elastomer. Here, strain due to a mismatch in expansion between two layers with different elastic modulus (plasma-oxidized surface of the PDMS specimen vs. unaltered bulk PDMS) causes upon release spontaneously the wrinkled surface.[2] In the transfer step a functionalized, monomer/ crosslinker/ initiator mixture coated silicon surface acts as counterpart. The polymerizable film is forced into the wrinkles. After polymerization and drying the microstructured hydrogel film is left behind as negative replica. In order to provide coordination sites for the tetrachloroaurate(III) ions the hydrogel is based on N-vinylimidazole.[3,4] To reduce the coordinated ions and simultaneously remove the supporting hydrogel a cold plasma is introduced. Cross sectional analyses were performed by atomic force microscopy (data not shown). The height of the structures decreased from about 200-250 nm to 50-100 nm upon plasma treatment, whereby the hydrogel is removed. Fig. 2 shows exemplary SEM images of the generated 1D gold structure revealing its granular structure. Additional EDX measurements confirmed the presence of gold (data not shown).


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FIG. 2: Exemplary SEM images of the generated 1D gold structures (inset: magnified area).

Local probe investigations were carried out within the SEM chamber by in situ electrical conductivity measurements.[5,6] Individual nanowires were analyzed with different tip-to-tip distances dtip-tip in order to investigate the distance dependent conductivity (Fig. 3).

FIG. 3: Exemplary SEM images recorded at tip-tip distances of 12.6 µm, 5 µm and 1 µm (top), as well as the corresponding current-potential plots I(U) for all distances (center); the resulting ohmic resistance plotted versus the tip-tip distance (bottom). (Printed with permission of Wiley-VCH from [1])

Fig. 3 shows a series of 13 I(U) curves recorded on a single gold nanowire at different tip-to-tip

spacings (12.6, 5, and 1 μm), illustrated on three exemplary SEM images. A current compliance (CC) limited the current at I=±1 µA. Before being regulated by the CC all I(U) curves exhibit a linear behavior. Starting from the largest tip-to-tip distance of 12.6 μm the conductivity was improved in the series of decreasing distances until direct contact of the tips was reached. The respective ohmic resistances were calculated from the slope of each I(U) curve and plotted versus dtip-tip. A linear dependency converging to the minimum resistance (dtip-tip=0 µm) was observed, which could be attributed to the internal resistance Rint of the setup. By subtracting Rint from measured resistances at each position, the resistivities ρ of all investigated, individual wires (data not shown here) were derived. Their resistivities range from 2.2·10-6 Ωm to 1.4·10-7 Ωm. The deviation results from variation in the granularity of the structures itself. Nevertheless, comparing the resistivity ρbulkAu=2.2·10-8 Ωm of bulk gold to the derived resistivities, our values are only one to two orders of magnitude higher and, hence, reveal that these structures are highly conductive.

Concluding, we developed a highly flexible, cost-efficient two-step process for the generation of uniform, large array of electrically conductive gold nanowire structures. After further development of the metal, the 1D-structures revealed a low resistivity with 3·10-7 Ωm on average. This value is close to bulk gold, indicating the generation of high quality gold nanowires.

The presented approach allows easy variation of polymers and thereby incorporating many coordination capabilities, finally enabling other inorganic nanowire compositions (e.g., Ag or Pt).

The presented research is part of the efforts of the Collaborative Research Group SFB985 “Microgels”, supported by the German Science Foundation (DFG).[7] Details of this work and supporting materials are published elsewhere.[1]

[1] P. Wünnemann, M. Noyong, K. Kreuels, R. Brüx, P. Gordiichuk, P. van Rijn, F. A. Plamper, U. Simon, and A. Böker, Macromol. Rapid Commun. 37, 1446 (2016)

[2] S. Hiltl, J. Oltmanns, and A. Böker, Nanoscale 4, 7338 (2012)

[3] O. Mergel, A. P. H. Gelissen, P. Wünnemann, U. Simon, A. Böker, and F. A. Plamper, J. Phys. Chem. C 118, 1326199 (2014)

[4] O. Mergel, P. Wünnemann, U. Simon, A. Böker, and F. A. Plamper, Chem. Mater. 27, 7306 (2015)

[5] J. Timper, K. Gutsmiedl, C. Wirges, J. Broda, M. Noyong, J. Mayer, T. Carell, and U. Simon, Angew. Chem. Int. Ed. 51, 7586 (2012)

[6] M. Noyong, K. Blech, A. Rosenberger, V. Klocke, and U. Simon, Meas. Sci. Technol. 18, N84 (2007)

[7] https://sharepoint.ecampus.rwth-aachen.de/vo/ microgels/aussen/Pages/default.aspx


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Fabrication of organometal halide perovskite layers via chemical vapor deposition

S. Sanders1, D. Stümmler1, N. Wickel1, P. Pfeiffer1, M. Weingarten1, M. Heuken1,2, A. Vescan1, and H. Kalisch1 1 GaN Device Technology, RWTH Aachen University, Germany 2 AIXTRON SE, Germany Organometal halide perovskites are promising materials for photovoltaic devices and have demonstrated a rapid increase in performance in the last decade. Recently, perovskite solar cells have passed the threshold of 20 % power conversion efficiency (PCE). Yet, they are still not competitive to their inorganic counterparts in terms of production scalability and lifetime. Preferable methods for large-area production are vapor phase based processes. In this work, we present a setup for the direct chemical vapor phase deposition (CVD) of methylammonium lead iodide (MAPbI3) employing nitrogen as carrier gas. X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements are carried out to investigate the crystal quality and structural properties of the resulting perovskite. By optimizing the deposition parameters, we have produced perovskite films with a deposition rate of 30 nm/h.

In the past years, organometal halide perovskite solar cells attracted great attention because of their rapid development towards efficiencies beyond 20%, making these type of solar cells competitive to existing solar cell technologies [1]. Usually, perovskite solar cells are processed via solution-based methods such as spin-coating or printing techniques. These methods require fabrication pathways which include employing materials with a good solubility and a system of orthogonal solvents. While printing technologies offer the possibility to transfer perovskite solar cells from the lab to industrial scale, efficiencies decrease with larger active areas. In general, solution-based fabrication processes suffer from strong statistical deviations of the device performance [2]. A promising alternative method for the fabrication of perovskite solar cells is vapor phase deposition of the perovskite layer as preliminary literature results indicate [3]. VTE as well as carrier gas based processes have been demonstrated to yield perovskite solar cells with high reproducibility and comparable performance to solution-processed cells.

Vapor phase deposition of the widely used perovskite MAPbI3 (figure 1, left) and its derivatives remains challenging as both precursors, methylammonium iodide (MAI) and lead iodide (PbI2), have significantly different thermodynamic

properties especially regarding their sublimation enthalpies and vapor pressures. While MAI has a high vapor pressure making it even possible to form perovskite films from PbI2 in a MAI-saturated atmosphere at normal pressure below 200 °C [4], lead iodide needs significantly higher temperatures and low total pressures for reasonable deposition rates [3].

FIG. 1: Crystal structure of the perovskite MAPbI3 [5] (left) and schematic of the deposition system (right).

For the chemical vapor phase deposition of the organometal halide perovskite (figure 1, right), a newly designed experimental setup has been constructed taking into account the different evaporation properties of the precursors MAI and PbI2. For MAI (Dyenamo AB), a bubbler type evaporation source was built and for the PbI2 precursor (Alfa Aeasar), a thermal evaporator was placed in a heated tube, both employing nitrogen as the carrier gas. In the deposition chamber (diameter of 6.3 cm and height of 25 cm), the two precursor-containing gas flows are merged and then directed with a nozzle onto the substrate surface. Prior to the deposition of perovskites, single deposition experiments for each precursor have been conducted on Si substrates to optimize the individual evaporation temperatures and deposition rates. The substrate is temperature-controlled in the range of 5-50 °C. The overall pressure of the deposition chamber has been varied between 10-20 hPa whereas both gas flows can each be controlled by mass flow controllers up to 500 sccm. All gas flows were set to their maxima in all conducted experiments. The deposition of the perovskites takes place on titanium oxide coated FTO (fluorine-doped tin oxide) substrates.

For the evaporation of MAI, temperatures between 100 and 150 °C are sufficient to achieve reasonable deposition rates. The experiments also show that the deposition of MAI is strongly


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pressure-sensitive. The rate of deposition can be enhanced by lowering the pressure to its minimum, which is 10 hPa for the experimental setup. Deposition rates of around 170-180 nm/h could be measured at 150 °C and 10 hPa. Compared to MAI, PbI2 showed a significantly lower deposition rate. While evaporation of PbI2 begins at temperatures of 350 °C, for reasonable deposition rates in this setup (30 nm/h), the temperature had to be chosen near the melting point of PbI2 (410 °C) because of the small evaporation area of the crucible.

To form perovskites (confirmed by XRD), simultaneous and alternating deposition have been tested. When both species were deposited at the same time, intermixing of both gas streams did not occur because of a laminar flow regime of the deposition process. Consequently, PbI2 and MAI were deposited on different neighboring areas of the substrate and only a small intersecting region on the substrate was covered with the perovskite (figure 2, left). Therefore, alternating deposition of both components has been investigated. Regarding uniformity and reproducibility, the best perovskite deposition was achieved when PbI2 was heated up to 400 °C and MAI to 150 °C at 10 hPa total chamber pressure with deposition cycles of 10 min of each component in the alternating deposition mode. This process yields a resulting 30 nm perovskite film in one hour of deposition time (measured with cross-sectional SEM). Compared to the deposition rate of MAI alone, the deposition rate of the resulting perovskite film in the alternating deposition mode is significantly lower. Therefore, we assume that PbI2 is deposited on the substrate and reacts in the MAI-saturated atmosphere after the MAI component flux is turned on. A similar reaction mechanism has been already shown in the hybrid chemical vapor phase deposition of perovskites [6]. The resulting perovskite has a characteristic red to dark-brown color and is homogeneously deposited on the surface area (figure 2, right).

FIG. 2: Vapor phase deposited perovskite with simultaneously deposited components (left) and perovskite formation with alternating deposition (right).

To investigate the influence of the substrate temperature on the resulting perovskite structure, several perovskite layers with substrate temperatures between 5 and 50 °C have been deposited. Comparing the SEM images of a perovskite deposited at 10 and 50 °C (figure 3), it can be clearly seen that perovskite formation tends to a higher porosity with decreasing temperature. Obviously, the substrate temperature has a critical

influence determinig the crystal structure of the resulting perovskite, and the kinetic processes of perovskite formation on the surface have to be investigated in further experiments. Furthermore, it is also known that higher temperatures lead to larger perovskite grains which are beneficial for solar cells with higher PCE [7]. Therefore, it is expected that higher substrate temperatures (beyond 50 °C) could enable perovskite formation with less porosity and better coverage of the surface. The vapor phase deposited perovskites suffer from a strong instability when released to ambient conditions. This is attributed to their high porosity compared to perovskites fabricated by solution processing.

FIG. 3: Comparison of perovskites formed at 10 °C substrate temperature (left) and 50 °C (right).

In this work, we produced uniform perovskite films at a deposition rate of 30 nm/h. Furthermore, the developed CVD process can be easily scaled up to larger substrates, thus rendering this technique a promising candidate for manufacturing large-area devices. Moreover, CVD of perovskite solar cells can overcome most of the limitations of liquid processing, e.g. the need for appropriate and orthogonal solvents. To enable the fabrication of homogeneous perovskite layers via simultaneous deposition, a heated mixing unit is required. Although it was possible to form perovskites from the vapor phase at a pressure of 10 hPa by alternating deposition, the produced films suffer from a pronounced instability due to a highly porous structure. Therefore, further research has to be performed concerning the influence of the processing conditions on the resulting structure of the perovskite.

[1] W. S. Yang, J. H. Noh, N. J. Jeon et al., Science 348, 1234 (2015)

[2] A. Barrows, A. Pearson, C. Kwak et al., Energy Environ. Sci. 7, 1 (2014)

[3] L. K. Ono, M. R. Leyden, S. Wang et al., J. Mater. Chem. A 4, 6693 (2016)

[4] B. Niesen, S. Moon, D. Nicolas et al., IEEE J. Photovoltaics 4, 1545 (2014)

[5] M. A. Green, A. Ho-Baillie, and H. J. Snaith, Nature Photonics 8, 506 (2014)

[6] M. R. Leyden, L. K. Ono, S. R. Raga et al., J. Mater. Chem. A 2, 18742 (2014)

[7] X. Ren, Z. Yang, D. Yang et al., Nanoscale 8, 3816 (2016)


75

Field-induced self-assembly of iron oxide nanoparticles

Z. Fu1, Y. Xiao2, A. Feoktystov1, V. Pipich1, M.-S. Appavou1, Y. Su1, E. Feng1, W. T. Jin1, and Th. Brückel1,2 1 Jülich Centre for Neutron Science JCNS at MLZ, Garching, Germany 2 Jülich Centre for Neutron Science JCNS-2, Forschungszentrum Jülich, Germany The magnetic-field-induced assembly of magnetic nanoparticles (NPs) provides a unique and flexible strategy in the design and fabrication of functional nanostructures and devices. We have observed and investigated the field-induced self-assembly of core-shell iron oxide NPs in toluene by means of small-angle neutron scattering (SANS). When sufficient magnetic field is applied, distinct Bragg peaks appear in SANS patterns, demonstrating the formation of well-ordered self-assembly of iron oxide NPs. The crystallinity of the self-assembly is found to improve with increasing magnetic field, along with the growth of large-scale NP aggregates. The crystal structure of the field-induced self-assembly of iron oxide NPs is identified to be face-centred cubic.

The intriguing phenomenon of self-assembly of colloidal magnetic nanoparticles (NPs) into well-defined ordered arrays has been attracting much attention because it provides an effective bottom-up strategy for the fabrication of functional

nanostructures and model systems [1]. Ordered arrays of magnetic NPs show different behaviour from that of the bulk and possess wide application potential for various purposes [2]. As a fast and reversible bottom-up approach among the various directed and template-assisted rational strategies, the magnetic-field-induced assembly of magnetic NPs has shown tremendous flexibility for the experimental production. Iron oxide NPs hold the advantage over many magnetic nanomaterials given their ease of preparation, low cost, and high chemical stability. It is highly desirable to study the field-induced self-assembly of iron oxide NPs from both a fundamental and applicational point of view.

Small-angle neutron scattering (SANS) is a powerful technique, which probes not only the form factor of individual NP, but also the spatial correlation and organization of the NP assembly. Due to the high penetration of neutrons in matter, SANS is well suited for the in situ investigations on the samples in liquid. The very-small-angle neutron scattering (VSANS) can detect large aggregates with real-space sizes from several hundred

FIG. 1: SANS patterns measured from iron oxide NP solution exposed to magnetic fields of 0.005 T (a), 0.1 T (b), 0.25 T (c), 0.5 T (d), 1 T (e) and 2.2 T (f). In (f), the calculated reflections for a face-centred cubic structure are shown as white circles and superimposed onto the experimental pattern for comparison.


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nanometers to several micrometers. Here, we exploit suspensions of oleic-acid-coated iron oxide NPs to study their crystallization driven by the magnetic field by means of SANS and VSANS.

The morphology of the NPs was characterized by TEM. The NPs are spherical in shape and relatively uniform in size. The average diameter of the iron oxide cores is around 17.1 nm. XRD and magnetization results suggest that the iron oxide core is composed mainly of maghemite [2].

SANS investigation was carried out on the KWS-1 instrument at the Heinz Maier-Leibnitz Zentrum (MLZ) in Garching. The form factor of core-shell iron oxide NPs was measured from a diluted sample and then fitted with the model for a core-shell spherical object. The radius of the core and the thickness of the shell are determined as 8.5 nm and 1.5 nm, respectively, in good agreement with the TEM result. The as-prepared NP solution was exposed to a vertical magnetic field to study the field-induced self-assembly of iron oxide NPs. The SANS patterns collected at various fields ranging from 0.005 to 2.2 T are depicted in Fig. 1. The SANS intensities in these patterns are radially averaged and plotted as a function of Q in Fig. 2a. The SANS pattern is isotropic at 0.005 T, showing no indication for the presence of locally ordered structures. Upon applying a field of 0.1 T, distinct Bragg peaks appear, demonstrating the formation of well-ordered superstructures of NPs. When the field is increased from 0.1 to 2.2 T, more particles are aligned due to the enhanced dipole-dipole attraction. The crystallinity of the NP self-assembly is improved, as indicated by the increased sharpness of the diffraction peaks, allowing a reliable inspection of the crystal structure. The diffraction peak positions represent a Q ratio of √3 :√4 :√8 :√11, corresponding to the (111), (200), (220) and (311) lattice planes of a face-centred-cubic (FCC) structure with a lattice constant of a = 29.4 nm. The magnetic field direction defines the [011] crystallographic direction, along which the nearest neighbors in the FCC structure locate. The orientation of the NP supercrystals is random with respect to the field direction, i.e., their [011] crystallographic direction. As a result, the diffraction intensity is distributed over circles in the reciprocal space, and Bragg reflections are observed at the intersections of the reciprocal circles and the Ewald sphere surface. In our SANS experiment, we detect only the reflections with the scattering vector Q = (Qx, Qy, 0), where Qx is perpendicular to the field direction. The measured intensity of an observable reflection is proportional to the multiplicity of the reflection, but inversely proportional to its Qx [3]. We have calculated the diffraction pattern (white circles in Fig. 1f) from a FCC structure with a lattice constant of 29.4 nm. The calculated pattern agrees well with the measured SANS pattern. The SANS investigations clearly show that the field-induced self-assembly of iron oxide NPs has a FCC type of structure [2].

FIG. 2a: Radially-averaged SANS intensities measured on KWS-1 as a function of Q; FIG. 2b: VSANS intensities measured on KWS-3 and integrated over an azimuth sector perpendicular to the field direction.

The self-assembly process has been explored further by using VSANS on KWS-3 at MLZ. As shown in Fig. 2b, the VSANS intensities are integrated over an azimuth sector perpendicular to the field direction and plotted as a function of Q. For fields above 0.02 T, the low-Q scattering profiles follow a Porod law (i.e., I(Q) ∝ Q-4), indicative of the presence of large-scale aggregates whose radiuses of gyration Rg are at least larger than the inverse of the minimum accessible Q value, i.e., Rg > 170 nm. As the field increases, the growth of the NP aggregates is seemingly consistent with the improvement of the crystallinity of self-assemblies.

In conclusion, well-ordered self-assemblies of iron oxide NPs have been achieved in solution when sufficient magnetic field is applied. The crystal structure of the self-assembly is determined to be FCC by means of SANS. Our research [2] sheds light on the creation of field-induced self-assembly of colloidal magnetic nanoparticles and shows the superiority of the SANS technique in studying the self-assembly phenomena of NPs in solution.

[1] F. X. Redl, K.-S. Cho, C. B. Murray, and S. O’Brien, Nature 423, 968 (2003)

[2] Z. Fu, Y. Xiao, A. Feoktystov, V. Pipich, M.-S. Appavou, Y. Su, E. Feng, W.T. Jin, and Th. Brückel, Nanoscale 8, 18541 (2016)

[3] A. Pal, V. Malik, L. He, B. H. Erne, Y. Yin, W. K. Kegel, and A. V. Petukhov, Angew. Chem. Int. Ed. 54, 1803 (2015)


77

KKRnano: Towards precise large-scale density-functional calculations

R. Zeller

Institute for Advanced Simulation, Forschungszentrum Jülich, Germany KKRnano is a computer code developed for precise large-scale density functional calculations. The aim is to treat systems with tens or hundreds of thousands of atoms in an efficient manner on the present and next generation of supercomputers. Particularly important achievements in KKRnano are (i) a large reduction of the computational cost, realized by accepting small losses in precision, and (ii) the capability to determine rather accurate ground-state energies, realized by an improved evaluation of the density-functional total-energy expression.

Materials development for advanced 21st century ap-plications will be supported more and more by an atom-by-atom quantum-mechanical understanding of their nanoscale properties. Often, density-functional theory (DFT) is used for this purpose because it simplifies the many-electron problem into the determination of the electron density. Nevertheless, while systems with a few hundred atoms can be treated routinely today, precise calculations for larger systems are a formidable task even on the largest supercomputers. This is so because the computing time in standard approaches increases with the third power of the number of atoms in the system. A way to avoid this problem is to give up some accuracy for speed by exploiting the so-called nearsightedness principle of electronic matter. The principle means that in systems without long range electric fields (and for fixed chemical potential) the density change at a point in space is negligibly affected if the electronic potential is changed sufficiently far away from this point. In KKRnano we exploit this fact by using a spatial truncation of the Kohn-Sham Green function as described in [1].

KKRnano is based on the Kohn-Korringa-Rostoker (KKR) Green-function (GF) method. In this method, the only part of the calculations, which requires computing time increasing cubically with system size, is the solution of the matrix equation

nn r nn r nn n n nLL LL LL L L L L

n L L

G G G t G

for the GF matrix elements nnLLG

. Here, r nnLLG

are the GF matrix elements of the known GF of a suitably chosen reference system. The upper indices n label the atomic sites and the lower indices , the angular-momentum quantum numbers and . In principle, an infinite number of values contributes to the summations over ′′

and ′′′ , while in practice only a finite number can be used, usually with 3.

In KKRnano the direct solution of the matrix equation is replaced by using the transpose-free quasiminimal residual (TFQMR) method as an iterative solver. This has two main advantages: (i) easy parallelization, because different values ′ do not mix, and (ii) only a quadratic increase of computing time, because the sparse matrices

r nnLLG

arising from the repulsive reference

system of the screened KKR method [2] can be exploited. In the iterative solution it is also conceptually simple to use the nearsightedness principle by neglecting small GF matrix elements

nnLLG

for atoms ′ which are outside a local

interaction zone (LIZ) around atom .

While the TFQMR iterations can be done without losing precision, the use of the nearsightedness principle necessarily reduces the precision. It is important to quantify this loss, because ground-state energy changes are calculated often by taking the difference of two large density-functional total energies. With system size these energies increase which necessitates to increase the precision per atom. In Fig. 1 it is demonstrated for the example of a disordered AgPd alloy that deviations arising from the spatial truncation of GF matrix elements can be kept below a few milli-electron Volt per atom if the LIZ contain more than a few hundreds of atoms.

With such precise results in mind, it is important to assess the fundamental precision available in the KKR method. In recent work [3] it was shown that the density in the KKR method can be expressed exactly by a finite number of terms and that the dependence on the angular variables is analytically given. In this work the potential is understood as a non-local angular-projection potential

ˆ ˆ ˆ ˆ( ) ( ) ( ) ( )maxl

n nL L LL

LL

V r Y Y V r

r r r r

around each atom. For practical applications, it is important that potentials of this type can be used to approximate any conventional local potential as well as desired because of the completeness relation of spherical harmonics. The exact expression for the density should lead to precise results for total energies and atomic forces in contrast to often stated view that the KKR method is not suitable for this purpose.


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FIG. 1: Total energy deviations for 16384 atoms in a periodic unit cell. The reference value (set to zero) is obtained without neglect of GF matrix elements. The blue and red curves are for the choices (i) neglecting all elements outside the LIZ or (ii) neglecting only elements with 0.

FIG. 2: Deviations from absolute total energies calculated with 8. The deviations are obtained if the indicated values of lmax are used during the self- consistency steps and then the total-energy functional is evaluated accurately using 8.

For total energies the essential problem is that the total-energy functional must be evaluated with a higher value of lmax than the one which is enough for the self-consistent determination of the density. In this way highly precise values for absolute total energies can be obtained as demonstrated in Fig. 2. For atomic forces the essential problem is not as often believed that the standard expression

1

4 ( )( ) ( )

2 1

l nnn n L

L nn lnn

r YV r d n

l

r R

r rr R

cannot be used for the spherical-harmonics expansion of the potential . The problem arises if one evaluates the integral above by using an expansion for ′ , where

is the

distance between atoms n and n1 . This expansion converges too slowly for neighbouring atoms and will be replaced in KKRnano by isoparametric integrations as described in [4].

FIG. 3: Wall-clock time on the Jülich supercomputer JUQUEEN for the TFQMR solver and for the Pois- son solver for one density-functional self-consistency step for a helical magnet MnGe in B20 structure. The number of atoms per rack is 8192 and two ways of distributing the work between MPI tasks and OpenMP threads are shown. The inadequate scaling of the Poisson solver arises because we have not yet replaced our old one by an improved one.

With the new understanding of the past KKR problems for total energies and atomic forces, KKR-nano can be expected to deliver rather precise large-scale density-functional calculations.

Because of this prospect considerable work was invested for modernizing the code mainly by Paul Baumeister (Jülich Supercomputing Centre) and by Marcel Bornemann (Institute of Advanced Simulation) with the aim to increase floating-point and communication performance of the code. The new code was tested (results obtained are shown in Fig. 3) during the JUQUEEN Extreme Scaling Workshop 2017 for the helical magnet MnGe, a system which shows unexplained three-dimensional skyrmion structures.

With next generation of supercomputers in mind, an efficient GPU version [5] of the TFQMR solver was developed by Thorsten Hater and Paul Baumeister within the Exascale Innovation Centre at the Jülich Supercomputing Centre for an IBM Power System S824L equipped with NVIDIA K40m cards. At present the code is also adapted for Intel Xeon Phi 7210 (Knights Landing) processors as they are available on the supercomputer QPACE3 in Jülich.

[1] A. Thiess, R. Zeller, M. Bolten, P. H. Dederichs, and S. Blügel, Phys. Rev. B 85, 235103 (2012)

[2] R. Zeller, P. H. Dederichs, B. Újfalussy, L. Szunyogh, and P. Weinberger, Phys. Rev. B 52, 8807 (1995)

[3] R. Zeller, J. Phys.: Condens. Matter 25, 105505 (2013) and Condens. Matter 27, 306301 (2015)

[4] A. Alam, S. N. Khan, B. G. Wilson, and D. D. Johnson, Phys. Rev. B 84, 045105 (2011)

[5] P. F. Baumeister, M. Bornemann, M. Bühler, T. Hater, B. Krill, D. Pleiter, and R. Zeller, in Euro-Par 2016, Lecture Notes in Computer Science 9833 pp. 77-89; P.-F. Dutot and D. Trystram (Eds.), Springer International Publishing, Switzerland (2016)


79

Quantum interference effects in molecular spin hybrids

T. Esat1,2, R. Friedrich3, F. Matthes1, V. Caciuc3, N. Atodiresei3, S. Blügel3, D. E. Bürgler1, F. St. Tautz2, and C. M. Schneider1 1 Peter Grünberg Institut-6, Forschungszentrum Jülich, Germany 2 Peter Grünberg Institut-3, Forschungszentrum Jülich, Germany 3 Peter Grünberg Institut-1 and Institute for Advanced Simulation-1, Forschungszentrum Jülich, Germany Molecular spin hybrids form upon chemisorption of aromatic molecules on ferromagnetic surfaces. Spin-dependent π-d hybridization combines the molecule and a few substrate atoms to a novel magnetic entity with prospects for molecular spintronics. Spin-sensitive scanning tunneling microscopy of single polycyclic TPT molecules on Co nanoislands reveals intramolecular variations of the spin polarization among structurally equivalent aromatic rings due to the superposition of spin-polarized hybrid states and the quantum interference pattern of the Co(111) surface state. The results stress the importance of quantum confinement when tailoring molecular spin hybrids on magnetic nanostructures.

In order to design miniaturized spin-based devices, e.g. for data storage, spintronics or quantum computation, magnetic properties of nano-structures are currently in the focus of interest and a strong motivation for research on molecular spintronics, since molecules can be synthesized in large numbers with identical properties. Single-molecule magnets with a large magnetic moment and strong magnetic anisotropy have attracted a lot of interest. However, up to now their magnetism is limited to cryogenic temperatures. An alternative approach is to tailor a so-called hybrid molecular magnet from a non-magnetic molecule by spin-dependent hybridization with a ferromagnetic building block. Besides the fact that the electronic and magnetic properties of the molecule are tuned, e.g. by inducing a magnetic moment, also the magnetic properties of the ferromagnetic building block are affected [1]. Such effects can lead to magnetic interlayer hardening and intralayer softening of the newly formed magnetic entity consisting of the chemisorbed molecule and its direct metal neighbors [1, 2]. Here, we study by spin-polarized scanning tunneling microscopy and spectroscopy (SP-STM/STS) the spin-dependent hybridization between the d-states of a Co bilayer nanoisland on Cu(111) and the π-orbital of a triphenyl-triazine (TPT) molecule and reveal the influence of quantum interference effects [3].

All measurements have been carried out in ultra-high vacuum using an STM operating at 4.3 K and in out-of-plane magnetic fields up to 3 T. Spin-sensitive Cr tips have been electrochemically etched from polycrystalline Cr rods. TPT molecules

were sublimated from a home-built Knudsen cell at about 415 K onto the sample held at 70 K.

TPT consists of a central triazine-like ring and three peripheral phenyl groups. In the gas phase it is non-magnetic and exhibits 3-fold rotational symmetry [Fig. 1(a)].

FIG. 1: (a) Adsorption geometry of TPT on Co(111). (b) dI/dV map of a Co nanoisland at +690 mV. The spatially modulated pattern is due to the sp electron confinement in the Co island. (c) STM topography of TPT molecules on a Co nanoisland. Insets: Magnified image of a TPT molecule overlaid with a true-scale representation of TPT in the gas phase.

The topmost atomic layer of Co(111) nanoislands shows 6-fold rotational symmetry, and the spin polarization is spatially modulated due to spin-dependent quantum interference pattern of the Co(111) sp surface state [4]. The periodicity of the modulation in Fig. 1(b) is of the order of 10 Å. Figure 1(c) shows an STM image of a Co island after the adsorption of TPT molecules that can be clearly identified by their size and shape, see insets. We observe two preferred adsorption orientations as expected considering the 6-fold rotational symmetry of the uppermost Co layer and 3-fold symmetry of TPT. The exact adsorption position of TPT on Co(111) is determined using STM images with atomic resolution of TPT and the substrate lattice that we obtained with CO-functionalized tips. We find that the N atoms (green color) of the triazine ring adsorb on top of Co atoms, leading to a symmetric adsorption geometry for all phenyl rings [Fig. 1(a)] and to two distinguishable orientations on the Co islands. This finding is corroborated by DFT calculations that reveal the highest binding energy (3.55 eV) for the experimentally observed adsorption geometry. The relative alignment of tip and substrate magnetizations can be controlled with the external field, which allows measuring differential conductance dI/dV P,AP spectra or maps for parallel (P) and antiparallel (AP) alignment.

The local spin polarization of the sample PS can be


80

calculated from spin asymmetry A by assuming aconstant polarization of the tip PT at a given bias voltage

⁄ ⁄

⁄ ⁄ ∝ (1)

Figure 2 shows the spin-resolved dI/dV spectra of the different aromatic rings of a TPT molecule. All TPT spectra show the same main features, but significantly differ from each other. The peak at -310 mV is located in the same energy region as the d-like surface state of Co(111) indicating that the molecule is strongly hybridized with the Co island, thus fulfilling the basic requirement for a hybrid molecular magnet. The strongest spin-dependent effect occurs for this hybrid state. All other states show hardly any spin polarization. Interestingly, the hybrid state exhibits different spin polarizations on the different aromatic rings of the TPT molecule. The spin polarization is inverted (green curve) or has the same sign (red curve) as the spin polarization of the d-like surface state (gray curve), or shows no spin polarization (blue curve). The spin asymmetry values at -310 mV for the different locations are given in Fig. 2 in the corresponding colors. The varying degree of spin polarization on the different aromatic rings cannot stem from a different hybridization of the aromatic rings with the ferromagnetic substrate as recently observed for TPT on Fe/W(110) [5], because TPT adsorbs highly symmetric on the Co islands (Fig. 1). These variations demonstrate that the state at -310 mV is not just the d-like Co surface state that shows through the molecules, but a new state formed due to the hybridization, because the spin polarization of the surface state does not change sign within an Co island.

FIG. 2: Spin-resolved dI/dV spectra acquired on the different aromatic rings of an adsorbed TPT molecule (red, green, blue, black crosses in the inset) and on bare Co (gray) for P and AP magnetization alignment of tip and Co island. The spin asymmetry value at -310 mV extracted for each spectrum using Eq. (1) is given in the corresponding color. Spectra are vertically shifted and the gray Co spectra are divided by 5.

Figure 3 shows the spatial variation of the spin asymmetry A at -310 mV, which according to Eq. (1) reflects the spin polarization of the sample at this energy.

FIG. 3: (a) STM image of a Co island with TPT molecules. (b) Spin asymmetry map calculated [Eq. (1)] from dI/dV maps taken in the framed area in (a). TPT representations are overlaid at the positions of the molecules extracted from the image in (a).

There is a significant spatial modulation of A within the Co island. The strongest effects occur around the TPT molecules, where A exhibits sign changes on a length scale of approximately 13 Å, as indicated in Fig. 3(b). Such sign changes are not observed in areas, where no molecules are adsorbed – there A only shows positive values, but is also modulated on a length scale of ≈13 Å from minimum to maximum. This length scale is very similar to that of the spatial modulation of the spin polarization of the Co(111) sp surface state [4]. Hence, the observed variations are most likely linked to the spin-dependent interference pattern on the Co nanoisland. This pattern depends on the island edges and other scattering centers (e.g. defects) and, thus, in general does not exhibit a 3-fold symmetry. Therefore, the influence of spin polarization pattern due to the interference of the Co(111) surface state is an obvious explanation for the observed broken 3-fold symmetry of the spin polarization within a single chemisorbed TPT molecule, which according to our STM images (Fig. 1) and DFT calculations forms a structurally 3-fold symmetric molecular spin hybrid. Since the wave function of the sp surface state cannot vanish abruptly at the edges of the TPT molecule, but rather extends into the molecule, a superposition of the hybridization-induced spin polarization of TPT and of the modulated sp surface state is detected above TPT in the spin-resolved STM measurements.

In conclusion, our results demonstrate the formation of a single molecular spin hybrid, i.e. a molecule that is spin-dependently hybridized with a ferromagnetic surface, using a polyaromatic molecule and reveal that the aromatic rings within the molecule can exhibit different spin polarizations [3]. In further studies it will be interesting to explore to what extent the spin properties of hybrid molecular magnets can be modified by spin-dependent quantum interference patterns.

[1] M. Callsen et al., Phys. Rev. Lett. 111, 106805 (2013)

[2] R. Friedrich et al., Phys. Rev. B 92, 195407 (2015)

[3] T. Esat et al., Phys. Rev. B 95, 094409 (2017)

[4] H. Oka et al., Science 327, 843 (2010)

[5] V. Heß et al., New J. Phys. 19, 053016 (2017)


81

Molecular characteristics of V18O42 nanoclusters in solution and adsorbed on the Au(111) surface

M. Moors1, O. Linnenberg2, A. Solé-Daura3, X. López3, C. Bäumer1, E. Kentzinger1,4, W. Pyckhout-Hintzen4,5, and K. Y. Monakhov1,2 1 Peter Grünberg Institut-7, Forschungszentrum Jülich, Germany 2 Institut für Anorganische Chemie, RWTH Aachen University, Germany 3 Departament de Química Física i Inorgànica, Universitat Rovira i Virgili, Spain 4 Jülicher Center for Neutron Science JCNS, Forschungszentrum Jülich, Germany 5 Institut of Complex Systems-1, Forschungszentrum Jülich, Germany Understanding the molecular characteristics of stoichiometric vanadium-oxo clusters (polyoxovanadates, POVs) both in solution and on surfaces is crucial for their usage in future application in the fields of catalysis as well as molecular electronics and spintronics. Therefore, we investigated the behavior of a (NEt4)5[V18O42(I)] compound as a representative for a mixed-valence POV solved in either water or organic solvents and after adsorption on an Au(111) surface using a wide range of experimental and theoretical techniques. While in pure water a strong agglomeration effect with intercalated H2O molecules could be observed, hardly any agglomeration was detected in pure organic solvents MeCN and DMF. Depending on the used solvent the adsorption on the Au(111) surface resulted in a diversely pronounced agglomeration tendency of the negatively charged particles. Furthermore, an oxidative effect of the metal substrate was found at low surface coverages.

Polyoxometalates (POMs) exhibit unique structural features and chemical, physical and optical properties, which have led to a growing fundamental and technological interest in the fields of solid-state devices for quantum computing (nanoscale quantum magnets) and magnetic sensing, single-molecule electronics, nano-semiconductors, and renewable energy conversion and storage [1-3].

Polyoxovanadates (POVs) are a subclass of POMs which combine a high thermal and chemical stability with exceptional redox properties. The in the recent study investigated [V18O42(I)]

5– cluster consists of 18 edge-sharing square-pyramidal O=VO4 units that form a spherical shell with an encapsulated iodide ion in the central void [4,5]. Formally, the cluster contains 8 V5+ and 10 V4+ atoms whose oxidation states are delocalized. This implies a rather easy change of the molecular redox state, which is confirmed by cyclic voltammetry measurements of the solved cluster as shown in Fig. 1. Hereby, altogether eight different molecular redox states can be detected. Molecular dynamics (MD) simulations predict a significant agglomeration tendency of the clusters

in aqueous solutions by intercalating H2O molecules.

FIG. 1: Cyclic voltammogram (1st and 2nd cycle) of (NEt4)5[V18O42(I)] (0.3 mM) in a 0.1 M TEAPF6-MeCN solution, measured with a scan rate of 50 mV·s–1 versus the Fc/Fc+ couple. It confirms the existence of 8 different molecular redox states.

While MD trajectories in pure or only small water impurities containing organic solvents like acetonitrile (MeCN) or N,N-dimethylformamide (DMF) indicate mainly the presence of monomolecular V18 species with the exception of sparse formation of dimers, higher water contents of ≥ 5 % increase the formation of dimers and trimers. Finally, in pure water the latter species get dominating and even higher oligomers can be found.

FIG. 2: Summary of the % of POV units forming different agglomeration state species averaged over 25 ns of MD trajectories in H2O and MeCN.

These theoretical predictions could be qualitatively confirmed by SAXS measurements. By assuming a


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rotationally averaged spherical geometry of the cluster and an effective hardsphere potential for the counterion interaction the particle radius Rp and the interparticle distance D could be determined by a simple Guinier approach. Hereby, water amounts of ≥ 5 % result in a significant increase of Rp and D indicating more and / or higher cluster oligomers in the solution.

Solvent (concentration)

Additional water content

RP [nm] D [nm]

DMF (1 ∙ 100 g∙L−1) 0 % 0.49 6.10 DMF (1 ∙ 101 g∙L−1) 0 % 0.49 6.94

0 % 0.63 6.24 MeCN (1 ∙ 101 g∙L−1) 1 % 0.63 6.06

5 % 0.72 8.44

TAB. 1: Particle radius Rp and interparticle distance D determined by SAXS measurements. Error bars are of the order of 0.1 %.

Depositing the [V18O42(I)]5– cluster on the (111)

orientated gold surface from solution shows an even stronger influence of the used solvent. While using MeCN leads to a rather homogenous contribution of small agglomerates or even single clusters, deposition from DMF results in significantly bigger agglomerates and a preferred adsorption at the step edges of the substrate terraces. This observation might be explained by the higher boiling temperature of DMF compared to that of MeCN, which results in an increased evaporation time of the solvent after the surface coating and, thus, in an increased surface mobility during the preparation process.

FIG. 3: STM images of a submonolayer POV covered Au(111) surface prepared by deposition (A) from a MeCN solution (concentration = 0.9 · 10–3 g·L–1; 1000 nm x 1000 nm; UB = 1.5 V, IT = 0.5 nA) and (B) from a DMF solution (concentration = 1.4 · 10–3 g·L–1; 900 nm x 900 nm; UB = 1.5 V, IT = 0.5 nA). (C) High resolution scan of (A) (100 nm x 100 nm, UB = 1.5 V, IT = 0.5 nA). (D) Height profile from (C) along two presumable cluster monomers and some fragments.

By using XPS the influence of the Au(111) substrate on the vanadium redox states can be determined. At high concentrations, and thus,

multilayer coverages the spectra show a slight excess of V4+ as it could be expected for the unmodified POV cluster and was also found for the bulk material. For lower concentrations only submonolayer coverages are obtained on the surface, so that the relative amount of clusters with direct Au contact is increased. In that case the observed ratio changes toward a higher amount of V5+, which indicates a partly oxidation of the mixed-valence cluster on the metallic surface.

Solvent (concentration)

Vanadium 2p3/2

Fitted peak

position [eV]

Relative vanadium content

[%]

Bulk

DMF (1.6 ∙ 10–2 g∙L−1)

V4+ V5+ V4+

515.4 516.7 515.5

7.0 5.1 7.9

V5+ 516.7 5.9 MeCN (1.6 ∙ 10–2 g∙L−1) V4+ 515.5 6.6

V5+ 516.7 6.0 MeCN (1.6 ∙ 10–3 g∙L−1) V4+ 515.6 2.3

V5+ 516.7 4.8

TAB. 2: Vanadium 2p3/2 XPS data and relative vanadium content for the bulk (NEt4)5[V18O42(I)] compound and deposited on the Au(111) surface using different solution concentrations.

In summary, the behavior of a mixed-valance POV cluster in solution and after adsorption on the Au(111) surface could be linked in this study by using a large variety of physical and theoretical means. Hereby, the proneness to agglomeration has shown to be strongly dependent on the solution medium. In aqueous solution dimers and trimers are the dominating species. Small water impurities caused for example by humidity have no significant influence. While the solvent polarity and the resulting charge stabilization is decisive for agglomeration in solution, this effect is subordinated for the adsorption behavior on surfaces. In that case surface mobility is the key factor for agglomeration, which can be enhanced by a lower evaporation speed of the solvent. Furthermore, the electronic structure of adsorbed POVs on the metallic surface undergoes redox modifications by the alteration of the cluster concentration-modulated surface coverage, which is an important aspect for future applications of immobilized POVs as, for example, molecular switches or heterogeneous catalysts.

______________________________________ [1] D.L. Long et al., Angew. Chem. Int. Ed. 49, 1736

(2010)

[2] X. López et al., Chem. Soc. Rev. 41, 7537 (2012)

[3] M. Ammam, J. Mater. Chem. A 1, 6291 (2013)

[4] K.Y. Monakhov et al., Chem. Soc. Rev. 44, 8443 (2015)

[5] O. Linnenberg et al., J. Phys. Chem. C 121, 10419 (2017)


83

On the formation of ZnO patches on a ZnPd/ZnO catalyst during methanol steam reforming

M. Heggen1, S. Penner2, M. Friedrich3, M. Armbrüster4, and R. E. Dunin-Borkowski1 1 Ernst Ruska-Centrum and Peter Grünberg Institut-5, Forschungszentrum Jülich, Germany 2 Institute of Physical Chemistry, University of Innsbruck, Austria 3 Fritz-Haber-Institute of Max-Planck-Society, Berlin, Germany 4 Institute of Chemistry, Technische Universität Chemnitz, Germany The high CO2 selectivity of ZnPd/ZnO, an outstanding catalyst in methanol steam reforming, was recently attributed to the interaction between small ZnO patches and ZnPd particles [1]. Yet, the detailed microstructure of this catalytic system and the formation mechanism of the ZnO patches inducing the high catalytic selectivity are unknown. We have uncovered the formation mechanism patches using aberration-corrected electron microscopy, electron energy loss spectroscopy and X-ray spectroscopy. It is demonstrated that the ZnO patches form by direct oxidation of the particles rather than by transport from the ZnO substrate, thus ruling out a classical strong metal-support interaction [2].

Hydrogen generation by methanol steam reforming (MSR) is a promising route to provide clean hydrogen for fuel cells in electric vehicles and other future energy applications. Due to their outstanding performance and high CO2 selectivity, ZnO-supported ZnPd particles have shown to be highly promising MSR catalysts. Recently, the origin of the high CO2 selectivity of ZnPd/ZnO was revealed by linking their catalytic properties with aberration-corrected high-resolution transmission electron microscopy (HRTEM) imaging of the catalyst at different stages of the MSR reaction [1] and comparison to catalytic data obtained on unsupported ZnPd model catalysts. While as-prepared ZnPd/ZnO shows only low selectivity towards CO2, the material exhibits very high CO2-selectivity (>95%) after a “selectivation period” in the methanol steam reforming reaction mixture [1]. It was shown that the high CO2-selectivity is intimately connected with the formation of small ZnO patches on ZnPd nanoparticles, subsequently causing a large amount of beneficial interface between ZnPd and ZnO. The formation of the large interface in turn enables efficient water activation, being a prerequisite for a high CO2-selectivity in MSR. However, whereas the presence of the ZnO patches was unambiguously revealed by aberration-corrected electron microscopy, the exact formation mechanism of the ZnO patches, and hence of the supposedly active and selective interface itself, still remained unclear.

Here, we present the results of a detailed microstructural study of ZnO-supported ZnPd nanoparticles, which focus on the structure and composition of the ZnPd nanoparticles leading to the formation of ZnO patches.

Scanning transmission electron microscopy (STEM) was performed using two FEI Titan scanning electron microscopes at the Ernst Ruska-Centre operated at 200 and 80 kV [3, 4]. Both microscopes are equipped with a Cs-probe corrector and a high-angle annular dark field (HAADF) detector. Electron energy loss (EEL) spectra were recorded analyzing the O-K, Pd-M4,5 and Zn-L2,3 edges. Compositional maps were obtained with energy-dispersive X-ray spectroscopy (EDX) using four large-solid-angle symmetrical Si drift detectors (FEI ChemiSTEM). For EDX elemental mapping, Pd-L, Zn-K and O-K peaks were used.

FIG 1: A) HAADF-STEM micrograph of a ZnPd particle showing non-uniform contrast. An EELS line scan across the particle was performed (large arrow). B) EEL spectrum profiles for Pd, Zn, and O and HAADF intensity profile. Dark spots (small arrows) in the HAADF image represent regions which are Pd-depleted and enriched in Zn. O is only present at the dark spots.


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Figure 1 shows an HAADF-STEM micrograph of a ZnPd particle and a respective EEL spectrum profile which was recorded across the particle along the arrow in Figure 1a representing the elemental distribution of Pd, Zn, and O. A comparison of the HAADF micrograph and the EELS data shows that the dark areas correspond to local Pd-depletion zones. The amount of Zn shows an increase from the outside of the particles, which is consistent with the geometrical increase of thickness of the ZnPd particle towards its center. At about 20 and 33 nm two distinct additional bumps are visible. From these observations it is obvious that the local Pd-depletion in the dark areas is accompanied by an enrichment of Zn. The O EELS signal is almost zero along most of the scan; only at about 20 and 33 nm two distinct peaks are noted. The presence of oxygen points towards the presence of ZnO in the dark areas.

The EELS analysis is supplemented by an EDX compositional analysis of two representative ZnPd particles. Figure 2 displays the respective HAADF-STEM image and the EDX maps showing the distribution of Pd, Zn, and O.

FIG 2: HAADF-STEM image of two ZnPd nanoparticles and EDX maps representing Pd (red), Zn (blue), and O (green).

The composition in the dark regions in the HAADF image (1, 2, and 6 in Figure 2) is on average Zn62Pd23O15 and Zn58Pd41O1 in the bright regions (3 and 4). Again, it is clearly visible that the bright regions are Pd-rich and generally have an O content close to zero. In contrast, the dark areas are Pd-depleted. Based on EELS and EDX compositional analysis several conclusions can be drawn: The composition of the bright regions (around Zn60Pd40) is slightly richer in Zn than the compositional limits of the tetragonal bulk ZnPd phase between about 39 and 50% Zn. HRTEM and previously published X-ray diffraction investigations [1], however, demonstrate solely the presence of the ZnPd and ZnO phase. Therefore, the bright regions are clearly assigned to the ZnPd phase. Most remarkable, the EDX and EELS investigations show the absence of oxygen in the bright regions. Hence, we can conclude that ZnPd phase regions exist which are not covered by ZnO. The dark areas are assigned to a mixture of the ZnPd and ZnO phase, and show, depending on the ratio of both phases, a stronger compositional variation than the bright areas. Most remarkable is the high amount of oxygen found at some of the dark spots. These results indicate the presence of

large amounts of ZnO even within the nanoparticles.

FIG 3: HAADF-STEM image of a large ZnPd particle with dark Pd-depleted regions. The Pd-depleted regions penetrate the surface where Zn can be oxidized (arrows).

Our results show that the formation mechanism of ZnO patches on top of the ZnPd nanoparticles can be described as follows: ZnPd particles show an inhomogeneous elemental distribution after initial reduction of the material before being exposed to MSR conditions. Pd-depleted Zn-rich regions extending towards the surface become capped by ZnO in the oxidative environment of the MSR reaction mixture. Coarsening of the Zn-rich areas inside the particles due to heating during the MSR reaction may assist the formation of localized ZnO surface plumes, which may extend deeply into the particles due to their channel-like appearance (Figure 3) [2]. Our results rule out a formation mechanism by transport from the ZnO substrate to the particle surface. This pathway is highly unlikely, since diffusion of oxidized Zn species from the support must include partial reduction of ZnO and the associated formation of substoichiometric Zn oxide species, subsequently partially encapsulating the ZnPd particles. Furthermore, if the ZnO patches would be the result of a true SMSI effect, complete coverage of the ZnPd particles would be expected, not being limited just to the Zn-rich areas. Our results provide direct evidence for a dynamical state of the catalyst in MSR conditions, adapting itself to the specific catalytic conditions, subsequently giving rise to lowered activation barriers for the associated catalytic reaction to CO2. _____________________________________________

[1] M. Friedrich, S. Penner, M. Heggen, and M. Armbrüster, Angew. Chemie 52, 4389 (2013).

[2] M. Heggen, S. Penner, M. Friedrich, R. E. Dunin-Borkowski, and M. Armbrüster, J. Phys. Chem. C 120, 10460 (2016).

[3] M Heggen, M Luysberg, and K Tillmann, Journal of large-scale research facilities (JLSRF) 2, A42, (2016).

[4] A. Kovács, R. Schierholz, and K. Tillmann, Journal of large-scale research facilities (JLSRF) 2, A43 (2016).


85

Néel-like domain walls in ferroelectric Pb(Zr,Ti)O3 single crystals

X.-K. Wei1,2, C.-L. Jia2,3, T. Sluka1, B.-X. Wang4, Z.-G. Ye4,5, and N. Setter1 1 Ceramics laboratory, EPFL-Swiss Federal Institute of Technology, Lausanne, Switzerland 2 Peter Grünberg Institut-5, Forschungszentrum Jülich, Germany 3 The School of Electronic and Information Engineering, Xi'an Jiaotong University, China 4 Department of Chemistry and 4D LABS, Simon Fraser University, Burnaby, British Columbia, Canada 5 Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education &

International Center for Dielectric Research, Xi’an Jiaotong University, China In contrast to the flexible rotation of magnetization direction in ferromagnets, the spontaneous polarization in ferroelectric materials is highly confined along the symmetry-allowed directions. Accordingly, chirality at ferroelectric domain walls was treated only at the theoretical level and its real appearance is still a mystery. Here we report a Néel-like domain wall imaged by atom-resolved transmission electron microscopy in Ti-rich ferroelectric Pb(Zr1-xTix)O3 crystals, where nanometer-scale monoclinic order coexists with the tetragonal order. The formation of such domain walls is interpreted in the light of polarization discontinuity and clamping effects at phase boundaries between the nesting domains. Phase field simulation confirms that coexistence of both phases as encountered near the morphotropic phase boundary promotes the polarization to rotate in a continuous manner. Our results provide a further insight into the complex domain configuration in ferroelectrics, and establish a foundation towards exploring chiral domain walls in ferroelectrics.

Oxide ferroelectrics, with coupling of polarization, strain, heat and electric field, are widely utilized in electronic devices such as multilayer capacitors, piezoelectric transducers, pyroelectric detectors, and non-volatile high-density memories. While it is well known that properties of the ferroelectrics such as piezoelectric effect and permittivity are substantially enhanced by mobile and stationary domain walls, the ferroelectric domain walls themselves were found recently to possess unique properties that differ thoroughly from the bulk domains, e.g., electronic conductivity and photovoltaic effect. These results raise much interest in further exploration of the properties of domain walls and their internal structures.

As known, magnetic 180 domain walls are typically Bloch type or Néel type, across which the magnetization vector rotates continuously in a plane parallel or normal to the wall plane respectively, while ferroelectric 180 domain walls are predominately Ising type. In spite of this, theoretical and computational studies predict plethora of wall structures in the oxide ferroelectrics, e.g., the Bloch-type walls in

rhombohedral BaTiO3 and tetragonal PbTiO3, where the polarization rotation is enabled by symmetry lowering at the walls. Driven by the flexoelectric effect, a higher-order electro-mechanical coupling, a Bloch-like component combined with or without a Néel-like component is also predicted at the 180 domain walls. However, the chiral components are negligible in comparison with the Ising component. The recent discovery of ferroelectric monoclinic phases close to the morphotropic phase boundary, e.g., in Pb(Zr1-

xTix)O3 (PZT), (1-x)PbMg1/3Nb2/3O3-xPbTiO3, and BiFeO3, where the polarization easily rotates within a crystallographic plane, offers new possibilities in the search of new types of domain walls.

FIG. 1: A. Schematic relation of the monoclinic unit cells with respect to the tetragonal unit cell. The PS denotes the spontaneous polarization in the monoclinic phase (space group Cm). B. Bright-field TEM image of the x = 0.60 PZT crystal. The inset is a dark-field TEM image recorded under two-beam condition using g = (110)T reflection in the [110] orientated specimen. C. The representative SAED pattern recorded along the [100]T zone axis. The diffraction spots are indexed with respect to the tetragonal structure. D. Reciprocal space mapping of the (002) reflection spot from C to show the spot splitting for the coexisting monoclinic and tetragonal phases.

Here, by means of direct imaging of electric dipoles using the NCSI technique combined with the ultrahigh-precision measurements, we report a


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Néel-like rotation of polarization at monoclinic domain walls in ferroelectric PZT (x = 0.54, 0.60) single crystals, where the monoclinic order coexists with the tetragonal order at nanometer scale. The formation mechanism of the domain wall is interpreted in the light of polarization discontinuity and clamping effect at the monoclinic-tetragonal phase boundaries.

Figure 1A shows the basic axes of the monoclinic and the tetragonal unit cell structures. The relations of their lattice parameters are aM ≈ bM ≈ √2aT, cM ≈ cT with a small tilting angle. The spontaneous polarization (PS) is oriented along the [001]T direction in the tetragonal phase (space group P4mm), but in the monoclinic phase, it is allowed to rotate flexibly within the (110)T plane33. Our TEM studies reveal that the monoclinic phase coexists with the tetragonal phase in the PZT (x = 0.54, 0.60) crystals. Figure 1B shows morphology of the x = 0.60 PZT specimen oriented along the [100]T direction. The observed nanometer scale ferroelectric domains are in agreement with earlier experimental results on the second-order nature of the ferroelectric phase transition in this composition range. Under two-beam conditions, diffraction contrast analysis using Pg > 0 relation (P is a component of PS and g is a scatting vector) reveals that sizes of the nesting domains are roughly less than 20 nm and the domain boundaries cannot be clearly identified, see the inset in Fig. 1B. In the selected area electron diffraction (SAED) pattern shown in Fig. 1C, the reflection spots can be registered as the tetragonal phase, with a tetragonality c/a ≈ 1.043. However, splitting of the reflection spots as shown in Fig. 1D, can be found by a careful inspection, further corroborating the coexistence of the monoclinic phase with the tetragonal phase.

To investigate details of the domain and domain wall structures, high-resolution TEM experiments were performed on the PZT crystals under the NCSI conditions. A particularly interesting feature of the atomic structure is seen in the region including the central plane of the wall between domains M-I and M-II, as denoted by green arrows in Fig. 2A, where the oxygen columns exhibit vertical displacements. Measurements of the relative displacements of oxygen columns reveal a continuous rotation of the displacements from leftward (slightly up) in M-II through upward at central plane to rightward (slightly up) in M-I. The displacement feature of oxygen atoms forms a structure similar to the Néel wall in ferromagnets [1]. Quantitative measurement of the atom displacements is obtained by means of iterative image simulations, where the simulated image with the best fitting to the experimental image is attached to the left-side of Fig. 2A.

Figure 2C shows the lattice parameter changes as a function of distance away from the central plane of the Néel-like wall. It is seen that the c-axis in the M-I and M-II domains is 0.415 nm and 0.410 nm, respectively, which decreases in a step manner across the wall center. While the a-axis in both domains is almost the same. As a result, the c/a

ratio in the M-I domain is 1.038 in average, it reaches a value of 1.048 near the central plane of the wall and extends into the M-II domain by 2 unit cells. This ratio decreases to about 1.024 in the rest part of the M-II domain as shown in Fig. 2D. Our statistical studies reveal that the c/a ratio in the monoclinic domains is not necessarily different from each other.

FIG. 2: A. Experimental image of the monoclinic domain wall viewed along the [100]M direction. Green arrows denote the central plane of the domain wall. The inset on the left hand shows a simulated image for thickness of 4.4 nm and defocus of 5.5 nm. B. Continuous rotation of the O2-displacement vectors across the domain wall compiled by δxO2-Zr/Ti and δyO2-Zr/Ti in E. C,D. The c, a axis and c/a ratio across the domain wall. E. The relative displacements of the O2 atoms parallel and normal to the domain wall plane. Blue and pink symbols represent the values measured from the Pb to Pb and from the Zr/Ti to Zr/Ti atom positions, respectively.

With respect to the centers of the nearest neighboring Ti/Zr atom columns, the rightward horizontal displacements of the O2 atoms (δxO2-

Zr/Ti) undergo a smooth reduction from about 12 pm in the M-I domain to zero at the central plane of the wall, see Fig. 2E. The displacements switch to the opposite direction (leftward) inside the M-II domain and further increase to 14 pm. Along the vertical direction, the displacements of O2 atoms gradually decay from the maximum of δyO2-Zr/Ti ≈ -21 pm at the central plane of the wall towards far ends of the two domains. Since the unit-cell dipole moment is proportional to the relative displacement (Fig. 2B), the atomic displacement data represents the transition behavior of the polarization across the domain wall. Phase-field simulations reveal that formation of such domain walls is attributed to polarization discontinuity and clamping effects at the tetragonal/monoclinic phase boundaries [2].

Discovery of the rotational polarization configuration not only establishes a foundation towards exploring chiral domain walls in ferroelectrics, but also highlights that miniaturized piezoelectric devices can be fabricated by creation of high density of such domain walls in the ferroelectric/piezoelectric materials [3].

[1] G. Chen et al., Nat. Commun. 4, 2671 (2013)

[2] X.-K. Wei, C.-L. Jia, T. Sluka, B.-X. Wang, Z.-G. Ye, and N. Setter, Nat. Commun. 7, 12385 (2016)

[3] H. Fu and R. E. Cohen, Nature 403, 281 (2000)


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Spin-wave and electromagnon dispersions in multiferroic MnWO4 as observed by neutron spectroscopy

Y. Xiao1, C.M.N. Kumar2, S. Nandi3, Y. Su3, W.T. Jin3, Z. Fu3, E. Faulhaber4, A. Schneidewind3, and Th. Brückel1,3 1 Jülich Centre for Neutron Science JCNS-2, Forschungszentrum Jülich, Germany 2 Jülich Centre for Neutron Science JCNS at SNS, Oak Ridge, Tennessee, USA 3 Jülich Centre for Neutron Science JCNS at MLZ, Garching, Germany 4 Heinz Maier-Leibnitz Zentrum, Technische Universität München, Garching, Germany Multiferroics with strong coupling between ferroelectric and ferromagnetic degrees of freedom have attracted intense research effort due to their application potential in tunable multifunctional devices. The cross-control between electric and magnetic dipoles in multiferroics is found to be accompanied by the dynamical motion of multiferroic domain walls. In addition, the dynamic magnetoelectric coupling in multiferroics will lead to the appearance of electric-dipole-active magnetic resonance. Here we report the observation of elementary magnetic excitations in multiferroic MnWO4 by high-resolution inelastic neutron scattering. A detailed analysis of the low energy excitations of MnWO4 shows that the spin-wave excitations in the collinear antiferromagnetic/paraelectric phase can be well described by a Heisenberg model with magnetic exchange couplings extending to the 12th nearest neighbor, although MnWO4 was considered as a moderately spin frustrated system. Moreover,the electromagnon excitation observed in both the paraelectric and ferroelectric phases supports the existence of strong spin-orbit interaction in MnWO4, despite the fact that Mn2+ is an S-state ion.

So-called spin-driven ferroelectrics, for which the inversion symmetry is broken in the ferroelectric phase due to the appearance of a particular magnetically ordered state, provide a path to the strong magnetoelectric (ME) coupling [1]. It was found that spin-driven ferroelectricity can occur in different magnetic materials with different types of magnetic ordering states. In order to understand magnetoelectric coupling in spin-driven ferroelectrics, three different microscopic models, namely, the exchange striction model, inverse Dzyaloshinskii-Moriya (DM) model, and spin-dependent p-d hybridization model, have been proposed to describe the observed ferroelectricity in different spin-driven ferroelectrics.

As a prototypical multiferroic material with spiral magnetic order, MnWO4 has been widely studied concerning its magnetic and ferroelectric properties. MnWO4 undergoes three successive magnetic transitions. The corresponding phases are labeled as AF1, AF2, and AF3. It is well

accepted that the inverse DM mechanism is relevant for modeling the ME coupling in MnWO4. However, it is argued that DM interaction is not the only driving force for the ferroelectric polarization; other single-site symmetric interactions are also shown to be involved in the magnetoelectric process in MnWO4. Besides, a theoretical study suggests a more complex scenario where multiferroicity in MnWO4 is caused by a competition of DM and isotropic exchange interactions. A deeper insight into the coupling between the electric and magnetic degrees of freedom can be gained by studying not only the respective order but also the excitation spectra.

Single-crystal neutron diffraction and inelastic neutron spectroscopy were performed on the cold-neutron triple-axis spectrometer PANDA operated by Jülich Centre for Neutron Science at the Maier-Leibnitz Zentrum in Garching, Germany. The observed low-energy excitations in the AF1 phase at 1.5 K along the [1 0 2] and [0 1 0] directions through the magnetic peak (1/4, 1/2, 1/2) are shown in Figs. 1(a) and 1(c), respectively. Because MnWO4 is an antiferromagnetic insulator with a rather large ordered Mn moment, we analyze inelastic neutron-scattering data in the linear spin-wave approximation with a Heisenberg Hamiltonian. The primitive magnetic unit cell of MnWO4 is composed of eight Mn spins in the collinear AF1 phase. Therefore, four twofold degenerate spin-wave branches are expected in zero field. However, the excitation spectrum at the zone center Q = (1/4, 1/2, 1/2) exhibits at least five resolvable excitations. This can be clearly seen in the individual energy scans plotted in Fig. 2(a). By considering different possibilities and evaluating the results of the refinements as a function of Q, we found that it is impossible to describe the two low-lying energy excitations located in the zone center at ω1 = 0.07(1) and ω2 = 0.45(1) meV within the Heisenberg model.

As demonstrated in Figs. 1(b) and 1(d), the dispersion as well as the intensities of the magnetic excitations can be well modeled as spin-wave excitations using the Hamiltonian, if the low-lying excitations ω1 and ω2 are excluded. The fitting results exhibit excellent agreement with the experimental data for the spin-wave excitations


88

and yield exchange parameters as J1 = −0.37(1), J2 = −0.002(1), J3 = −0.17(1), J4 = −0.21(1), J5 = −0.011(5), J6 = −0.34(1), J7 = −0.11(1), J8 = −0.010(5), J9 = −0.20(1), J10 = −0.12(1), J11 = −0.042(1), J12 = −0.016(1) meV, and DS = 0.06(1), where all parameters are given in units of meV. All obtained exchange parameters are negative, which indicates that three dimensional antiferromagnetic exchange interaction is the dominant interaction in MnWO4.

FIG. 1: The measured spin-wave spectrum compared to calculations based on a Heisenberg model that describes the antiferromagnetic ground state of MnWO4.

In the AF1 phase of MnWO4, the Mn2+ spins are aligned collinearly along the easy axis with the spin direction alternating along the a axis as ↑↑↓↓. The ↑↑↓↓ spin configuration can be considered as the special case of a modulated structure. As for a spiral magnet, one phason mode and two rotation modes may contribute to the low-energy excitation. The observed two modes ω1 and ω2 in Fig. 2(a) can be attributed to the electromagnon excitations and they relate to the phason and rotation modes, respectively. In Figs. 2(b) and 2(c), the dispersion relation along the [1 0 2] direction as well as the intensity change of electromagnon mode ω2 extracted from the individual energy scans are plotted. Interestingly, an energy dip in the dispersion relation and a peak in the intensity are observed at Q = (0.26(1), 1/2, 0.52(2)), which corresponds exactly to the magnetic propagation vector we find for the incommensurate AF2 phase. The minimal energy gap for the electromagnon ω2 is associated with the magnetoelectric coupling effect in multiferroic MnWO4. In contrast to the dispersion along the [1 0 2] direction, both energy and intensity of the electromagnon ω2 along the [0 1 0] direction evolute monotonically away from Q = (1/4 1/2 1/2), as shown in Figs. 2(d) and 2(e), indicating anisotropic dispersion behaviors of electromagnons in MnWO4.

FIG. 2: Representative magnetic excitation spectra around Brillouin zone center.

Compared to the spin-wave energy gap, the energy gaps of 0.07(1) and 0.45(1) meV for the two observed electromagnon excitations are relatively small, but of the same order of amplitude. If the inverse DM interaction is considered as the mechanism responsible for the multiferroic properties in MnWO4, the observed electromagnon excitations, which are associated with the magnetoelectric coupling, may arise from the DM exchange interaction instead of the Heisenberg interaction. Moreover, although the two observed electromagnon excitation modes cannot be described within the Heisenberg model, they do exhibit dispersive behavior characteristic for collective excitations.

In summary, we present results of a comprehensive neutron scattering study of the elementary magnetic excitations in multiferroic MnWO4. In addition to the well-known spin-wave excitations, we demonstrate the existence of electromagnon excitations in both the paraelectric AF1 and the ferroelectric AF2 phases. The analysis of the low-energy excitation spectra implies the existence of collective electromagnon excitations, which reflect the strong ME coupling [2].

[1] Y. Tokura, S. Seki, and N. Nagaosa, Rep. Prog. Phys. 77, 076501 (2014)

[2] Y. Xiao, C.M.N. Kumar, S. Nandi, Y. Su, W.T. Jin, Z. Fu, E. Faulhaber, A. Schneidewind, and Th. Brückel, Phys. Rev. B 93, 214428 (2016)


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Neutron diffraction to distinguish between symmetry lowering and Renninger effect: An example of multiferroic Ba2CoGe2O7

A. Sazonov1,2, M. Meven1,2, G. Roth1, R. Georgii3, I. Kézsmárki4, V. Kocsis5,4, Y. Tokunaga5,6, Y. Taguchi5, Y. Tokura5,7, and V. Hutanu1,2 1 Institute of Crystallography, RWTH Aachen University, Germany 2 Jülich Centre for Neutron Science JCNS at MLZ, Garching, Germany 3 MLZ and Physics Department E21, Technical University of Munich, Garching, Germany 4 Department of Physics, Budapest University of Technology and Economics, and MTA-BME Lendület Magneto-optical Spectroscopy Research Group, Budapest, Hungary 5 RIKEN Center for Emergent Matter Science (CEMS), Saitama, Japan 6 Department of Advanced Materials Science, University of Tokyo, Kashiwa, Japan 7 Department of Applied Physics, University of Tokyo, Japan For a symmetry-consistent theoretical descrip-tion of the multiferroic phase of Ba2CoGe2O7 a precise knowledge of its crystal structure is a prerequisite. In our previous synchrotron X-ray diffraction experiment on multiferroic Ba2CoGe2O7 at room temperature, forbidden reflections were found that favor the tetragonal-to-orthorhombic symmetry lowering of the compound. Here, the results are reported of room-temperature single-crystal diffraction studies with both hot and cold neutrons to differentiate between genuine symmetry lowering and multiple diffraction (the Renninger effect). A comparison of the experimental multiple diffraction patterns with simulated ones rules out symmetry lowering. Thus, the structural model based on the tetragonal space group P-421m was selected to describe the Ba2CoGe2O7 symmetry at room temperature.

Recently, multiferroic behaviour and static magnetoelectric effects have been observed in many members of the melilite family, e.g. Ca2CoSi2O7, Sr2CoSi2O7, Ba2MnGe2O7 and Ba2CoGe2O7 (see e.g. Ref. 1, and references therein). The dynamic magnetoelectric effect, also observed in these compounds, drastically changes the optical properties of multiferroics compared with conventional materials. For instance, quadrochroism at the magnetoelectric spin excitations of multiferroic Ca2CoSi2O7, Sr2CoSi2O7 and Ba2CoGe2O7 was recently discovered [2].

Owing to the lack of complete structural phase diagrams in the case of melilites, the high-symmetry melilite phase is often used as a basis for theoretical calculations and experimental investigations (see e.g. Ref. 2, and references therein). However, the melilite family shows a variety of structural phase transitions, including incommensurate phases, depending on

temperature and chemical composition. Knowledge of the precise crystal structure is a very important input for understanding magnetoelectric phenomena in multiferroics. For instance, the main features of the magnetoelectric behavior in Ba2CoGe2O7 were predicted by symmetry considerations without referring to any specific microscopic mechanism [3]. Thus, the exact structural model is often an essential starting point for further theoretical and experimental research.

The results of our previous synchrotron X-ray diffraction study on Ba2CoGe2O7 at room temperature and below [4] are in accordance with a symmetry lowering from the high-temperature tetragonal structure with space group P-421m to the orthorhombic space group Cmm2. The assumption of the symmetry reduction was mainly based on the presence of reflections forbidden in P-421m [4]. The origin of the observed superstructure reflections in Ba2CoGe2O7 was never cross-checked by other methods.

In order to differentiate between genuine symmetry lowering and multiple diffraction or the Renninger effect (see [5, 6], and references therein), neutron diffraction with both short (hot neutrons) and long (cold neutrons) wavelengths was applied. We present here the results of the model selection based on the multiple diffraction patterns [7].

Single-crystal neutron diffraction studies were performed on the two diffractometers HEiDi (short wavelength λ = 0.793 Å) and MIRA (long wavelength λ = 4.488 Å) at the FRM II reactor, Heinz Maier-Leibnitz Zentrum, Germany. Multiple diffraction patterns (the so-called ψ scans) were simulated before the experiment in order to select the most appropriate wavelength and positions in reciprocal space. The instrumental parameters used in the simulation are the horizontal and vertical beam divergences and the wavelength spread of the incident beam. These were


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estimated from the geometry of the instrument. The Ba2CoGe2O7 sample-specific parameters, like the mosaic spread and the mosaic block radius, were set manually. Fractional atomic coordinates and anisotropic atomic displacement parameters were obtained from the structure refinement using the present single-crystal neutron diffraction data.

Fig. 1 shows a comparison of the simulated multiple diffraction pattern for the forbidden (300) reflection and the experimental data collected at HEiDi with λ = 0.793 Å. Good agreement between the calculated and experimental data indicates that both instrumental and sample-specific parameters were reasonably selected. As can be seen from Fig. 1, the very broad peaks on the scan make it impossible to separate the case of symmetry lowering from that of multiple diffraction. There is no single point in the diffraction pattern without an overlap of neighboring reflections and the intensity never drops down to zero. As a result, the possible contribution caused by symmetry reduction could be well hidden by the multiple diffraction part. A comparison of the simulated and experimental intensities is not reliable and therefore could not be used to solve the problem.

FIG. 1: Multiple diffraction pattern of the forbidden (300) reflection according to calculation (top, blue curve) and the single-crystal neutron diffraction experiment (bottom, red curve) with a short wavelength λ = 0.793 Å.

In contrast with the short-wavelength case, cold neutrons make it possible to overcome the problem with overlapping peaks in the ψ scans. A change of just one single parameter, the wavelength λ, in the simulation process from 0.793 Å to 4.488 Å drastically modifies the entire multiple diffraction pattern. The number of peaks is reduced and they become well separated from each other. Regions of zero intensity appear on the simulated curve. The experimental values for the (100) reflection agree well with the calculation, as can be seen in Fig. 2. We were able to reproduce the zero-intensity regions experimentally. Therefore, the observed intensity at the positions of the forbidden reflections could

be explained solely by multiple diffraction. This rules out the symmetry lowering scenario in Ba2CoGe2O7 and supports the tetragonal space group P-421m as the correct description of the true structure at room temperature.

FIG. 2: Multiple diffraction pattern of the forbidden (100) reflection according to calculation (top, blue curve) and the single-crystal neutron diffraction experiment (bottom, red curve) with a long wavelength λ = 4.488 Å.

In conclusion, the single-crystal neutron diffraction experiments with both hot and cold neutrons were performed to differentiate between genuine symmetry lowering and multiple diffraction (the Renninger effect). It was found that the scattered intensities detected at the positions of the forbidden reflections are entirely due to multiple diffraction. Thus, the crystal structure of Ba2CoGe2O7 at room temperature can be described by the tetragonal space group P-421m without any symmetry lowering [7]. The reported structure can serve as a profound experimental and theoretical basis to develop microscopic models describing the multiferroic nature and the peculiar magnetoelectric phenomena in melilites.

[1] H. Murakawa, Y. Onose, S. Miyahara, N. Furukawa and Y. Tokura, Phys. Rev. B 85, 174106 (2012)

[2] I. Kezsmarki, D. Szaller, S. BordACS, V. Kocsis, Y. Tokunaga, Y. Taguchi, H. Murakawa, Y. Tokura, H. Engelkamp, T. Room, and U. Nagel, Nat. Commun. 5, 1 (2014)

[3] J. M. Perez-Mato and J. L. Ribeiro, Acta Cryst. A67, 264 (2011)

[4] V. Hutanu, A. P. Sazonov, H. Murakawa, Y. Tokura, B. Nafradi, and D. Chernyshov, Phys. Rev. B 84, 212101 (2011)

[5] M. Renninger, Z. Phys. 106, 141 (1937)

[6] E. Rossmanith, Acta Cryst. A62, 174 (2006)

[7] A. Sazonov, M. Meven, G. Roth, R. Georgii, I. Kézsmárki, V. Kocsis, Y. Tokunaga, Y. Taguchi, Y. Tokura, and V. Hutanu, J. Appl. Cryst. 49, 556 (2016)


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Mott transition and spin-orbit effects in Ca2RuO4

E. Pavarini1,2 and G. Zhang2 1 Peter Grünberg Institut-2, Forschungszentrum Jülich, Germany 2 Institute for Advanced Simulation-3, Forschungszentrum Jülich, Germany We study the effects of spin-orbit and Coulomb anisotropy on the electronic and magnetic properties of the Mott insulator Ca2RuO4. We show that, contrary to a recent proposal, the Mott metal-insulator transition is not induced by the spin-orbit interaction. We confirm that, instead, it is mainly driven by the change in structure from long to short c-axis layered perovskite. We show that the magnetic ordering and the anisotropic Coulomb interactions play a small role in determining the size of the gap. The spin-orbit interaction turns out to be essential for describing the magnetic properties. It not only results in a spin-wave gap but it also enlarges significantly the magnon bandwidth.

The 4d4 layered perovskite Ca2RuO4 (Fig. 1) is made of planes of corner-sharing RuO6 octahedra. It belongs to the Ca2−x SrxRuO4 family, well known for its exotic electronic properties, including, among others, spin-triplet superconductivity, Hund’s coupling physics, heavy-fermion and spin-glass behavior as well as a series of structural, magnetic and electronic phase transitions. Ca2RuO4 itself exhibits a peculiar paramagnetic metal- insulator transition (MIT) at TMIT = 360 K, basically concurrent with the change from L-Pbca (long c axis) to S-Pbca (short c-axis) structure at TS = 356 K. Similar transitions have been reported when Ca is partially replaced by Sr (x ≤ 0.2) or under pressure. The origin of the MIT has been intensively investigated, both experimentally and theoretically [1]. Electronically, Ca2RuO4 is characterized by 2/3-filled t2g bands. Because of the layered structure, the ratio R between the xz/yz and xy band-width takes the value R ~ 0.5. There is a general agreement that the MIT is caused by strong correlations, i.e., by the screened Coulomb interaction tensor part of the Hamiltonian. Its actual nature has been hotly debated, however. Early on, an orbital-selective Mott transition (OSMT) scenario has been proposed, in which the orbital polarization p = nxy − (nxz + nyz )/2 changes from the value p = 1 below the transition (xy orbital order) to p = −1/2) in the metallic phase. An alternative proposal was a single Mott transition, assisted, however, by the tetragonal crystal-field splitting, εCF = εxz/yz − εxy > 0. From angle-resolved photoemission (ARPES) data, for x = 0.2 a new type of OSMT (p = 1/4) was inferred; other ARPES experiments reported, however, three metallic bands and no OSMT.

.

FIG. 1: Crystal structure of Ca2RuO4

Later, accurate local-density approximation + dynamical mean-field theory (LDA+DMFT) calculations have shown that the change in crystal structure from L-Pbca to S-Pbca is a decisive factor, leading to a reduction of the bandwidth ratio R and to an enhancement of the crystal-field splitting; in the metallic L-Pbca phase the orbital polarization is p ~ 0 (no orbital order) and three metallic bands are obtained, in line with ARPES result. Very recent ARPES data for the insulating phase of Ca2RuO4 appear also in line with LDA+DMFT calculations. Recently, these conclusions has been however challenged by a LDA+U study, which proposes a different scenario. In the latter, it is the Coulomb-enhanced spin-orbit interaction, neglected in LDA+DMFT studies of the MIT so far, to actually induce the transition. Indeed, several works point to a relevant role of the spin-orbit coupling for the electronic structure of layered ruthenates. To further complicate the picture, it has been recently shown that, surprisingly, the effects of the anisotropic Coulomb interactions – i.e., the terms with symmetry lower


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FIG. 2: DMFT spectral functions in the various cases.

than O(3) – are crucial to reproduce the experimental Fermi surface of Sr2RuO4. This suggests that they could also play an important role in the metal- insulator transition of Ca2RuO4, perhaps changing the actual form of the ground state. It becomes therefore necessary to systematically re-analyze the metal-insulator transition, explicitly accounting for the effects of both spin-orbit interaction and Coulomb anisotropy.

Remarkably, even for the magnetic phase the role of the spin-orbit interaction is unclear. Once more, competing scenarios have been proposed. In the first, the spin-orbit interaction can be treated as a perturbation modifying the spin Hamiltonian for the Hund’s rule S = 1 ground multiplet; in this picture the magnetic interactions are described via a S = 1 Heisenberg- like magnetic-exchange model, – both an isotropic and a strongly anisotropic Heisenberg Hamiltonians have been put forward – plus a spin-orbit induced single-ion anisotropy term, described via a tensor D. In the second scenario, the spin-orbit interaction leads to the formation of a zero total angular momentum state (jt = 0), and magnetism is therefore of the Van Vleck type.

In this work [2], we analyze the problem by using the LDA+DMFT approach including explicitly the spin-orbit interaction and the anisotropic Coulomb interaction. We use a generalized continuous-time quantum Montecarlo solver (interaction expansion flavor). We perform calculations for two set of screened Coulomb parameters available in the literature, one calculated via the so-called constrained random-phase approximation (CRPA) and the other via the constrained local-density

approximation (cLDA). The LDA+DMFT results that we obtain with these two sets are qualitatively similar.

Our calculations show that the change of structure from L-Pbca → S-Pbca is the most influential factor in determining the metal-insulator transition in Ca2RuO4, whereas spin-orbit coupling, magnetic ordering and the anisotropic Coulomb interactions are not decisive. Furthermore, contrarily to what static mean-field LDA+U calculations suggest, we find that the spin-orbit interaction slightly reduces the gap in the xy-orbitally-ordered insulating S-Pbca phase.

FIG. 3: Spin-wave dispersion.

By means of many-body perturbation theory, we then compute the inter-site magnetic couplings and the spin- orbit-generated single-ion anisotropy tensor D. With this realistic set of parameters we calculate the magnon dispersion via spin-wave theory. The quantitative agreement with experiments is better when cRPA screened Coulomb parameters are used, but qualitatively the results are similar for both cRPA and cLDA parameter sets. We find that the D tensor is key in determining the spin dynamics of Ca2RuO4: it not only contributes to half the bandwidth, but, surprisingly, it also yield a sizable gap at the Γ point via the small in-plane anisotropy Daa − Dbb ∼ 1 meV.

[1] See complete list of references in [2] for information on previous works.

[2] G. Zhang and E. Pavarini, Phys. Rev. B. 95, 075145 (2017).

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93

Surface atomic structure and growth mechanism of monodisperse 100-faceted strontium titanate zirconate nanocubes

H. Du1,2, C.-L. Jia1,3, and J. Mayer1,2 1 Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich, Germany 2 Central Facility for Electron Microscopy (GFE), RWTH Aachen University, Germany 3 Peter Grünberg Institute-5, Forschungszentrum Jülich, Germany The highly sensitive and selective properties of monodisperse faceted nanocrystals inherently stem from the atomic and electronic structures on the faceted surfaces. For elemental nanocrystals, the atomic structure on the surfaces is merely determined by the geometric shape itself. However, for compound materials such as alloys and complex oxides, atomic details on the faceted surfaces need to be studied on the atomic level. Here, we demonstrate that the surface atomic structure of faceted nanocrystals of complex oxides, 1 0 0-faceted strontium titanate zirconate nanocubes, can be unambiguously resolved by aberration-corrected scanning transmission electron microscopy. The resolved surface atomic details reveal a layerwise growth process of the nanocubes, thereby allowing an in-depth understanding of the growth mechanism.

Strontium titanate (SrTiO3), strontium zirconate (SrZrO3) and their solid solutions (SrTi1−xZrxO3) are important members in the class of perovskite structures with a general formula ABO3 (Fig. 1a). These materials are of great technological and fundamental importance not only because of their interesting properties, but also because of their ability to combine and to adjust these properties by chemical substitution with a wide variety of cations. However, despite the success of the synthesis of the 100-faceted BaTiO3, SrTiO3, and Ba1−xSrxTiO3 nanocubes, whether the 100 facets of the nanocubes are terminated with AO (SrO) or BO2 (TiO2) is a question still remaining open for speculation and investigation. A comprehensive understanding of the growth mechanisms of these faceted nanocubes has not been achieved. Direct experimental evidence for the atomic structure on these nanocube surfaces has become one of the key steps in exploring the growth mechanisms.

In this work, we report on detailed studies of monodisperse 100-faceted nanocubes of SrTi1−xZrxO3 (x = 0.25 to 0.5) [1], which were synthesized using the oil-water two-phase solvothermal method (Fig. 1b) [2]. The surface atomic structure of the monodisperse faceted

nanocrystals is determined by means of aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). On the basis of the structural features on the faceted surfaces, a deeper insight into the growth mechanisms could be obtained [1].

Pure perovskite nanocrystals of SrTi1–xZrxO3 were obtained by syntheses over the compositional range for x from 0.0 to 0.5. Nanocrystals of ZrO2 appeared when x reaches about 0.6 and gradually become the dominant products with further increase of Zr content. For cubic perovskites such as SrTiO3, it is well known that the 100 faces have the lowest energy, however, the 100 faces were not well developed at low-level (x < ~ 0.25) Zr doping under the studied synthesis conditions. For x from 0.25 to 0.5, the synthesized SrTi1–xZrxO3 nanocrystals appeared to be monodisperse nanocubes with edge length of about 10 nm. Fig. 1c and 1e show the TEM and HAADF-STEM images of the nanocubes by taking SrTi0.75Zr0.25O3 as a representative example.

FIG. 1: Synthesis of perovskite oxide nanocubes by an oil-water two-phase solvothermal synthesis. (a) Structural model of ABO3 perovskite structure. (b) Schematic illustration of the strategy for the oil-water two-phase solvothermal synthesis. (c, d) Bright-field TEM images, (e, f) HAADF-STEM images of the nanocubes (SrTi0.5Zr0.5O3), at lower and higher magnification, respectively.

The surficial terminating atom planes were investigated by HAADF-STEM imaging. Fig. 2a shows a HAADF-STEM image of a nanocube taken in the [001] zone axis, which clearly reveals


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cation columns, and the single crystalline nature of the nanocube. The majority of the surfaces of the nanocubes are parallel to the 001 planes. Owing to its Rutherford scattering nature, the intensity of the HAADF images depends on the composition through Zζ of the scattering cross section, where Z is the atomic number and ζ is close to 2 depending on the actual value of the collection angle of the HAADF detector. As a result, in the image of SrTi0.75Zr0.25O3 nanocubes the intensity for the Ti/Zr-O atomic columns appear weaker than that for the Sr columns. The presence of residue surfactant molecules and the amorphous carbon support mainly contributed to the background. By fitting the intensity distribution of the two types of atomic columns with two-dimensional Gaussian functions, the integral peak intensity for each column was obtained. For convenience of identification, the Gaussian peaks from fitting were encoded in green and red colors for Sr and Ti/Zr-O columns, respectively, and overlaid over the HAADF image. As shown in Fig. 2b, the surface terminating cationic columns of the nanocubes appear to be the Sr columns in all the four 100 facets.

FIG. 2: Surface atomic structure of the nanocubes. (a) HAADF-STEM image of a SrTi0.75Zr0.25O3 nanocube in the [001] zone axis. The Sr columns appear brighter than the Ti/Zr-O columns. (b) HAADF-STEM image overlaid with color-scale two-dimensional Gaussian peaks from fitting the intensity distribution of each column. (c, d) Maps of integrated peak intensity of the Sr and Ti/Zr-O columns, respectively. Arrows in (d) indicate Zr-rich columns showing exceptional brightness. (e, f) Histogram of the intensities of the Sr and Ti/Zr-O columns, respectively.

It is noted that the intensities for the Sr atomic columns distribute statistically into two separate regions of values (Fig. 2c and 2e), a low intensity region for the columns on the surfaces and a high intensity region for the columns in the bulk. In contrast, those for the Ti/Zr-O columns are in a broad and continuous region (Fig. 2d and 2f). The presence of residue surfactant molecules and the amorphous carbon support as discussed above may unavoidably broaden the distribution of the column intensity, but this should affect both the Sr and the Ti/Zr-O columns simultaneously at a

similarly extent. Therefore, the broad and continuous distribution of the intensity values for the Ti/Zr-O columns can be understood as the result of the inhomogeneity in the number of the Zr atoms occupying the B-sites in individual columns.

FIG. 3: Layerwise growth process of the nanocubes. (a) Sketch of a layerwise growth process for a 1 0 0 facet of the perovskite ABO3 structure presuming a faster growth rate for the AO layer (green) than that for the BO2 layer (blue). The gray, yellow, and red symbols indicate the possible atom sites for the BO2 layer growth in the order of increasing preference. Oxygen was omitted in the model for clarity. (b) HAADF-STEM image of a SrTi0.75Zr0.25O3 nanocube with a growth step, averaged from 2 frames and denoised by a nonlinear filter.35 (c) HAADF-STEM image overlaid with color-scale two-dimensional Gaussian peaks from fitting the intensity distribution of each column. (d) Magnified image of the growth step. (e) Magnified image of the growth step overlaid with structural model.

The effects of Zr-substitution can be understood by a layerwise growth process. Along the <001> axis, the structure of SrTi1–xZrxO3 (ABO3) (Fig. 1a) can be described as a layered structure with alternating stacking sequence of a SrO (AO) layer and a Ti1–

xZrxO2 (BO2) layer. We presume that the growth rate of the AO layers is much faster than the BO2 layers, which appears to be valid for the studied system. In a layerwise growth process, a growing BO2 layer (B1) will be covered by an AO layer (A1) before its finish (Fig. 3a). Whereas growth of a new BO2 layer on the terminating AO layer (A1) is less favorable when a growing layer with steps (B1) is available. As a result, the layer with a faster growth rate (AO) will terminate the surface rather than the layer with a slower growth rate (BO2). The observed growth steps at the surfaces of nanocubes provide direct evidence supporting the layerwise growth process for the nanocube facets (Fig. 3b – 3e).

In conclusion, the success of determination of the surface atomic structure of faceted nanocrystals provides insights into the growth mechanisms of the nanocrystals [1].

[1] H. Du, C.-L. Jia, and J. Mayer, Chem. Mater. 28, 650 (2016)

[2] H. Du, S. Wohlrab, M. Weiß, and S. Kaskel, J. Mater. Chem. 17, 4605 (2007)


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An electric field triggered thermal runaway model for volatile resistive threshold switching in NbO2

C. Funck1,2, S. Menzel1, N. Aslam1, H. Zhang1, A. Hardtdegen1, R. Waser1,2, and S. Hoffmann-Eifert1 1 Peter Grünberg Institut-7, Forschungszentrum Jülich, Germany 2 Institut für Werkstoffe der Elektrotechnik 2, RWTH Aachen University, Germany Volatile threshold switching devices have attracted great attention for the use as selector devices in passive crossbar arrays. These devices show a hysteretic abrupt jump in the current-voltage characteristic and thus offer a very high selectivity. As this nonlinearity appears for either voltage polarity, threshold switches are an ideal selector for bipolar resistive switching redox-based resistive memories (ReRAM) applicable to functional logic-in-memory concepts and non-boolean computing frameworks. Here, an alternative physical model for the threshold switching is presented that overcomes the shortcomings of the previous explanation given by an insulator-to-metal transition. The alternative model, is based on an electric field induced thermal runaway effect, which correctly predicts the experimentally observed narrow opening of the hysteresis and the magnitude of the current jump in the threshold-type current-voltage characteristic of NbO2-based devices. The model is also applicable to a wider class of materials showing threshold switching, but do not show a temperature-induced insulator-to-metal transition.

The fast growing field of information technology demands for new types of nonvolatile memories that fulfill the requirements for low power consumption, fast switching kinetics, high endurance and long retention. One class of new memory comprises the redox-based resistive switching memories (ReRAM), which have the potential to replace the common FLASH memory technology. If ReRAMs are integrated into passive crossbar arrays, the highest possible integration density of 4F2 will be achieved, with the minimum feature size F. The integration density might even be increased by stacking multiple layers of passive matrices to a three dimensional (3D) structure. The size of an array, however, is restricted by the so-called sneak path problem. This serious issue can be alleviated by the integration of a selector device in addition to the passive ReRAM.[1] Among the different selectors threshold switching devices are highly interesting as they are favorable for low-voltage operation and often provide an inherent combination of the threshold-type selector property and the memristive non-volatile switching in a single ‘1Th1M’ cell. Threshold devices show an

abrupt resistance change at a certain voltage stress VTh,ON. At this voltage, the resistance changes from a high resistive state (OFFTh) to a low resistive state (ONTh). In contrast to the memristive operation, the threshold switching is volatile: The resistance state switches to OFFTh as soon as the voltage is reduced below the turn-off voltage VTh,OFF. Volatile threshold switching is observed in thin films of VO2, NbO2, and Ti2O3.[1] To date, the threshold switching behavior in thin film devices of the described transition metal oxides is mostly explained by the temperature driven insulator-to-metal transition (IMT). However, this explanation bears major conflicts.

Revisiting the threshold switching phenomenon in NbO2 thin film based devices we were able present an axisymmetric two-dimensional (2D) simulation model for simulating threshold switching (see Fig. 1), which successfully reproduce the experimental data obtained from threshold switching devices with and w/o memristive switching contribution.[2] The utilized nano-crossbar devices, obtained from 10 nm amorphous Nb2O5 sandwiched between Pt and Ti/Pt electrodes, revealed threshold-type switching behavior after an electroforming step that results in the formation of an oxygen deficient Nb2O5-x/NbO2 filament.

FIG. 1: Axisymmetric two-dimensional simulation geometry for the 1Th1M structure with the applied boundary conditions. The colour code illustrates the temperature distribution within the cell for the OFFTh state at the threshold voltage VTh,ON.


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FIG. 2: Illustration of the FTTR model. In the low electric field range A the net activation energy for the carriers is too high to be overcome due to the small thermal energy. At high electric fields in the range B, the net activation energy is lowered strongly by the Poole-Frenkel effect C. Due to the increased temperature the carriers are able to become excited. The exact combination of barrier lowering and temperature depends on the total serial resistance.

Our proposed model uses a conduction mechanism that is described by polaron hopping in NbO2 [2], with an additional Schottky or Poole-Frenkel like activation energy barrier lowering. Thus, the occurrence of the CC-NDR effect in our simulations is not restricted to a specific conduction mechanism. In fact, the important features are the combination of an electric field based increase in the conductivity combined with a Joule-heating induced thermal runaway of the electronic conductivity. Thereby, these two effects should be correctly outbalanced in order to obtain a CC-NDR and threshold switching. Hence, we suggest to call the model used in this study (see Fig. 2) more generally field triggered thermal runaway (FTTR) model.

The follow-up analysis of the multidimensional simulation data discloses additional detailed information important for the deeper understanding of the different stages during threshold switching.[3] An important simulation result addressing the design of future threshold switching selector devices is that width and magnitude of the threshold hysteresis are determined by the value of an intrinsic series resistance. As soon as the threshold element turns on the resistance of the threshold switching region decreases. Thus, the

emphasis of the voltage divider is shifting from the threshold element towards the series resistance. In addition, the widely known S-type I-V curve characterizing the CC-NDR effect is induced by the internal series resistance. The transition from the negative to the positive slope in the S-type switching curve originates from the change in the dominating conduction mechanism. For high voltages the current is limited by the intrinsic series resistance. Parametric simulations demonstrate that the threshold switching voltage increases linearly with the thickness of the threshold switching layer. Moreover, in our studied Nb2O5-

x/NbO2 system the threshold variation also affects the hysteresis as it comes along with the variation of the internal series resistance. In addition, the simulation results provide further insights into the spatial extent of the threshold switching. We show that the temperature and the conductance obtain maximum values in the center of the filament. However, the threshold switching occurs homogenously. The threshold region in the ONTh range is rather constricted due to the radially decreasing temperature in the conducting NbO2 filament towards the insulating surrounding Nb2O5 material. Consequently, it is not the whole radial threshold region that contributes to the current transport in the ONTh state. This aspect may become increasingly important with the further decrease of the minimum feature size F of the ReRAM and the selector devices.

In conclusion we presented a multi-dimensional model for the threshold switching in NbO2-based devices, which is based on a field triggered thermal runaway (FTTR) effect. The axisymmetric 2D simulations reproduce correctly the experimental observations, including the voltage controlled threshold switching, the S-type NDR effect, and the temperature dependence of the threshold switching. The simulations based on the FTTR effect overcome the contradictions that arise in explaining the threshold switching in NbO2 by means of the common IMT model, i.e. the too small hysteresis and the too strong temperature dependence. The FTTR model for this study is based on an intrinsic polaron hopping conduction mechanism, which includes a PF-like barrier lowering. In general, every conduction mechanism that contains a field-dependent barrier lowering component should be applicable to threshold switching behavior. The physical origin should base on the same self-acceleration process of carrier generation and Joule heating that lead to the FTTR behavior.

[1] G. Burr, R. Shenoy, K. Virwani, P. Narayanan, A. Padilla, B. Kurdi, and H. Hwang, J. Vac. Sci. Technol. B 32, 040802 (2014)

[2] C. Funck, S. Menzel, N. Aslam, H. Zhang, A. Hardtdegen, R. Waser, and S. Hoffmann-Eifert, Adv. Electron. Mater. 2, 1600169/1-13 (2016).

[3] C. Funck, S. Hoffmann-Eifert, R. Waser, and S. Menzel, Int. Conf. On Simulation of Semiconductor Processes and Devices (SISPAD), Nuremberg, Germany, Sept. 6-8, 319 (2016)


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On the interrelation of gradual and abrupt SET switching in valence change memory cells

K. Fleck1, C. La Torre1, N. Aslam2, S. Hoffmann-Eifert2, and S. Menzel2

1 Institut für Werkstoffe der Elektrotechnik 2, RWTH Aachen University, Germany 2 Peter Grünberg Institut-7, Forschungszentrum Jülich, Germany Understanding the switching dynamics of the resistive switching phenomenon in valence change memories (VCM) is crucial for the design of optimized memory cells. In this work the SET switching dynamics of Pt/SrTiO3/TiN nanocrossbar VCM cells are studied on a timescale from 10 ns to 104 s. It is found that the resistance change during the SET transition can be divided into two regimes: a linear degradation regime followed by an abrupt resistance decrease. By comparing the experimental data to the simulation results of a dynamical electro-thermal model, the physical origin of the resistance progression during the SET process is identified. The combination of the steady current increase and the concurrent Joule heating leads to a positive feedback. Thus, the initial slow resistance degradation becomes faster and faster, which finally evolves into the abrupt resistance decrease.

In recent years the resistance switching effect in transition metal oxides based on the valence change mechanism (VCM) attracted great attention due to its potential application in redox-based resistive switching memories (ReRAMs) [1]. A VCM cell typically consists of a transition metal oxide sandwiched between a high work function metal electrode, which is also called active electrode, and a low work function metal electrode (ohmic electrode). The valence change mechanism relies on the movement of positively charged oxygen vacancies and a concurrent local redox reaction in the cation sublattice. By applying a negative potential to the active electrode, oxygen vacancies move towards this electrode and accumulate there. The accumulation of oxygen vacancies leads to a decrease of the potential barrier at the active electrode and the cell switches to a low resistive state (LRS). By reversing the voltage polarity the cell is reset to the high resistive state (HRS).

The dynamics of the SET switching process are highly nonlinear, i.e. the switching time decreases several order of magnitude in time when the applied voltage is increased only slightly [2]. This nonlinearity origins from the temperature-acceleration of the oxygen vacancy drift due to local Joule heating [3]. Typically, the SET process is characterized by an abrupt resistance decrease, similar to a runaway process. But also a gradual transition from the HRS to the LRS can be

observed. Herein, we report an experimental study of the SET kinetics of resistive switching cells based on SrTiO3 covering 12 orders of magnitude between 10 ns and 104 s. An accurate analysis of the transient currents reveals that the SET process starts with a slow gradual resistance change that is followed by a much faster runaway. This is a development that is not only present in the SrTiO3 devices but also in other devices based on Ta2O5 or HfO2 [4].

Fig. 1 shows the pulse schemes applied to the devices under test, namely Pt/SrTiO3/TiN nanocrossbar devices with 100x100 nm² junction area, along with two recorded current transients during SET switching. The 10 µs pulse in Fig. 1c shows a rather abrupt jump of the current from one current plateau to the other (red line). In contrast, the 500 ms-long pulse shown in Fig. 1d exhibits first a gradual increase in the current, termed pre-SET slope, followed by a rapid transition to the LRS. The question is whether this two distinct transition regimes are connected to different physical processes or can be consistently explained by a single process.

FIG. 1: Measurement schemes for (a) the pulse measurements below 1 s pulse length and (b) the quasi-static measurements. Voltage and current transients showing two SET events on different timescales are shown in (c) and (d). Reproduced from [4].


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To understand the origin of the two-fold SET transition a physical compact model is developed. Fig. 2 shows the equivalent electrical circuit diagram of the model. In this model the switching filamentary region is divided into a plug region and disc region representing the drift region and the Schottky contact at the active Pt interface. The disc resistance Rdisc and the current through the Schottky diode depend on the internal state variable, which is given by the oxygen vacancy concentration in the disc region.

The change of the oxygen vacancies in the disc depends on the ionic current between plug and disc. This current Iion is described by the Mott-Gurney law for ion hopping

A Vo discion Vo Vo 0

B B

Δexp sinh

2

W az eEI Az ec a

k T k Tn

æ ö æ ö÷ ÷ç ç÷ ÷= -ç ç÷ ÷ç ç÷ ÷ç çè ø è ø,

where A is the filament cross-section area, zvo is the charge number of the oxygen vacancies, a the hopping distance, ΔWA the hopping barrier, Edisc the electric field over the disc region, and cVo the average concentration in disc and plug. The change of the disc concentration Ndisc is then given by [4]

discion

Vo disc

d 1

d

NI

t z eAl=- .

Here, ldisc is the length of the disc region, representing the depletion zone of the Schottky junction. The temperature T changes during switching due to Joule heating and is thus a function of the dissipated power. Further details of the model and the used simulation parameters are given in [4].

FIG. 2: Equivalent circuit diagram representing the electrical model of the TiN/SrTiO3/Pt device. As in the measurements, the voltage is applied to the bottom electrode. Reproduced from [4].

The simulation model nicely fits the experimental data, namely the dependence of the SET time, the pre-SET slope and the transition time on the

applied voltage [4]. Simulation model and experimental data coincide over several orders of magnitude in time. Besides the dependence of the above mentioned quantities on the voltage, the experimental data shows a clear correlation between pre-SET slope and the inverse of the SET time as illustrated in Fig. 3. The simulation results are again consistent with the experimental data.

FIG. 3: The pre-SET-slope (absolute values) plotted against 1/tSET shows the direct correlation of both parameters. The solid red line represents the simulation and the dashed black line a hyperbolic fit of the measurement. Reproduced from [4].

By analyzing the simulation data the appearance of the gradual resistance degradation can be explained. The gradual increase in the current is induced by an increase in the disc concentration. The current increase leads to an increase in the dissipated power and the filament temperature increases. As the ionic current is highly temperature dependent a positive feedback results. The resulting abrupt resistance jump is thus a consequence of a thermal runaway process. For comparison, one simulation neglecting Joule heating was performed. The simulated SET transition in this case does not show any runaway, but rather a gradual resistance degradation over time [4].

In conclusion, the gradual and the abrupt SET transition in VCM cells could be unified into one picture of resistive switching. Both have the same nature of a steady increase of the oxygen vacancy concentration close to the active interface. The resistance change begins slowly and gets steadily faster due to a thermal runaway process [4].

[1] J. J. Yang, D. B. Strukov et al., Nat. Nanotechnol. 8, 13 (2013)

[2] S. Menzel, M. Salinga et al., Adv. Funct. Mater. 25, 6306 (2015)

[3] S. Menzel, M. Waters et al., Adv. Funct. Mater. 21, 4487 (2011)

[4] K. Fleck, C. La Torre et al., Phys. Rev. Applied 6, 064015 (2016)


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Dynamic modelling of the RESET process in valence change memory cells

A. Marchewka1, B. Rösgen2, K. Skaja2, V. Rana2, R. Waser1,2, and S. Menzel2 1 Institut für Werkstoffe der Elektrotechnik 2, RWTH Aachen University, Germany 2 Peter Grünberg Institut-7, Forschungszentrum Jülich, Germany The working principle of valence change memories relies on the migration of ions, typically oxygen vacancies, in transition metal oxides and a concurrent resistance change. While the set transition from a high to a low resistance state progresses very abruptly, the reset transition is rather gradual. In this work a dynamic numerical model is presented that is applied to simulate the gradual reset switching in Ta/Ta2O5/Pt nanocrossbar devices. By analyzing the simulation results the origin of the gradual reset transition can be attributed to the temperature-accelerated oxygen-vacancy motion being governed by drift and diffusion processes that approach an equilibrium situation.

Resistive switching devices based on the valence change mechanism (VCM) are considered as potential candidates for their use in future non-volatile computer memory [1]. VCM devices consist of a simple metal-insulator-metal structure. Typically one of the electrodes, the active electrode (AE), consists of a high work function metal, and the other out of a low work function metal forming and ohmic contact with an insulating transition metal oxide, e.g. Ta2O5, HfO2 or SrTiO3. The VCM switching mechanism relies on the migration of positively charged oxygen vacancies in the transition metal oxide, associated redox-reactions and changes in the electronic barriers at the contacts [1].

Typically, the set operation is characterized by an abrupt increase in current, whereas the reset operation takes place in a gradual manner. The latter can be used to achieve multiple intermediate resistance states by changing either the reset “stop” voltage in sweep measurements [2] or the reset voltage amplitude in pulse experiments [3]. To date, this key phenomenon known as gradual reset has been reported in various experimental studies, but a comprehensive physical understanding is missing.

Here, we present a combined experimental and numerical modeling approach to develop a fundamental understanding of this process. Time-resolved pulse measurements are performed to study the reset kinetics in TaOx-based nano-crossbar structures. The results are analyzed using a two-dimensional dynamic model of non-isothermal drift-diffusion transport in the mixed

electronic-ionic conducting oxide including the effect of contact potential barriers.

Fig. 1 shows the transient current response of the fabricated Ta/Ta2O5/Pt nanocrossbar devices on 1 µs-long reset pulses with varying amplitude. All transients show a current decay that slows down over time. By increasing the voltage amplitude the decay time decreases exponentially as illustrated in Fig. 1b.

FIG. 1: (a) Measured transient reset currents for different reset voltage amplitudes Vpulse ranging from −1.3 V to −1.8 V. (b) Decay time τ50 for different reset voltage amplitudes Vpulse, extracted from the transient currents. Reproduced from [4].

To analyze the experimental results, we developed a two-dimensional axisymmetric physical simulation model. The model accounts for drift, diffusion and thermodiffusion of electronic and ionic charge carriers. It further includes the effect of contact potentials and related barrier formation as well as Joule heating effects. Fig. 2 shows the model geometry. The drift-diffusion equation for the ionic an electronic charge carries and related rate equations are solved within the Ta2O5 layer only. Initially, a filamentary region with a high


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oxygen vacancy concentration is assumed as illustrated in Fig. 2a. To solve for the local temperature -the simulation domain comprising the entire layer stack as shown in Fig. 2b is used. Further details of the model are given in [4].

FIG. 2: Model geometry. (a) Computational domain of the Ta2O5 layer with initial and boundary conditions used in the drift-diffusion simulation. (b) Computational domain comprising the layer stack of 75 nm SiO2, 25 nm Pt, 5 nm Ta2O5, 5 nm Ta, and 25 nm Pt used for the temperature calculation, along with the boundary conditions for the heat equation. A typical temperature distribution is shown as an example. Reproduced from [4].

Fig. 3 shows that the simulation model is able to reproduce the current transients and the decay times very well. Our analyses show that the filament dissolution during the reset operation can be characterized by a lateral change due to axial movement of the oxygen vacancies and a subsequent formation of a depleted gap in front of the high work function metal electrode interface. The potential barrier at this Schottky-like contact is modified, which results in a concomitant decrease in current. In addition, we are able to explain the gradual reset transition. A crucial prerequisite for the gradual behavior of the reset current is a moderate sensitivity of the current to the underlying reset mechanism. Further, the oxygen-vacancy concentration gradient building up during the reset process counteracts the driving force of the applied electric field that forces the oxygen vacancies move towards the ohmic-like counter electrode, decelerating the oxygen-vacancy migration and provoking a smooth fading of the reset current.

FIG. 3: Comparison between simulation and measurement of (a) transient currents for pulse voltages of −1.3 V, −1.4 V, −1.5 V and −1.6 V, (b) 50% decay times as a function of pulse voltage. Reproduced from [4].

Our study reveals that the gradual nature of the reset can be attributed to oxygen-vacancy drift and diffusion processes approaching equilibrium [4].

[1] R. Waser, R. Dittmann et al., Adv. Mater. 21, 2632 (2009)

[2] F. Nardi, S. Larentis et al., IEEE Trans. Electron Devices 59, 2461 (2012)

[3] J. H. Hur, K. M. Kim et al., Nanotechnology 23, 225702/1 (2012)

[4] A. Marchewka, B. Roesgen et al., Adv. Electron. Mater. 2, 1500233/1 (2016)


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Dynamics of the metal-insulator transition in donor-doped SrTiO3

A. F. Zurhelle1, R. Meyer1, R. A. De Souza2, M. Andrä3, S. Menzel3, D. N. Mueller4, R. Waser1,3, and F. Gunkel1,3 1 Institute of Electronic Materials, RWTH Aachen University, Germany 2 Institute of Physical Chemistry, RWTH Aachen University, Germany 3 Peter Grünberg Institut-7, Forschungszentrum Jülich, Germany 4 Peter Grünberg Institut-6, Forschungszentrum Jülich, Germany As a model material, donor-doped strontium titanate (n-STO) is applied in gas sensing and resistive switching applications, and serves as substrate for functional films. In the literature, n-STO is mostly referred to as a degenerate n-type semiconductor. However, ionic defect formation can drive a chemically controlled metal-insulator transition (MIT). Here, we employ dynamical numerical simulations to examine the high-temperature MIT in n-STO over a large range of time and length scales. Our continuum model reveals that n-STO, upon oxidation, develops a kinetic space charge region (SCR) in the near-surface region. The formation of the SCR in which electrons are strongly depleted occurs within nanoseconds, i.e., it yields a fast MIT in the near-surface region during the oxidation process. As a result of charge (over-)compensation by Sr vacancies incorporated at the surface of n-STO, this SCR is much more pronounced than conventionally expected. In addition, we find an anomalous increase of O vacancy concentration at the surface upon oxidation caused by the SCR.

As we elaborate in this project, the electrical properties of donor-doped SrTiO3 (n-STO) are profoundly affected by an oxidation-induced metal-insulator transition (MIT). This MIT occurs upon oxidation driven by a large change in the oxygen partial pressure at elevated temperatures: In the initial state (Fig.1a), the positive charges of the Nb donors (NbSr

∙ ) are balanced by conduction electrons (e′), such as in a classical semiconductor. Upon oxidation, however, oxygen from the gas phase gets reduced by the free electrons in the near-surface region (Fig. 1a,b). Together with Sr ions from the topmost surface, nuclei of a SrO secondary phase are formed (leaving Sr vacancies behind). This surface reaction consumes free electrons from the bulk, yielding a transition from n-type to insulating phase.

The details of this oxidation process have been discussed in the present study [1]. We reveal an unexpected and significant “overoxidation” of the surface, the formation of a correspondingly enhanced space charge region, and, in the long term, the diffusion of Sr vacancies into the bulk under time- and position-variable ambipolar

acceleration (Fig.1b,c).At the end of the process, the positive charge of ionized Nb dopants is balanced ionically by Sr vacancies, effectively acting as intrinsic codopants (ionic charge balance). In that case, n-STO turns into a fully insulating state.

The motion of all defects during oxidation is calculated with a continuous medium model, based on the Nernst–Planck equations, the continuity equations, and the Poisson equation, with surface lattice disorder equilibria serving as time-dependent boundary conditions. The model is solved using an implicit finite element approach. In contrast to earlier models, we consider a global charge neutrality condition, in order to accommodate the interactions of ionic defects with the intrinsic internal electric field formed during the oxidation process. This approach allows for the formation of kinetically controlled space charge layers at the surface evolving during the equilibration process due to the different diffusion coefficients of the species involved. In return, this leads to time variant surface defect concentrations (cf. Fig.2).

We consider a change in oxygen partial pressure from reducing condition (electronically charge-balanced) to oxidizing (cation vacancy charge-balanced) conditions triggering the oxidation process at high temperatures. The evolution of all involved electronic and ionic defect concentrations are illustrated in Fig. 2. The kinetic model discloses

FIG. 1: Simplified sketch of the oxidation-induced metal-insulator transition of donor-doped SrTiO3.


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a threefold re-equilibration process of (1) the electronic band structure, (2) the oxygen sublattice, and (3) the cation sublattice taking place on very different time scales. The main results of our study can be summarized as follows.

(i) On the short-term scale (up to 100 ps), the establishment of the electronic equilibria yields a rapid formation of an electron depletion SCR and a corresponding potential barrier at the surface. The SCR formation is driven by the fast formation of Sr vacancies only in the surface layer, while there is no diffusion of Sr vacancies into the bulk.

(ii) Because of the active surface equilibrium reaction during the SCR formation, the Sr vacancy surface density exceeds the new equilibrium by more than one order of magnitude. We call this effect an “overoxidation.”

(iii) As a result of overoxidation, the SCR is much more pronounced than in the classical model. This is associated with a correspondingly enhanced drop in the conductivity and thus a particularly pronounced MIT in the near-surface region.

(iv) On the midterm scale (up to a few milliseconds), the equilibration of the oxygen sublattice takes place. The electron depletion profile as well as the electrical properties of the

material do not change during this process, since the O vacancies are minority carriers.

(v) Driven by the strong internal electric field in the SCR, O vacancies are enriched in the near-surface region on the midterm scale. Oxygen vacancies are forced to move against their concentration gradient, so that the profile reveals an anomalous “uphill diffusion” shape, in agreement with 18O diffusion data reported in the literature.

(vi) On the long time scale (up to days), the equilibration process is dominated by the defect kinetics of the Sr sublattice. The initial surface excess of Sr vacancies fades while the width of the SCR increases along with the diffusion front of the Sr vacancy profile. The amplitude of the SCR decreases until it completely vanishes in the end. The electron density profile, as well as the O vacancy profile, can be regarded as being in quasi-equilibrium states, dictated by the slowly evolving Sr vacancy profile.

(vii) Initially, the strong electric field in the SCR accelerates the (very slow) Sr vacancy diffusion by a factor of about 100. This field acceleration vanishes as the SCR fades in the course of time.

The formation of the pronounced electron depletion SCR at the surface of n-STO and the local equilibration of the cation sublattice imply severe consequences for the electrical properties of n-STO thin films and the interfacial band structure in epitaxial thin-film devices based on n-STO substrates. In particular, the effects of surface carrier depletion and concentration profiles, as well as interface bend banding, may be seen from a different perspective, now considering the near-surface and near-interface lattice disorder of n-STO. Based on this model, the understanding of the resistive switching behavior of n-STO, as well as the strategies to tailor electronic properties of n-STO thin films and interfaces [2], may be refined. The model may also contribute to a link between the macroscopic understanding of the dynamics of the lattice disorder and surface structures observed on the atomistic level after dedicated surface treatments. Moreover, the processes described by the model will help to refine the understanding of the formation of grain boundary potential barriers in donor-doped ceramics, such as PTCRs , resistive sensors, and internal boundary layer capacitors.

The presented model is not limited to the investigated material but might describe a general mechanism, which can be found in a variety of different ionic materials with similar lattice disorder, as being elaborated in ongoing projects at PGI-6/7 and Institute of Electronic Materials (IWE2), RWTH Aachen.

[1] R. Meyer, A. F. Zurhelle, R. A. De Souza, R. Waser, and F. Gunkel, Phys. Rev. B 94, 115408 (2016)

[2] F. Gunkel, R. Waser, A. H. H. Ramadan, R. A. De Souza, S. Hoffmann-Eifert, and R. Dittmann, Phys. Rev. B 93, 245431 (2016).

FIG. 2: Calculated concentrations at the surface over three time regimes. The concentrations of Sr vacancies (green), electrons (red), and O vacancies (blue) are shown for the surface (dark colors) and for the bulk (light colors).


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Investigating the resistive switching properties of ultrathin TaOx films with Scanning Tunneling Microscopy

M. Moors1, K.K. Adepalli2,3, Q. Lu3, A. Wedig1, C. Bäumer1, K. Skaja1, B. Arndt1, H.L. Tuller3, R. Dittmann1, R. Waser1,4, B. Yildiz2,3, and I. Valov1,4 1 Peter Grünberg Institut-7, Forschungszentrum Jülich, Germany 2 Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, USA 3 Department of Materials Science and Engineering, Massachusetts Institute of Technology, USA 4 Institut für Werkstoffe der Elektrotechnik 2, RWTH Aachen University, Germany The redox state and, thus, the electrical resistance of ultrathin tantalum oxide (TaOx) films were manipulated by the electric field of a Scanning Tunneling Microscope (STM) tip. Therefore, 2-3 nm thick amorphous TaOx films were sputter deposited on a Ta/SiO2/Si substrate. The primarily insulating films got sufficiently conducting for STM studies by annealing them under Ultra High Vacuum (UHV) conditions. The resistive switching (RS) properties and their physical origin strongly depend on the oxygen content and the pretreatment of the TaOx films. In freshly reduced samples the movement of Ta cations dominates the switching process leading to an electrochemical metallization (ECM) switching mechanism, whereas post-oxidation in air results in an oxygen vacancy driven valence change mechanism (VCM).

Most surface related studies considering RS in transition metal oxides prefer local conductance atomic force microscopy (LC-AFM) as investigation method due to its independence of the sample conductance. However, using an electric field induced by applying an increased potential between the sample and a STM tip for switching between different redox states has several advantages like the lack of any physical contact and the potentially higher resolution [1]. Furthermore, also information about the electronic structure of devices can be obtained by tunneling spectroscopy, which may deliver important information for the evaluation of the underlying switching mechanism [2]. In recent studies we could show that this technique can also be applied on materials considered as macroscopic insulators like TaOx, HfOx and TiOx [3,4].

Freshly sputter deposited TaOx films exhibit a rather low surface conductance resulting in significant charging effects during STM experiments. Annealing the samples to 500 °C in UHV for 2-3 h results in a surface reduction and leads to sufficient conductivity for stable STM measurements.

Fig 1. shows the STM topography image of an UHV annealed TaOx film (a) before and (b) after scanning a small area in the image center with an

increased negative tip voltage of -5 V. Applying such a highly negative potential to the tip results in an local increase of the electronic states density and, thus, in a higher surface conductivity (→ LRS). The observation of a metal-like I-V curve taken on the freshly switched surface region in combination with the switching polarity indicates an ECM-type switching mechanism via Ta cation migration and reduction at the surface.

FIG. 1: (a-c) STM images (1000 nm x 1000 nm, taken with VTip = -3.0 V and IT = 0.5 nA) of the UHV-annealed TaOx film (a) before and (b) after scanning the marked area at the center with VTip = -5.0 V; (c) after additionally scanning the marked area in (b) with VTip = +5.0 V; (d) I-V curves on bright regions corresponding to LRS (ON state) and on unmodified regions corresponding to HRS (OFF state) on TaOx thin films after annealing in UHV. The arrows in (a), (b) and (c) show the features on the surface that are the same, and thus, used as fiducial markers in the consecutive images.

The switching behavior changes completely after exposure of the vacuum annealed samples to the atmosphere under ambient conditions (see Fig. 2). In that case switching to LRS is realized at positive tip voltages, while the RESET process to HRS


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occurs at a negative tip potential. This switching polarity is in accordance to the well-established oxygen vacancy driven mechanism known for classical VCM cells. Hereby, oxygen anions are attracted by the positive tip and then reduced at the surface. This results in a local increase of oxygen vacancies and a higher surface conductivity of the reduced n-conducting oxide. This mechanism is also supported by the I-V curves taken on the LRS area as shown in Fig. 2d. In contrast to the vacuum annealed samples these curves still have a semiconducting behavior with a smaller bandgap than the ones taken on HRS areas. This indicates a surface reduction under the influence of a positive tip polarity without the formation of a metallic species on the surface.

FIG. 2: (a-c) STM images (1000 nm x 1000 nm, taken with VTip = -3.0 V and IT = 0.5 nA) of the atmosphere exposed TaOx film (a) before and (b) after scanning the marked area at the center with VTip = +5.0 V; (c) after additionally scanning the marked area in (b) with VTip = -5.0 V; (d) I-V curves on bright regions corresponding to LRS (ON state) and on unmodified regions corresponding to HRS (OFF state) on TaOx thin films after exposure of the preannealed sample to the atmosphere.

The reason for that change of the resistive switching characteristics can be associated with the Ta oxidation state. X-Ray Photoelectron Spectroscopy (XPS) measurements shown in Fig. 3 indicate a minimal Ta(V) content in the vacuum annealed samples, while at the same time the peaks of lower oxidized Ta species increase significantly in intensity. Subsequent air exposure again leads to an increased Ta(V) content and a decrease of Ta(IV) and Ta(II) species, which is comparable to the situation in the freshly deposited TaOx films. Thus, it can be concluded that the air exposure of the vacuum annealed samples leads to an instant reoxidation of the surface. We assume that the Ta cation mobility is increased in the stronger reduced oxide films and dominates under these conditions the switching process,

while in the more oxidized films the movement of oxygen anions is the prevailing factor.

FIG. 3: Quantitative results on the concentration of (a) Ta(V) species and (b) Ta(IV), Ta(II) and Ta(0) species under different conditions from peak fittings of Ta 4f XPS spectra.

In conclusion, it could be shown by STM and XPS measurements that the resistive switching properties of thin TaOx films strongly depend on their non-stoichiometry caused by the sample history. With decreasing oxidation state of Ta in the oxide layer the switching mechanism changes from a VCM-like to an ECM-like behavior.

[1] Y. Yang, Y. Takahashi, A. Tsurumaki-Fukuchi, M. Arita, M. Moors, M. Buckwell, A. Mehonic, A. J. Kenyon, J. Electroceram., doi:10.1007/ s10832-017-0069-y (2017)

[2] Y. L. Chen, J. Wang, C. M. Xiong, R. F. Dou, J. Y. Yang, and J. C. Nie, J. Appl. Phys. 112, 023703. (2012)

[3] A. Wedig, M. Luebben, D. -Y. Cho, M. Moors, K. Skaja, V. Rana, T. Hasegawa, K. K. Adepalli, B. Yildiz, R. Waser, and I. Valov, Nat. Nanotechnol. 11, 67 (2016)

[4] M. Moors, K. K. Adepalli, Q. Lu, A. Wedig, C. Bäumer, K. Skaja, B. Arndt, H. L. Tuller, R. Dittmann, R. Waser, B. Yildiz, and I. Valov, ACS Nano 10, 1481 (2016)


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Quantifying redox-induced Schottky barrier variations in memristive devices via operando spectromicroscopy with graphene electrodes

C. Bäumer1, C. Schmitz1, A. Marchewka2, D. N. Mueller1, R. Valenta1, J. Hackl1, N. Raab1, S. P. Rogers3, M. I. Khan1, S. Nemsak1, M. Shim3, S. Menzel1, C. M. Schneider1, R. Waser1,4, and R. Dittmann1 1 Peter Grünberg Institut-7, Forschungszentrum Jülich, Germany 2 Institut für Werkstoffe der Elektrotechnik 2, RWTH Aachen University, Germany 3 Department of Materials Science and Engineering and Materials Research Laboratory, University of

Illinois, Urbana, USA The continuing revolutionary success of mobile computing and smart devices calls for the development of novel cost- and energy-efficient memories. For this purpose, memristive devices are attractive because of high switching speed and device density. Upon electrical stimulus, complex nanoscale redox processes are suspected to induce a resistance change in these devices. Quantitative information about these processes, which has been experimentally inaccessible so far, is essential for further advances. Here we use operando spectromicroscopy to verify that redox reactions drive the resistance change. We find that small changes in donor concentration at electrode-oxide interfaces cause a modulation of the Schottky barrier and lead to orders of magnitude change in device resistance.

Resistive switching in transition metal oxides can be attributed to the motion of mobile donor-type defects, such as oxygen vacancies, and the corresponding valence change in the transition metal cation [1,2]. The quantitative details of the related nanoscale redox-processes, which are believed to take place in so-called switching filaments, however, are still elusive due to the lack of analytical methods that provide quantitative information about the electronic structure with sufficient spatial resolution and sensitivity to detect small variations. Therefore, experimental advances in the operando characterization of nanoscale electronic structure changes during switching are critical prerequisites. For this purpose, the development of operando photoelectron emission microscopy (PEEM) studies, which allow for spectroscopic evaluation with high spatial resolution and interface- or surface sensitivity [2], is highly desirable. The main challenge in the implementation of operando PEEM characterization is to overcome the high surface sensitivity of photoemission, which limits the probing depth to few nanometers and practically prevents access to the active region covered by

top electrodes. Therefore, we used uncovered single layer graphene as the top electrode for SrTiO3-based memristive devices, allowing simultaneous electrical biasing and imaging inside PEEM instruments. We find that graphene scarcely dampens the photoelectron intensity of buried layers, making it the ideal electrode material to study buried layers using surface sensitive spectromicroscopy. More importantly, we were able to achieve comprehensive understanding of the microscopic processes during the resistance change [3]. Fig. 1a shows the experimental setup.

FIG.1: a) Device and measurement setup schematic. An epitaxial SrTiO3 layer (blue) is sandwiched between a Nb:SrTiO3 bottom electrode (violet) and a graphene top electrode (grey honeycomb lattice). The graphene electrode is contacted through a metal lead, which is electrically separated from the continuous bottom electrode, allowing for biasing inside PEEM instruments. b) Forming step (blue line) and following reset-set (green line) operation for a device with a graphene top electrode.

A SrTiO3 memristive device with a graphene top electrode was contacted with a metal lead, allowing for biasing inside the PEEM instrument. A


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typical so-called forming step (first electrical stimulus) and switching cycle between a low resistance state (LRS) and a high resistance state (HRS) are shown in Fig. 1b. Using soft X-ray illumination at the UE56/1-SGM beamline at BESSY II synchrotron (Berlin, Germany), spatially resolved X-ray absorption measurements were performed after careful calibration of the available X-ray absorption edges with reference samples. We investigated the same device during a switching cycle LRSHRSLRSHRS by acquiring O K-edge image stacks after each switching event. Upon close examination of the entire device area, we found a region of interest (ROI) exhibiting reduced intensity in the LRS at the first peak of the O K-edge, which we found to be indicative for the number of oxygen vacancies (red spot in Fig. 2a). The same ROI shows much weaker contrast at peak A in the HRS (Fig. 2b,c), indicating that this region is a switching filament. The corresponding change in oxygen vacancy concentration, in turn, exhibits a direct correlation with the device resistance (Fig. 2d,e). We therefore conclude that we found the fingerprint of the active switching filament that shows a large number of oxygen vacancies in the LRS, and a lower concentration in the HRS. These oxygen vacancies donate electrons into the conduction band of SrTiO3. Comparison of Fig. 2 to the reference measurements yields approximate values of 1.5×1021 electrons/cm3 (LRS) and 6.7×1020 electrons/cm3 (HRS). Although experimental uncertainties limit the accuracy of assigning absolute values, these results provide valuable quantitative information on the charge-carrier density differences between different resistance states. They finally yield direct evidence that resistive switching in transition metal oxides is driven by a nanoscale redox reaction: an oxygen-vacancy-driven valence change in the Ti leads to a charge carrier density modification by a factor of 2-3.

FIG. 2: a, b) PEEM images of the switching filament in the LRS, and HRS, respectively (indicated by the black arrow) for a photon energy of 531.6 eV. Scale bars are 1 µm. c) O K-edge for the switching filament in panels a and b (green and blue lines for the LRS and HRS, respectively) and the surrounding device area (black lines). d) Device resistance and e) oxygen vacancy concentration (VO) of the filament as a function of the device state.

Using the extracted filament dimensions and carrier densities as input into an existing device simulation model [4], we calculated the I–V characteristic of the device under investigation in both resistance states. For this purpose, we

compared simulated I-V curves with experimental read-out sweeps at low biases. The simulations include drift-diffusion transport of the electronic carriers in the SrTiO3 thin-film and the Nb:SrTiO3 bottom electrode and a Schottky barrier at the top electrode interface. The low-voltage I–V characteristics of the LRS and HRS are calculated accordingly using static distributions of doubly ionizable donor-type oxygen vacancies in the SrTiO3 thin-film. The donor concentrations of both states are derived from the electron densities determined in the PEEM experiments, i.e. the experimentally quantified values serve as an input parameter for the simulations and are not allowed to vary. All other parameters are identical in the simulations of the LRS and the HRS.

FIG. 3: a) Experimental read-out sweeps (green and blue data points for the LRS and HRS, respectively) of the device in Fig. 2 with simulated I-V characteristics (green and blue lines). b) Profiles of the energy of the conduction band edge WC(x) as a function of depth at zero bias for the LRS and the HRS.

Our results show that spatially confined changes in the donor concentration by a factor of 2-3 at the electrode-oxide interface lead to changes of more than two orders of magnitude in the device resistance (Fig. 3a), induced by the modulation of the effective Schottky barrier height and width (Fig. 3b). Considering the experimental uncertainties and simplicity of the model, the simulation yields a remarkable agreement with the experimental data. We conclude that the resistance change in SrTiO3-based devices is indeed caused by a spatially confined redox reaction. This reaction, in turn, leads to a measurable and quantifiable valence change between the HRS and the LRS, confirming the so-called valence change mechanism for resistive switching in transition metal oxides. Through the change of the effective Schottky barrier height and width at the electrode-oxide interface, small donor concentration changes lead to orders of magnitude change in resistance. The direct correlation between the experimental quantitative description of a switching filament by photoemission electron microscopy and nanoionic device simulations provides a significant step towards the design of memristive devices and circuits for applications in future electronics.

[1] R. Waser, M. Aono, Nat. Mater. 6, 833 (2007)

[2] C. Bäumer et al., Nat. Commun. 6, 9610 (2015)

[3] C. Bäumer et al., Nat. Commun. 7, 12398 (2016)

[4] A. Marchewka, R. Waser, and S. Menzel, Int. Conf. on Simulation of Semiconductor Processes and Devices (SISPAD), 9-11 September, Washington D.C, USA, 297 (2015)


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Hafnium carbide formation in oxygen deficient hafnium oxide thin films

C. Rodenbücher1, E. Hildebrandt2, K. Szot1,3, S. U. Sharath2, J. Kurian2, P. Komissinskiy2, U. Breuer4, R. Waser1,5, and L. Alff2 1 Peter Grünberg Institut-7, Forschungszentrum Jülich, Germany 2 Institute of Materials Science, Technische Universität Darmstadt, Germany 3 A. Chełkowski Institute of Physics, University of Silesia Katowice, Poland 4 Central Institute for Engineering, Electronics and Analytics ZEA-3, Forschungszentrum Jülich, Germany 5 Institut für Werkstoffe der Elektrotechnik 2, RWTH Aachen University, Germany

On highly oxygen deficient thin films of hafnium oxide (hafnia, HfO2-x) contaminated with adsorbates of carbon oxides the formation of hafnium carbide (HfCx) at the surface during vacuum annealing at temperatures as low as 600 °C is reported. Using X-ray photoelectron spectroscopy the evolution of the HfCx surface layer related to a transformation from insulating into metallic state is monitored in situ. In contrast, for fully stoichiometric HfO2 thin films prepared and measured under identical conditions, the formation of HfCx was not detectable suggesting that the enhanced adsorption of carbon oxides on oxygen deficient films provides a carbon source for the carbide formation. This shows that a high concentration of oxygen vacancies in carbon contaminated hafnia lowers considerably the formation energy of hafnium carbide. Thus the presence of a sufficient amount of residual carbon in resistive random access memory devices might lead to a similar carbide formation within the conducting filaments due to Joule heating.

HfO2 (hafnia) has been studied extensively in the last decades as a high-k dielectric being used as replacement of the standard gate dielectric SiO2 in order to produce high-density logic and memory devices. Furthermore, it was found that HfO2 can undergo a local insulator-metal (IM) transition under the influence of an electric field opening up the possibility to use it as resistive switching material in redox-based random access memories (ReRAM). During resistive switching in HfO2, a local redox reaction associated with a formation or modification of conducting filaments takes place related to the movement of oxygen vacancies. Furthermore a global insulator-metal transition on a macroscopic scale has been found in oxygen deficient hafnium oxide thin films grown by molecular beam epitaxy (MBE). The oxygen deficiency in these films is created during growth under oxygen deficient conditions leading to a homogeneous distribution of oxygen vacancies. With increasing amount of oxygen vacancies, the band gap is consistently reduced and defect states inside the gap eventually form a defect band at the Fermi level [1]. ReRAM devices based on these films show forming-free resistive switching and the

forming voltage is independent on the thickness of the highly oxygen deficient HfO2-x layer.

Here we discuss the reaction of carbon impurities to hafnium carbide and its influence on hafnium oxide-based resistive switching. The role of carbon impurities is of extremely high importance, as they are inherent to most of today’s deposition techniques commonly used for fabricating complementary metal oxide semiconductor (CMOS) devices, such as atomic layer deposition (ALD) based on organic precursors. Based on an in-operando hard X-ray photoelectron spectroscopy study, it was concluded that carbon impurities are detrimental to the cell endurance. In contrast to that, it was suggested that intentional doping with carbon could also be exploited in order to enhance the performance of resistive switches. We study the effect of a controlled thermal treatment under ultra-high vacuum (UHV) conditions on the chemical reactions of residual carbon, both, for stoichiometric and oxygen deficient films of hafnium oxide (Fig. 1). Studying the effect of post annealing on carbon impurities is important, as during CMOS processing several heat treatments of the whole device are performed, and the sample is locally heated during multiple resistive switching.

FIG. 1: Illustration of the formation of HfCx on oxygen-deficient hafnia thin films upon UHV annealing.

HfO2-x thin films were grown by MBE on sapphire substrates under various oxygen flow rates. In this way, a series of oxygen deficient hafnium oxide thin films with various oxygen vacancy concentrations was synthesized. All samples were investigated by XPS before, during, and after in situ annealing up to 700 °C in UHV to study the electronic structure as a function of annealing time and temperature. Additionally, LC-AFM has been applied to obtain information on film morphology and local electrical conductivity.

XPS spectra measured on the oxygen deficient thin films obtained at medium temperatures only


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FIG. 2: Influence of the UHV reduction process on a highly oxygen deficient HfO2-x thin film (grown under 0.3 sccm oxygen flow rate on c-cut sapphire). (a) Evolution of elemental ratios Hf/O and C/Hf during the stepwise annealing. (b) Valence band spectrum, O1s core spectrum, C1s core spectrum, and Hf4f spectrum after annealing at 700 °C for 24 h (maximal reduction). (b) LC-AFM current maps before and after UHV annealing.

show the expected peaks of HfO2-x contaminated with a small amount of carbon on the surface due to ex situ transfer. However, after annealing significant changes occur as can be seen at the Hf/O and C/Hf ratios calculated from XPS (Fig. 2a). At first, the Hf/O ratio does not change significantly and the C/Hf ratio increase due to the thermal desorption of organic adsorbates. At 700 °C however, the Hf/O as well as the C/Hf ratio increases significantly indicating the formation of hafnium carbide. This is supported by the core level spectra obtained after annealing for 24 h at 700 °C. In the C1s an additional peak labelled b can be seen and also in the Hf4f spectrum two new doublets appear labeled B and C (Fig. 2b). The valence band spectra show conducting states at the Fermi level indicating the formation of a conducting surface layer. All these features support our interpretation that HfCx is formed upon annealing of oxygen deficient thin films. When the same annealing experiment is repeated for fully oxidized HfO2 samples (1.5 sccm oxygen flow during growth), a formation of HfCx does not occur. Also when deficient HfO2-x films are annealed in situ directly after deposition, no HfCx is observed suggesting that only the exposure to air with subsequent annealing provides sufficient carbon species on the surface for a large spectral weight of HfCx. Once HfCx has been formed the electronic transport properties of the surface have been changed as identified by LC-AFM (Fig. 2c). The as received and ex situ transferred oxygen deficient HfO2-x film reveals an insulating surface due to reoxidation while the annealed films have a highly conducting surface due to the metallic behavior of HfC.

In summary, we have shown that hafnium carbide forms in strongly oxygen deficient hafnium oxide thin films at temperatures of several 100 degrees,

well below its thermodynamic equilibrium formation temperature. The evolution of hafnium carbide increases the surface conductivity significantly. A sufficient source of carbon impurities for carbide formation under the described conditions at the sample surface are adsorbed carbon species during ex situ processes. Given the large amount of oxygen vacancies inside conducting filaments and the high temperatures due to Joule heating in the electroforming and resistive switching process, the role of carbon impurities in general is important for ReRAM devices. In particular, for HfO2-x (or similar materials) based devices the formation of HfC has to be taken into account when a sufficient amount of carbon is available. Details of the work are given in [2].

We thank A. Besmehn for XPS measurements and A. Dahmen for carbon implantation. We gratefully acknowledge G. Cherkashinin for fruitful discussions as well as D. Leisten and T. Pössinger for technical assistance. This work was supported by the DFG (SFB 917 and AL560/13-2), the BMBF (contract 16ES0250) and ENIAC JU (PANACHE).

[1] E. Hildebrandt, J. Kurian, and L. Alff, J. Appl. Phys. 112, 114112 (2012)

[2] C. Rodenbücher, E. Hildebrandt, K. Szot, S. U. Sharath, J. Kurian, P. Komissinskiy, U. Breuer, R. Waser, and L. Alff, Appl. Phys. Lett. 108, 252903 (2016)


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Space charge effects at complex oxide interfaces

F. Gunkel1, R. A. De Souza2, S. Hoffmann-Eifert3, R. Waser3, and R. Dittmann3 1 Institut für Werkstoffe der Elektrotechnik 2, RWTH Aachen University, Germany 2 Institute of Physical Chemistry, RWTH Aachen University, Germany 3 Peter Grünberg Institut-7, Forschungszentrum Jülich, Germany Low-dimensional electron transport along complex oxide heterointerfaces has attracted enormous attention in recent years. While the formation of these 2-dimensional electron gases (2DEGs) often is attributed to electronic charge transfer triggered by a built-in electric field, the ionic defect structure at these interfaces is still being discussed controversially. In a recent report, we addressed the thermodynamic processes associated with built-in electric fields and derived ionic defect concentration profiles established at polar/non-polar oxide interfaces. The specific ionic-electronic defect structure stabilized within the interfacial space charge layer strongly depends on ambient oxygen partial pressure applied during sample fabrication and on the strength of the built-in electric field. We compare the thermodynamic ground states obtained for various oxide heterostructure systems and discuss implications for the low-dimensional electron transport. As we demonstrated, electron mobility as well as the magnetic signature of the electron gas can be systematically controlled by thermodynamic means, i.e., the control of the ionic-electronic defect structure.

Various material systems forming a 2-dimensional electron gas (2DEG) at interfaces to strontium titanate are being highly debated in the literature at present. The details of their formation, however, has not yet been fully resolved – in particular, whether or not all material systems share a common formation mechanism and a common ionic-electronic structure has been an open question. By characterizing the thermodynamic ground states of complex oxide heterointerfaces, we were now able to identify two distinct classes of oxide heterostructures [1]: for epitaxial perovskite/perovskite heterointerfaces, such as LaAlO3/SrTiO3, NdGaO3/SrTiO3, and LAST/SrTiO3,

FIG. 1: Sketch of 2DEG formation at oxide interfaces to SrTiO3, triggered by charge transfer (left) or ion transfer (oxygen vacancy formation, right) [1].

FIG. 2: Calculated defect concentration profiles expected from charge transfer across the LaAlO3/SrTiO3. The concentrations of Sr vacancies (red) and electrons (black) are plotted for various pO2 environments at a temperature of 950 K [2].

we found the 2DEG formation being based on charge transfer into the interface, stabilized an electric field in the space charge region (Fig. 1, left). In contrast, for amorphous LaAlO3/SrTiO3 and epitaxial γ-Al2O3/SrTiO3 heterostructures, the 2DEG formation mainly relies on the formation and accumulation of oxygen vacancies. (Fig. 1, left) This class of 2DEG structures exhibits an unstable interface reconstruction associated with a quenched non-equilibrium state.

The ionic-electronic structure of charge transfer-based interface systems has been studied in more detail by applying a numerical simulation model based on the electrostatics of the interface as well as on the thermodynamic defect formation processes in the bulk of SrTiO3 [2]. Our model therefore combines (1) the charge transfer deriving from the polar catastrophe picture and (2) the defect chemistry in SrTiO3. It hence represents a unified model, considering both defect formation processes (driven by energy and entropy) and electrostatics.

The results corroborated that charge transfer into the interface is the underlying doping mechanism in epitaxial perovskite/perovskite interfaces. As a result, a thermally stable, mixed electronic-ionic defect profile is established within the space charge layer (SCL). Depending on ambient conditions, the negative charge density in the n-


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type SCL is accommodated by electrons and/or acceptor-type strontium vacancies (Fig.2). The balance between 2DEG and ionic defects is controlled by pO2.We find an electron gas that is confined mainly within 5–10 nm from the interface. The concentration of strontium vacancies scales on a similar length scale emphasizing that strontium vacancy formation at the interface is essentially important to explain the 2DEG’s low carrier density found, e.g. in LaAlO3/SrTiO3 after oxidation.

As we found for the related NdGaO3/SrTiO3 heterostructures, the low temperature transport properties of such 2DEG systems are greatly affected by the ionic defect profiles established during high temperature growth, too [3]. By analyzing the low-temperature Hall effect in detail, we revealed a complex B-field behavior of the Hall effect, consisting of a superposed non-linear contribution owing to a multi-channel transport and a second non-linear contribution owing to the anomalous Hall effect and indicating a magnetic signature of the 2DEG (Fig. 3). We introduced a general model describing the entire B-field behavior of the Hall effect in magnetic electron systems exhibiting an inherently nonlinear conventional Hall effect. [3]

FIG. 3: Anomalous Hall effect observed in NdGaO3/ SrTiO3 heterostructures. As shown in Ref. [3], this signature of emerging magnetism directly correlates with the interface´s cationic defect structure.

We find that the conventional transport parameters scale systematically with growth parameters. In particular, we find a continuous enhancement of electron scattering with increased growth pressure, and thus increased concentrations of cationic Sr vacancies at the interface. As a result, the electron mobility found for the 2DEGs degrades with increasing amount of cationic disorder. At the same time, we observe an enhancement of the anomalous Hall effect contribution with increasing cationic disorder. It is found that the critical temperature at which the anomalous Hall effect arises (Tc), as well as its magnitude, can be controlled by the growth conditions during sample fabrication. The systematic trend found for samples grown at various oxygen partial pressure indicates a correlation between defect structure and

anomalous Hall effect and, thus, an influence of defect structure on the magnetic properties of the NdGaO3/SrTiO3 interface. In fact, we revealed a trend that excludes oxygen vacancies as the only origin of magnetism in these samples. Our analysis represents the first comprehensive modeling of the entire nonlinear behavior of the Hall effect in two-dimensional electron gases at oxide interfaces, involving both multiple channel conduction and magnetism. The results of these studies seem to converge into a generalized understanding of 2DEGs at complex oxide interfaces – charge transfer triggered by electrostatic discontinuities, such as engineered at the interfaces, can result in the formation of thermally stable and intrinsic 2DEGs.

However, under typical growth conditions, the electric field established at the interfaces attracts also cationic defects that accumulate in the interfacial space charge region. Their concentration is controllable via thermodynamic means, such as annealing.

The specific defect structure established has severe but also diverse impact on the resulting electrical properties of the 2DEG: For one, in order to generate highest electron mobility values, the formation of Sr vacancies should be suppressed as much as possible by avoiding high temperatures and high oxygen pressures during fabrication. However, in order to generate additional functionality, namely a magnetic response of the 2DEG, cationic disorder seems to be favourable. Increased defect concentrations thus seem to promote magnetism in these systems. This link does not necessarily imply that the cation vacancies themselves directly mediate magnetic ordering. However, there may be an indirect correlation between defects and magnetism, e.g., via the increased scattering rate further localizing the electrons at increased defect density or via mechanical strain induced by vacancies. The altered magnetic properties we observed may thus be related to a structural distortion, e.g., ionic displacements and buckling or lattice spacing changing with the cationic defect configuration and growth parameters.

[1] F. Gunkel, S. Hoffmann-Eifert, R. A. Heinen, D. V. Christensen, Y. Z. Chen, N. Pryds, R. Waser, R. Dittmann, ACS Appl. Mater. Interfaces 9 (1), 1086 (2016)

[2] F. Gunkel, R. Waser, A. H. H. Ramadan, R. A. De Souza, S. Hoffmann-Eifert, R. Dittmann, Phys. Rev. B 93, 245431 (2016)

[3] F. Gunkel, C. Bell, H. Inoue, B. Kim, A. G. Swartz, T. A. Merz, Y. Hikita, S. Harashima, H. K. Sato, M. Minohara, S. Hoffmann-Eifert, R. Dittmann, H. Y. Hwang, Phys. Rev. X 6, 031035 (2016)


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Impact of oxygen exchange reaction at the ohmic interface in Ta2O5-based ReRAM devices

W. Kim1, S. Menzel1, D. J. Wouters2, Y. Guo3, J. Robertson3, R. Waser1, 2, and V. Rana1 1 Peter Grünberg Institut-7, Forschungszentrum Jülich, Germany 2 Institut für Werkstoffe der Elektrotechnik 2, RWTH Aachen University, Germany. 3 Engineering Department, University of Cambridge, Cambridge CB2 1PZ, UK In this paper, the impact of different ohmic metal-electrode materials, namely W, Ta, Ti, and Hf on the switching characteristics of Ta2O5-based ReRAM device is investigated. The resistive switching devices with Ti and Hf electrodes having negative defect formation energy, show an early RESET failure during the switching cycles. This process is attributed to the accumulation of oxygen vacancies in the switching layer, leading to a permanent breakdown to the low resistive state. In contrast, the defect formation energy of Ta and W electrodes w. r. t. the Ta2O5 switching layer is positive. This results in a highly stable resistive switching behavior. Based on these findings, an advanced resistive switching model, wherein the oxygen exchange reaction at the ohmic electrode interface also plays a vital role in determining the resistance states, is presented.

Bipolar redox-based resistive random access memory (ReRAM) consists of an insulating switching layer stacked between two asymmetric metal electrodes: one low work function (WF) ohmic electrode (OE), such as W, Ti, Hf, or Ta, and a high WF (inert) metal electrode, such as Pt [1]. The resistive switching mechanism relies on the motion of mobile ionic defects under the applied electrical field. While motion of metal cations has been evidenced and their role cannot be excluded, the most widely accepted model is based on (double) positively charged oxygen vacancy defects Vö. These VO‘s are introduced in the switching layer during the initial forming step, which results in the formation of a filamentary region with a high concentration of oxygen vacancies between these two electrodes. In this study, we analyzed Ta2O5-based ReRAM devices using different OE materials (W, Ta, Ti and Hf). The OE materials are chosen for the defect formation energy of oxygen vacancy (EVO) with respect to the Ta2O5 layer. For Hf and Ti, this EVO is negative, whereas it is positive for Ta and W. In this experiment, we have observed that the OE not only affects the forming voltage but also the RESET process. The devices with the Hf and the Ti electrode show an early RESET failure, whereas the devices with the Ta and the W-OE show highly reliable switching behavior. The Pt/Ta2O5/W

ReRAM devices show faster RESET process than the Pt/Ta2O5/Ta, which leads to a higher resistance ratio under the same bias conditions. These observations indicate that oxygen interchange with the OE plays an important role during the resistive switching process.

FIG. 1: (a) A schematic cross-sectional diagram of the prepared ReRAM device with different ohmic electrode materials (W, Ta, Ti and Hf). (b) Scanning electron microscope image of the 2 × 2 μm2 ReRAM device in passive crossbar configuration.

Fig.1 shows the schematic diagram and SEM image of the Ta2O5 ReRAM device having different ohmic electrode materials (W, Ta, Ti and Hf). The feature size of the device was 2 x 2 µm2. The details of the experiment are described in reference 3. The influence of the OE on the forming voltage was investigated by measuring 50 devices for each device stack. The forming process was performed by a positive DC voltage sweep (+3 V) applied to the OE with a current compliance of 1.0 mA. The forming statistics of the four different device stacks are given in Fig. 2(a). A clear dependence of the forming voltage on the ohmic electrode material is observed.

FIG. 2: (a) Dependence of the forming voltage of the Ta2O5

ReRAM device on different OE (W, Ta, Ti and Hf). (b)Interfacial defect formation energy for oxygen vacancydefects (EVO) in Ta2O5 and HfO2 as function of the oxygenchemical potential [2]. This chemical potential isdetermined by the ohmic electrode material present asindicated here for Ta, W, Ti and Hf.


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These forming voltage trends correspond with the oxygen vacancy defect formation energies at the respective OE/Ta2O5 interfaces as shown in fig. 2(b). During the switching, the ReRAM devices with the Ti- and the Hf-OE show a failure in the RESET process within the first 20 cycles, shown in Fig. 3(a). The gray lines show the switching behavior of the Ta2O5 ReRAM with the Ti- and the Hf-OE before failure. After the RESET failure the ReRAM devices exhibit ohmic I-V behavior and are stuck in the LRS regime. In contrast, stable switching operation has been observed with the W- and the Ta-OE under the same biasing conditions as shown in Fig. 3(a). Average switching curves are shown in red for the W-OE and in blue for the Ta-OE devices. The occurrence of RESET failure or stable switching clearly correlates with the EVO at the OE/Ta2O5 interface, suggesting that oxygen exchange takes place at this OE/Ta2O5 interfaces during the switching process [2]. Fig. 3(b) shows the effect of the VRESET-STOP on the high resistance (RHRS) for the W- and the Ta-OE based Ta2O5 devices in the range of -1.4 V to -2.0 V. The W-OE devices consistently yield higher RHRS than the Ta-OE devices. At the VRESET-STOP = -2.0 V, the resistance value was 1.2 MΩ and 200 kΩ for the W- and the Ta-OE device, respectively.

FIG.3: (a) Switching curves of Ta2O5 ReRAM with different OE. Early RESET failures with Hf and Ti electrodes, whereas stable switching cycles of SET and RESET with Ta and W electrodes. (b) Change in RHRS and resistance ratio depend on the VRESET-STOP increment for Ta- & W-OE Ta2O5 ReRAM device.

The experimental results indicate that the OE plays an active role during the switching process in VCM ReRAM devices. In analogy to the forming process, an oxygen exchange reaction occurs during switching at the OE/metal-oxide interfaces. In addition, oxygen exchange reaction takes place at the OE/oxide interface. Hence, the total amount of the VO’s within the filament changes during the switching process. This modified switching model is illustrated in Fig. 4. During the SET transition, the oxygen vacancies drift towards the high WF metal electrode and decrease the Schottky barrier height. This is corresponding to oxygen moving towards the OE interface, where due to the oxygen exchange reaction oxygen is extracted from the metal-oxide and incorporated in the OE. By this, the electrode may be locally oxidized forming a sub-oxide, while the total amount of the VO in the metal-oxide increases. During the RESET process, the opposite movement of the VO’s result in their depletion at the high WF metal electrode and

accumulation at the OE electrode. Due to the oxygen exchange reaction at the electrode, oxygen is incorporated in the oxide layer lowering the amount of vacancies at the OE interface. Thus, the total amount of the VO’s decreases in the oxide layer. The rate of this oxygen exchange reaction depends on the EVO.

FIG.4: Illustration showing the movements of oxygen vacancies and oxygen ions during the SET and the RESET process.

A faster extraction of the oxygen ion from the switching oxide layer (SET) is achieved for a lower (positive) EVO values. In contrast, the re-incorporation of oxygen ions into the switching oxide layer (RESET) is fasterfor higher (positive) EVO. Thus, a higher EVO is beneficial for faster RESET operation. The early RESET failure for the Ti- and Hf-OE devices can be explained by the negative value of EVO. Kinetic barriers is lowered by the electric field while oxygen extraction from the Ta2O5 strongly is enhanced during the SET process while oxygen re-incorporation during the RESET process is strongly reduced. Hence, the total amount of the oxygen vacancies is steadily increasing from cycle-to-cycle, reducing the resistivity of the switching layer. Eventually, the current and the dissipated power during the RESET process is so high that oxygen may get extracted at the high WF Pt electrode (i.e., we get a spurious SET at this Pt electrode), resulting in an overall RESET failure process. For the Ta- and W-OE devices, the oxygen exchange during the SET and the RESET process is better balanced and thus stable switching is obtained. In this study, Ta2O5-based ReRAM devices have been studied with W-, Ta-, Ti- and Hf-OEs. Based on our experimental observations, a modified switching model is proposed. In contrast to the previous models, the experimental results show that oxygen exchange reactions with the ohmic electrode not only occur during deposition and the electroforming process, but also during each SET and RESET process. The positive EVO for the Ta- and W-OE leads to highly stable switching process in the Ta2O5 ReRAM. The W-OE devices show an increased RHRS compared with the Ta-electrode under identical RESET conditions. More details of this experiment can be accessed in [3].

[1] R. Waser and M. Aono, Nature Materials 6, 833 (2007)

[2] J. Robertson and S.J. Clark, Physical Review B 83, 075205 1 (2011)

[3] W. Kim et al., Nanoscale 8 (41), 17774 (2016)


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Impact of defect occupation on current-voltage characteristics in amorphous Ge2Sb2Te5

M. Salinga1,2 and M. Kaes1,2 1 Institute of Physics IA, RWTH Aachen University, Germany

2 IBM Research - Zurich, Rüschlikon, Switzerland Both phase change materials and ovonic threshold switching materials are known for their highly non-linear current-voltage characteristics. Despite intensive research, the origins of this non-linearity are still debated today. The most widely used conduction models invoke the inter-trap distance. In all modelling approaches so far this inter-trap distance is treated as a constant. In our recent study [1] we investigate the relation between the electronic density of states of amorphous Ge2Sb2Te5 and the current-voltage characteristics of micro-scale Ge2Sb2Te5-devices. By manipulating the occupation of defect-states with both temperature and illumination we demonstrate that it is not simply the distance between defect states that controls the temperature- and field-dependent electrical-transport. Instead, it is the density of occupied defect states that must be used as the relevant parameter in the transport models.

Chalcogenides can be switched reversibly between a highly-resistive amorphous (reset) and a highly-conductive crystalline (set) state. While the transition from the set- to the reset-state is usually obtained via melt-quenching, the set-transition is characterized by a breakdown of the resistance that occurs at threshold fields of 20 to 50 V/µm [2]. This breakdown leads to a substantial increase of current which can then induce crystallization through substantial Joule-heating. The time-scale on which crystallization can occur separates the so-called phase-change materials (PCM) from the ovonic-threshold switching materials (OTS). PCMs such as GeTe can crystallize within a few nanoseconds [3], whereas the atomic structure of OTS-materials remains unchanged even under significant Joule-heating. However, both kinds of amorphous materials share a prerequisite for resistive switching i.e. a highly non-linear current-voltage characteristic. Despite intensive research, the origins of this non-linearity are still debated today.

Many attempts to explain the non-linearity of subthreshold conduction in OTS and in PCM have resorted to the Poole-Frenkel (PF) mechanism [4-5]. In essence, this model describes how the electric field enhances the emission probability of charge-carriers trapped in the Coulomb potential of a defect state in the bandgap.

The enhancement of the emission probability occurs because the energetic barrier for emission is lowered by the electric field. Due to this lowering, the number of charge-carriers emitted into the band and thus the conductivity increases. If the density of sites with Coulomb potentials is sufficiently high, their overlap may lead to an additional barrier-lowering [4]. Consequently, conduction models invoke a parameter describing this defect density - the inter-trap distance s [6].

This parameter is crucial for modelling device-characteristics as the subthreshold current depends on it in an exponential way. In all modelling approaches so far, e.g. [8-10], this inter-trap distance describes the electronic states in a rather unspecified manner i.e. without drawing a connection to a specific defect. Moreover, it is treated as a constant. Especially the latter is conceptually puzzling as the Coulomb interaction of a trapped charge-carrier with a neighbouring defect must depend on the charge-state of that defect. The charge-state of a defect in turn should depend on its state of occupation. Thus, the fundamental temperature-dependence of the occupation function should affect the inter-trap distance.

To investigate the connection between the inter-trap distance and the occupation of defects, we deliberately change the occupation of defects in the prototype PCM, Ge2Sb2Te5 (GST), by illuminating our devices with light. We assess the impact of illumination and temperature by measuring the electrical transport in micro-scale devices of amorphous GST in darkness and under illumination. To span a wide temperature range and at the same time prevent (partial) crystallization and structural relaxation of the amorphous state, we perform our measurements below room temperature i.e. between 140 K and 300 K.

We derive temperature-dependent values for the inter-trap distance under illumination and in darkness by analysing our data with a PF-based model for subthreshold transport (FIG.1). We then relate this inter-trap distance to the electronic Density of States (DoS) and its occupation by calculating the average distance s between the various trap states in amorphous GST based on a DoS reported in literature.


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FIG. 1: Field- and temperature-dependent device current. Crosses and circles mark the transition fields FO from Ohmic to Poole, and Ft from Poole to Poole-Frenkel field-dependence in the model fits (black lines).

To extract the inter-trap distances from measured current-voltage characteristics (IV-curves), we employ a previously introduced transport model, which proved to be capable of reproducing the salient features of IV-curves of amorphous GeTe and GST in a wide range of temperatures (200 K to 400 K) in the melt-quenched as well as in the as-deposited state. It can even capture the effects of structural relaxation in melt-quenched devices (see [1] and references therein). In this model multiple-trapping transport is combined with field-enhanced emission, building on works by Hill [4] and Pillonet [11]. For a detailed description we refer to the original publication of our work in Scientific Reports.

The model can describe the three different regimes of field-dependence that have been observed in PCMs: While the conductivity is constant at low fields (Ohmic regime), a Poole-Frenkel (PF) type behavior is observed at high fields. Between these two regimes, the conductivity exhibits an exponential field-dependence (Poole). The field of transition from Poole to Poole-Frenkel depends on the inter-trap distance [12]. This transition field Ft is marked in the fit to an experimentally recorded IV-curve serving as a graphical representation of the inter-trap distance s (cf. FIG.1). In a similar way, the field at which the transition from the Ohmic to the Poole regime occurs also depends on s, but additionally on temperature [6].

Our model does yield a very good description of the measured current-voltage characteristics both in darkness (FIG.1) and under illumination (shown in our original work [1]).

Comparing the temperature-dependence of the extracted values for the inter-trap distance slight and sdark (FIG.2) we observe that our general expectation from the calculation of occupied states regarding s is matched by the results from experiments: a) the inter-trap distances slight and sdark are matched well within the accuracy we expect for our DoS (estimated from the variation of the DoS-spectroscopy data from different studies)

at high temperatures but differs more strongly at low-temperatures, b) sdark increases with decreasing temperature (above 200 K) and c) slight decreases with decreasing temperature.

FIG. 2: Comparison of modelled inter-trap distances (lines) and values extracted from the fit of the transport model to the measured conductivity data (circles) for two conditions: in darkness (black) and under illumination (red).

These observations demonstrate that the inter-trap distance depends on temperature and illumination. In light of the above, we formulate our main claim: it is not the mere density of defect-states but rather their state of occupation that must be employed in transport-models for PCMs.

These results are of great fundamental and practical value. The understanding of the relation between the density of states and the inter-trap distance constitutes a crucial step towards tailoring the characteristic current-voltage curve of PCM and OTS devices by material design. Clearly, the temperature-dependence of the inter-trap distance – hitherto not accounted for – must be consequential in a physically correct simulation of switching in such devices.

[1] M. Kaes and M. Salinga, Scientific Reports 4, 31699 (2016); DOI: 10.1038/srep31699

[2] D. Krebs et al., APL 95, 082101 (2009).

[3] G. Bruns et al., APL 95, 043108 (2009).

[4] R. M. Hill, Phil. Mag. 23, 59–86 (1971)

[5] Le Gallo et al., J. Appl. Phys. 119, 025704 (2016)

[6] Le Gallo et al., New J. Phys. 17, 093035 (2015)

[7] A. Sebastian et al., J. Appl. Phys. 110, 084505 (2011).

[8] D. Ielmini and Y. Zhang, J. Appl. Phys. 102, 054517 (2007).

[9] J. Chen et al. International Symposium on VLSI Technology, Systems, and Applications 2012

[10] R.G.D. Jeyasingh et al. IEEE T. Electron. Dev. 58, 4370 (2011)

[11] A. Pillonnet and R. Ongaro, Rev. Phys. Appl. 25, 229–242 (1990).

[12] R. Ongaro & A. Pillonnet, IEE Proc.-A 138, 127–137 (1991).


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Study of time resolved plasma dynamics in a hollow cathode Z-pinch EUV source

F. Melsheimer1,2, M. Ranis1,2, and L. Juschkin1,2 1 Peter Grünberg Institut-9, Forschungszentrum Jülich, Germany 2 Experimental Physics of EUV, RWTH Aachen University, Germany Gas discharge EUV sources are compact and versatile sources of short wavelength radiation for small-scale applications. The emission of EUV radiation is originated from a high voltage discharge within a low-pressure gaseous environment. Dynamics of the discharge-produced plasma and its emission depends strongly on electrode geometry and electrical properties as well as gas pressure and composition. In order to optimize the emission spectrum and tune such source for needs of different applications, deep understanding of physics of the discharge is essential. To analyze the discharge plasma dynamics inside the electrode system we implemented 5-ns-step time-resolved imaging with 2 ns exposures. The focus of the experiment addressed here is on the investigation of the influence of the initial gas composition on plasma dynamics. The obtained 2D plasma images allowed performing a reconstruction of the spatial distribution of the plasma radiation in 3D.

Extreme ultraviolet radiation is attributed to photons in 25 eV - 250 eV energy range (wavelength between 5 nm and 50 nm). The short wavelength enables high spatial resolution in microscopy and lithography. The exceptionally strong light-matter interaction in this spectral range enables strong elemental selectivity and high contrast between different materials. The gas discharge produced plasma (DPP) EUV sources represent an affordable low-maintenance solution for lab based applications. Spectral tunability is easily achieved by utilization of different fuel gases so the output emission spectrum can be tailored to the desired application. Source development and optimization of the emission properties requires detailed knowledge of plasma dynamics inside the electrode system. Pinching process during the discharge and subsequent EUV emission are happening on nanosecond time-scale thus for diagnosing plasma conditions and emission origins a time-resolving imaging setup is needed.

A common model to describe the plasma kinetics of a gas puff z-pinch is the snow-plow model [1]: Due to the skin effect the conduction starts at a thin shell around the systems axis of symmetry. The moving charges are affected by the strong magnetic field of the current, thus pushed inward

due to the Lorentz force. While moving towards the central axis, this shell associated with high B-field gradient grabs, ionizes and compresses gas inside the shell, hence the name “snow-plow”. As the shell’s radius decreases, the increased thermal pressure opposes the magnetic pressure causing plasma stagnation near the axis. Possible instabilities in plasma formation can weaken the achievable magnetic pressure and thus the diameter of the stagnating plasma. The axially unconfined plasma is accelerated towards the ends of the electrode system. After the end of the current pulse, plasma recombines and the process can start over again.

Control over plasma compression, stagnation phase and expansion plays the key role in optimizing the discharge source. To monitor this dynamics with suitable spatial and temporal resolution, an imaging setup is implemented (

FIG.). A 100 µm diameter pinhole is used to project the image of the plasma onto a microchannel plate (MCP), while simultaneously providing suitable vacuum separation between the discharge and detector vacuum compartments. The CsI-coated MCP (5-30 nm spectral sensitivity range) is gated by a 2 ns 1 kV high voltage pulse [2]. Synchronization is achieved by monitoring the rise of the discharge current (dI/dt rate of ~2e11 A/s). The obtained trigger signal is delayed with a cable delay line by the desired time. The chosen time step between frames is 5.1 ns (1 m RG58). The amplified electrons exiting the MCP are accelerated onto a phosphor screen. The fluorescent screen is photographed with a DSLR camera, integrating over 200 discharges.

FIG. 1: Experimental setup for time resolved plasma imaging synchronized by the current rise.


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Due to limited possibilities for matched termination, part of the gating pulse is reflected several times within the cable, leading to ghosting artefacts within the recorded data. This is validated by cable alternation and will be eliminated in future measurements. The recorded images contain projections of the emission distribution during the compressing and pinching phases as well as during the following expansion (Fig. 2).

FIG. 2: Normalized photographs of phosphor screen. Projection at 5° off-axis; 115 ns, 130 ns and 160 ns after current rise for argon; Central spot in the left image is ghosting artefact caused by reflected gating pulse.

Considering axial symmetry, the 3D distribution can be retrieved from off-axis measurements. A python code utilizing a positively constrained linear least squares solver is implemented to retrieve the 3D distribution. For validation, the axial projection is compared to the on-axis measurements. The measurements are performed for oxygen, argon, oxygen/argon mixture and xenon working gases.

The measured compression speed is between 40 and 60 km/s for all gases. The peak emission power corresponds to the maximum compression of the plasma as expected from theory. The time to the pinching phase scales with the average atomic weight of the fuel gas as expected from the snow-plow model. The whole mass of the shell is accelerated during the compression phase while the current and magnetic force are nearly identical for different fuels. Heating of the plasma is related to ohmic losses and thus absolute current. In this experiment, the maximum current is reached at 200 ns, consequently the later pinching of Xenon leads to more efficient conversion and higher peak emission power (which scales with density squared) as visible in Fig. 3.

FIG.3: Plasma diameter and relative emission power of argon and xenon discharges. Current maximum is reached at 200 ns.

In Argon, the plasma diameter decreases a second time as peak current is approached. Similar behavior was also observed by Rosier [3]. The lateral evolution of the emission distribution shows significant differences between working gases. In argon discharge, for example, the pinch moves with a measured velocity of 75 km/s towards the cathode.

FIG. 4: Exemplary results of reconstruction for argon discharge (2.2 kV, 10 sccm). Top: Evolution of radial plasma distribution. Snow-plow visible as V-shape between 100-130 ns. On axis signal before 120 ns is caused by ghosting artefact. Bottom: Lateral plasma evolution. Cathode is at -15 – -5 mm, anode at 0 – 10 mm. During pinch phase radiating plasma moves with 75 km/s to the cathode.

Such fast ion motion causes severe ablation of the electrode material. After the pinching phase, the plasma expands freely along the axis. Especially the light oxygen, but also argon and their mixture are expelled towards the front.

The experimental setup and analysis technique represent a new tool and method within the group and for related projects. It enables visualization and investigation of three-dimensional plasma dynamics on nanosecond time-scale. Future development of discharge and laser heated discharge plasma (LHDP) sources will benefit from this ready-to-use setup. A current step for improving the tool is the fabrication and implementation of phase-shifting Fresnel zone plates as imaging optics. While drastically improving the spatial resolution, using the zone plate will also increase the number of photons on the detector and consequently the signal to noise ratio.

[1] D. D. Ryutov, M. S.Derzon, and M. K. Matzen, Rev. Mod. Phys. 72 (1), 167 (2000).

[2] J. Hauck, R. Freiberger, and J. Juschkin, Proc. SPIE 8076 (2011); doi:10.1117/12.887017

[3] O. Rosier, PhD thesis, RWTH Aachen, (2004)


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Strain compensated ZnSe/CdSe/ZnSe quantum wells

T. Rieger1, T. Riedl2, E. Neumann3, D. Grützmacher1, J. K. N. Lindner2, and A. Pawlis1 1 Peter Grünberg Institut-9, Forschungszentrum Jülich, Germany 2 Department of Physics, University of Paderborn, Germany 3 Peter Grünberg Institut-8, Forschungszentrum Jülich, Germany The thickness of ZnSe/CdSe/ZnSe quantum wells on GaAs (001) substrates is limited to 2 − 3 monolayers due to the high lattice mismatch. We demonstrate that a dedicated strain com-pensation technique involving In0.12Ga0.88As pseudo substrates is capable to increase the critical thickness up to CdSe thicknesses of 6 monolayers. The latter allow to design quantum structures which emit light within the entire visible spectrum. The strain compensation effect is investigated by high resolution transmission electron microscopy (TEM) and supported by molecular statics simulations. The model approach with the experimental measurements is sufficiently general to be also applied to other highly mismatched material combinations for the design of advanced strained heterostructures.

Optically active devices based on ZnSe/CdSe heterostructures are in principle capable to cover the whole visible spectrum via a variation of the dimensions. However, the lattice mismatch between ZnSe and CdSe of about 7 % limits the thickness of coherently grown CdSe layers on ZnSe to ~ 2 monolayers (MLs). Between 2 and 3 MLs CdSe, Stranski-Krastanov quantum dots develop and plastic relaxation takes place for thicknesses exceeding 3 MLs, which leads to dominant non-radiative recombination in the devices. [1] In a recent publication, efficient light emission from ZnSe/CdSe/ZnSe quantum wells (QWs) with CdSe thicknesses up to 6 MLs was observed [2]. This was achieved with a dedicated strain compensation technique involving In0.12Ga0.88As buffers inducing alternatingly strained ZnSe and CdSe layers. Due to the alternating strain in the ZnSe and CdSe, the strain of the entire ZnSe/CdSe/ZnSe heterostructure with respect to the In0.12Ga0.88As buffer is compensated. Here, we investigate the strain compensation by TEM and compare experimentally determined strain profiles to simulated ones.

The ZnSe/CdSe/ZnSe strain compensated quantum wells are grown on 2 µm thick In0.12Ga0.88As pseudo substrates using molecular beam epitaxy. Furthermore, the QWs are embed-ded in 50 nm thick Zn0.9Mg0.1Se barriers, which are lattice matched to the In0.12Ga0.88As. The ZnSe layers are 11 monolayers (MLs) thick while the thickness of the CdSe is varied between 2 and 5 MLs. The structure is schematically depicted in Fig. 1a.

FIG. 1: (a) Schematic of the entire strain compensated structure, (b) ML distance from molecular statics simulation, (c) material specific strain and (d) Lagrange strain of the 5 ML QW.

The strain compensation was analyzed by high resolution TEM using lamellae prepared with focused ion beam. Geometric phase analyzes [3] (GPA) was utilized to extract the Lagrange strain in the QW, i.e. the strain relative to a reference region, from the TEM images. Here, the Zn0.9Mg0.1Se was used as the reference. The results are compared to molecular statics simulations based on the Tersoff potential [4] providing the atomic positions at minimum energy. Additionally, a simple set of analytic equations can be used to obtain an average lattice constant of the ZnSe/CdSe/ZnSe stack and to describe the strain. The average lattice constant is the weighted average of the individual lattice constants of ZnSe and CdSe, i.e.

(1)

where and are the thicknesses of the ZnSe and the CdSe, respectively, and and

are the bulk lattice constants. The average lattice plane distance in z-direction , is then obtained by straining this pseudo crystal with lattice constant to the Zn0.9Mg0.1Se barriers, i.e.,

, 1 ,

, (2)


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Here, , and , are the weighted averages of the individual ZnSe and CdSe layers. Using this strain compensation within the ZnSe/CdSe/ZnSe stack, the lattice mismatch of the pseudo crystal with lattice constant relative to the Zn0.9Mg0.1Se is significantly reduced compared to pure CdSe. Figure 1 b-d display the calculated ML distance, the material specific strain in growth direction and the Lagrange strain Δ , , respectively.

Figure 2a displays a HRTEM micrograph of a 5 MLs thick CdSe QW. No structural defects in the heterostructure are observed. In Fig. 2b a HRTEM micrograph with superimposed GPA Δ strain map of a 3 MLs thick CdSe quantum well is depicted. The GPA resolution is 2 nm. No gradient is visible indicating coherent growth. However, in the Δ strain map shown in Fig. 2c a gradient is present, i.e., in growth direction Δ first switches from zero (Zn0.9Mg0.1Se) to negative values (ZnSe, tensile in-plane strain compared to the Zn0.9Mg0.1Se) and subsequently to positive (CdSe, compressive in-plane strain compared to the

Zn0.9Mg0.1Se), after that negative (ZnSe) and finally, zero (Zn0.9Mg0.1Se). The CdSe layer (positive Δ ) has a uniform thickness, no clustering or formation of quantum dots is observed, and even the QW with 5 MLs CdSe is smooth (see Fig. 2a). In Fig. 2d a profile of the experimentally determined Δ map is displayed (red full line). The black dotted line indicates the Δ profile obtained from the molecular statics simulations. For the black full line, this profile was averaged over 7 MLs, reproducing the resolution used for GPA. As obvious, the profile averaged over 7 MLs reproduces the experimentally determined profile rather well while the original profile (not averaged, dotted line) indicates significantly larger Δ values. In order to further compare the molecular statics simulations with the experimental Δ profile, we have applied the GPA with the identical resolution as used for the experimental analysis to a simulated projection view of the heterostructure. The atomic coordinates in the projection view are taken from the molecular statics simulations. Figure 2e,f displays the projection view image, Fig. 2g shows the GPA Δ map of the projection view. The Δ map from the projection (Fig. 2g) resembles that one from the HRTEM image (Fig. 2c). Thus, the molecular statics simulations are well suited to describe the strain state within the ZnSe/CdSe/ZnSe layer stack.

The lattice parameters of the ZnSe/CdSe/ZnSe layer stack in z-direction , calculated using Eq. 2 are in excellent agreement with the average lattice parameters of the molecular statics simulations, indicating that Eqs. 1 and 2 represent an intuitive, simply and valid description of the layer stack.

In conclusion, we have demonstrated that ZnSe/CdSe/ZnSe quantum wells with CdSe thicknesses up to 5 MLs grow fully coherent due to the dedicated strain compensation technique involving alternatingly strained layers. The presented strain compensation mechanism can be easily applied to other highly lattice mismatched semiconductor heterostructures. The detailed description of this work has been recently published in Ref. [5].

[1] M. Strassburg, T. Deniozou, A. Hoffmann, R. Heitz, U. Pohl, D. Bimberg, D. Litvinov, A. Rosenauer, D. Gerthsen, S. Schwedhelm, K. Lischka, and D. Schikora, Appl. Phys. Lett. 76, 685 (2000).

[2] A. Finke, M. Ruth, S. Scholz, A. Ludwig, A. Wieck, A., D. Reuter, and A. Pawlis, Phys. Rev. B 91, 035409 (2015).

[3] M. Hytch, Microsc., Microanal., Microstruct. 8, 41 (1997).

[4] J. Tersoff, Phys. Rev. B 37, 6991 (1988)

[5] T. Rieger, T. Riedl, E. Neumann, D. Grützmacher, J. K. N. Lindner, and A. Pawlis, ACS Appl. Mat. Int. 9, 8371 (2017).

FIG. 2: (a) HRTEM image of the 5 MLs thick CdSe QW, (b) HRTEM image of the 3 MLs thick CdSe QW with superimposed GPA strain map, (c) HRTEM image of the 3 MLs thick CdSe QW with superimposed

GPA strain map, (d) profile of the 3 MLs thick CdSe QW in comparison to results from simulations, (e) schematic of the stacking sequence of the projection view shown in (f). The projection view image of the strain compensated structure is generated with atomic coordinates determined from molecular statics (MS) simulations, (g) strain map of the projection view.


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Fermi surface manipulation by external magnetic field demonstrated for a prototypical ferromagnet

E. Młyńczak1,2, M. Eschbach1, S. Borek4, J. Minár4,5, J. Braun4, I. Aguilera3, G. Bihlmayer3, S. Döring1, M. Gehlmann1, P. Gospodarič1, S. Suga1,6, L. Plucinski1, S. Blügel3, H. Ebert4, and C. M. Schneider1 1 Peter Grünberg Institut-6, Forschungszentrum Jülich, Germany 2 Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Krakow,

Poland 3 Peter Grünberg Institut-1 and Institute for Advanced Simulation-1, Forschungszentrum Jülich, Germany 4 Department Chemie, Ludwig-Maximilians-Universität München, Germany 5 New Technologies-Research Centre, University of West Bohemia, Pilsen, Chech Republic 6 Institute of Scientific and Industrial Research, Osaka University, Japan The electronic band structure near the Fermi level determines numerous vital properties of metallic materials, being responsible for their thermal, magnetic, and electronic transport behavior. Here, we consider the details of the near-surface electronic band structure of a prototypical ferromagnet, Fe(001). Using high-resolution angle-resolved photoemission spectroscopy, we directly demonstrate openings of the spin-orbit-induced electronic band gaps near the Fermi level. The band gaps, and thus the Fermi surface, can be manipulated by changing the remanent magnetization direction. The effect is of the order of ∆E ≈ 100 meV and ∆k ≈ 0.1 Å−1. We show that the observed dispersions are dominated by the bulk band structure. First-principles calculations and one-step photoemission calculations suggest that the effect is related to changes in the electronic ground state and not caused by the photoemission process itself. The symmetry of the effect indicates that the observed electronic bulk states are influenced by the presence of the surface, which might be understood as related to a Rashba-type effect.

The influence of the spin-orbit interaction on the electronic band structure of a ferromagnet is very subtle. It causes the mixing of the spin character and a magnetization-dependent opening of minute energy gaps (about 100 meV) but only at specific points in the reciprocal space. The consequences of these delicate modifications are, however, tremendous. The spin-orbit gaps (SOG) located at the Fermi level, referred to as magnetic monopoles in momentum space [1], are responsible for the magnitude of the intrinsic anomalous Hall effect [1,2], anisotropic magnetoresistance [3], and occurrence of the magnetocrystalline anisotropy (MCA).

Density functional theory calculations of the electronic band structure of the ferromagnetic iron reveal the regions in the Brillouin zone, where the bands cross and where SOG occurs. Figure 1 (b)

shows the results of the GW calculations of bulk bcc Fe, along Г-H direction in the bulk Brillouin zone [Fig. 1 (a)]. Two directions are distinguished: parallel and perpendicular to the magnetization (M) (plotted for positive and negative k values, respectively).

FIG. 1: Bulk and (001) surface Brillouin zones of bcc Fe. High symmetry points 1 to 4 are defined with respect to the magnetization direction depicted by an arrow. (b) Relativistic bulk band structure of bcc Fe, calculated along the H-Γ (positive k) and H’-Γ’ (negative k) using the GW method.

We can see that the occurrence of SOG depend on the magnetization direction. Very interesting situation is encountered close to the Fermi level, where three bands cross in the magnetization dependent manner [Fig. 1 (b), region marked by green rectangles]. Even though the SOG were known to exist thanks to the theoretical calculations, their experimental observation was challenging. To achieve this, we needed to use the Fe sample of the very high crystalline quality and perform measurements using instrument that allows high energy- and k-resolution [4].

The 100-ML Fe films (1 ML ≈ 1.43 Å) investigated in this study were deposited at low temperature (T=50K) by molecular beam epitaxy onto the Au(001) single-crystal surface. After deposition, the Fe films were briefly heated up to 300 °C. Before


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each ARPES measurement, the samples were remanently magnetized. The external magnetic field was applied to the thin-film sample by a permanent magnet. ARPES measurements were performed for samples magnetized along [100], [-100], [010], and [0-10], which will be referred to as UP, DOWN, RIGHT, and LEFT, respectively. All the ARPES spectra discussed in this study were collected in the energy region close to the Fermi level (within a binding energy range of 200 meV), utilizing the higher-energy neon emission line of hν = 16.85 eV. The analyzer was set to an energy resolution of 10 meV for all the presented spectra.

Figure 2 presents the collection of the cuts through the Fermi surface, measured in the vicinity of the point of the surface Brillouin zone. The Fermi sheets visible here are formed by the bands marked by the green rectangles in Fig. 1 (b).

FIG. 2: Electronic structure of Fe(001) close to the point of the SBZ for four different in-plane easy magnetization directions measured at hν= 16.8 eV.

Clearly, for the magnetization RIGHT and LEFT, a spin-orbit gap is observed. For the magnetization RIGHT, the SOG is observed for the α+ sheet [Fig. 2 (a)], while for the magnetization LEFT, it is visible for the α− sheet [Fig. 2 (b)]. The size of the observed spin-orbit gap is of the order of 0.1 Å−1. However, when the magnetization direction points UP or DOWN [Figs. 2 (c) and 2 (d), respectively], both α+ and α− are rather symmetric with respect to the Г (kx = 0 Å−1) line, and no clear gap is visible. The difference observed between these two magnetization directions is the shift of the spectral weight towards higher ky values for the magnetization DOWN, as compared to UP.

GW based on density functional theory calculation of the bulk electronic structure of bcc Fe predicts the occurrence of SOG at the Fermi surface close to the point for the k vectors parallel to the magnetization direction [grey solid line in Fig. 3 (a)]. The results of the experiment performed for different magnetization directions can be arranged with respect to the fixed magnetization direction when the direction of light incidence is neglected. Such arranged fragments of the Fermi surface are shown in Fig. 3 (a). Clearly, the symmetry of the experimentally obtained Fermi sheets is not as high as the symmetry of the bulk electronic structure. For example, for magnetization LEFT and RIGHT, there is a clear difference between Fermi sheets α+ and α−, which is not expected in the bulk.

FIG. 3: (a) Fermi surfaces derived from Fig. 2 (a)-(d) arranged with respect to the fixed magnetization direction (arrow). The gray line depicts the electronic band structure of the bulk for k = 2.2 Å−1 calculated using the GW method. The dashed vertical line marks the mirror plane. (b) Result of the one-step model photoemission calculation that shows spectral intensities within the entire surface Brillouin zone. The arrow marks the magnetization direction.

In order to understand the experimental result, it is necessary to consider the symmetry of the entire system. Even though in the experiment we are observing bulk electronic bands, due to the strong surface sensitivity of angular resolved photoemission in this photon energy range, the spectra are influenced by the presence of the surface. In addition, the in-plane magnetization breaks the symmetry even further.

To get more insight into the observed effects, we compared the results of the experiment with the result of the one-step model photoemission calculations [Fig. 3 (b)]. This calculation incorporates the entire experimental geometry including light incidence direction and photon energy. Very good agreement concerning the intensity of the photoemission features between experiment and one-step model calculations is revealed, especially for the γ Fermi sheet, close to the H point. The k-space region near the points is also relatively well reproduced by the calculations and the overall symmetry reflects the experimental one.

We interpret the observed effect as a result of the opening of spin-orbit interaction- and magnetization-related band gaps, the existence of which is essential for the emergence of fundamental magnetic phenomena such as magnetocrystalline anisotropy, anisotropic magnetoresistance, the anomalous Hall effect, and x-ray magnetic linear dichroism. What is more, the detected electronic band gaps might play a substantial role in spin dynamics.

[1] Z. Fang, N. Nagaosa, K. S. Takahashi, A. Asamitsu, R.Mathieu, T. Ogasawara, H. Yamada, M. Kawasaki, Y. Tokura, and K. Terakura, Science 302, 92 (2003)

[2] Y. Yao, L. Kleinman, A. H. MacDonald, J. Sinova, T. Jungwirth, D. Wang, E. Wang, and Q. Niu, Phys. Rev. Lett. 92, 037204 (2004)

[3] L. Berger, Physica (Amsterdam) 30, 1141 (1964)

[4] E. Młyńczak, M. Eschbach, S. Borek, J. Minár, J. Braun, I. Aguilera, G. Bihlmayer, S. Döring, M. Gehlmann, P. Gospodarič, S. Suga, L. Plucinski, S. Blügel, H. Ebert, and C. M. Schneider, Phys. Rev. X 6, 041048 (2016)


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Challenges of neuromorphic computing

J. Jordan1, I. Blundell1, M. Denker1, M. Diesmann1,2,3, J. M. Eppler4, S. Grün1,5, A. Morrison1,4,6, D. Plotnikov7, J. Senk1, T. Tetzlaff1, and S. J. van Albada1 1 Institute of Neuroscience and Medicine-6 and Institute for Advanced Simulation-6 and JARA-Institute

Brain Structure Function Relationships, Forschungszentrum Jülich, Germany 2 Department of Psychiatry, Psychotherapy and Psychosomatics, Medical Faculty, RWTH Aachen

University, Germany 3 Department of Physics, RWTH Aachen University, Germany 4 Simulation Lab Neuroscience, Bernstein Facility for Simulation and Database Technology, Institute for

Advanced Simulation, Germany 5 Theoretical Systems Neurobiology, RWTH Aachen University, Germany 6 Institute of Cognitive Neuroscience, Faculty of Psychology, Ruhr-University Bochum, Germany 7 Chair of Software Engineering, RWTH Aachen University, Germany

By enabling fast and energy-efficient simulations of large-scale neural networks, neuromorphic computing offers exceptional opportunities for computational neuroscience research and machine learning applications in areas out of reach for current general-purpose computers. Inspired by the structure and dynamics of biological nervous systems, neuromorphic computing aims for high levels of efficiency through asynchronous, massively parallel data processing. Consequently, neuromorphic systems depart from the traditional deterministic von-Neumann architecture. This introduces new challenges for users, including variability of the computing substrate, new programming paradigms, generation of massive data sets, and a correspondingly increased importance of metadata tracking. Tackling these issues requires new modeling approaches, tools, and workflows to be developed alongside neuromorphic hardware to make this technology easily accessible and exploit its full potential.

Neuromorphic hardware systems provide an alternative computing technology for investigating the properties of networks of simplified models of nerve cells and are promising candidates for building smart machines following the computational principles of biological nervous systems. Compared to simulations running on traditional high-performance computing (HPC) systems, neuromorphic hardware promises to be more energy-efficient and faster. To achieve this, neuromorphic design principles typically sacrifice determinism and accuracy in favor of efficiency and speed. This poses unprecedented challenges for users that need to be taken into account to leverage the full potential of this new computing substrate. Currently, two neuromorphic systems are being developed for neuroscientific research supported by the EU ICT Flagship Human Brain Project [1]: the BrainScaleS system, a mixed-signal system developed at the University of Heidelberg, and the SpiNNaker system, a purely digital manycore system developed at the University of Manchester. Neuromorphic mixed-signal systems such as Brain-ScaleS offer a tremendous increase

in simulation speed, up to five orders of magnitude over traditional HPC systems. However, due to inevitable fluctuations in the production process, all analog components show intrinsic temporal and spatial variability, in strong contrast to digital systems where users can expect identical outcomes for two simulations with identical initial conditions. In [2] we investigated the influence of spatial heterogeneities on network dynamics, specifically on correlations in the spiking activity of recurrent networks emulated on the Spikey neuromorphic system, a precursor to the BrainScaleS system. While correlations are inevitable in finite recurrent networks due to shared presynaptic input, theoretical studies have shown that inhibitory feedback actively suppresses these. Since the functional performance of neural networks often depends on the level of correlations in the neural activity, it is important to understand how properties of the neuromorphic hardware influence the dynamic suppression of correlations. We found that inhibitory feedback suppresses correlations even in highly heterogeneous networks emulated on mixed-signal hardware. However, while heterogeneities suppress the amount of shared-input correlations, the overall level of correlations increases due to diminished effective feedback in highly heterogeneous networks. This demonstrates that a systematic investigation of the influence of specific hardware properties on dynamical features of neural network models is required before one can successfully port these from the domain of numerical simulations to mixed-signal systems.

SpiNNaker is a purely digital neuromorphic system based on ARM processors linked by an asynchronous distributed communication infrastructure designed to simulate neural network dynamics in real time. In contrast to other neuromorphic architectures, it possesses a high level of configurability: neuron and synapse models are not hardwired, but can be defined by users via an appropriate implementation in C code. However, the SpiNNaker hardware currently only offers fixed-point single-precision arithmetic. Separating the model definition from platform-specific code would allow users to focus on the


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specific neuroscientific models instead of implementation details on a specific computing platform. In [3] we describe NESTML, a domain-specific language that supports domain experts in creating neuron models for the neural simulation tool NEST. NEST is an open-source software tool designed for the simulation of large-scale networks of single-compartment spiking neuron models on workstations and HPC systems. NESTML allows the specification of neuron models either in mathematical notation as a simple string or as an algorithm written in a built-in procedural language inspired by Python’s syntax. From this description, a code generator creates optimized code that can be dynamically loaded into NEST. All knowledge about optimizations for a specific platform can hence be incorporated into the code generator and thus does not require the user to be aware of the implementation details. Extending NESTML with a backend to generate code for the SpiNNaker platform would allow users to just as easily generate new models with optimized code for this neuromorphic system, without requiring extensive knowledge of the hardware architecture.

Figure 1: Workflow overview: A network simulation of a cortical microcircuit model is run using both NEST (1) and SpiNNaker (2). Simulation results are transferred to a common storage (3) and compared utilizing functionalities of the Elephant library (4). Complex analysis results are visualized to gain further insight (5). The middleware UNICORE is used to access HPC systems.

The constraints put into place by network simulation engines and specifications of neuromorphic hardware lead to a situation where the detailed validation of neuron, synapse, and network models simulated on different systems is a necessity. However, such validations become meaningful only when their results can be reliably compared independently of the data source. This requires a top-level validation framework that builds on validation tests constructed from standardized comparisons and analysis methods. We recently presented a concrete implementation

of such a validation workflow [4], covering diverse topics such as HPC-based simulation using the NEST software, access to the SpiNNaker neuromorphic hardware platform, complex data analysis using the Elephant library, and interactive visualization methods for facilitating further analysis (see Fig.1). Using the Elephant tool based on the universal Neo data model for electrophysiological data, it is guaranteed that identical types of analysis results enter the validation. The validation work based on this prototype is continued conceptually, will enter the universal validation frameworks constructed by the HBP, and forms an important aspect of constructing integrated data processing workflows [5].

A major aspect in the development of workflows that support a reproducible research environment is the accurate, automated, and serviceable tracking of metadata which describe the circumstances under which data was acquired, and how it has been processed. The degree of complexity of electrophysiological experiments has increased dramatically over the last years, in terms of experimental design, amount of recorded data, and sophistication of analysis protocols. In [6] we presented the first detailed practical description of how currently available tools, in particular the open metadata markup language (odML), can be used to describe a complex electrophysiological experiment. Given the positive evaluation and generic nature of this framework, it is natural to extend its application beyond the scope of experimental laboratories, and describe similar data generated in silico. A major challenge here is to include data describing the simulation, which is often present in a procedural fashion, in the key-value-based experimental descriptions of metadata. Recording metadata for simulations carried out on neuromorphic computing is arguably the most challenging scenario, as it combines the need to describe the formal setup of the model and simulation with metadata describing the neuromorphic system itself, which is likely to resemble descriptions obtained from experimental recording systems. In order to grant scientists a chance to truly compare data levied across these different sources, an efficient, unified approach to handling metadata is of significant importance.

This work was partially supported by the Helmholtz Portfolio Theme “Simulation and Modeling for the Human Brain”, JARA and JARA-HPC Seed Fund “NESTML”, EU Grants BrainScaleS and Human Brain Project, the “Initiative and Networking Fund” of the Helmholtz Association, Priority Program 1665 of the DFG, the German Neuroinformatics Node (G-Node, BMBF Grant), ANR-GRASP, CNRS, and RIKEN-CNRS Research Agreement.

[1] https://www.humanbrainproject.eu/en/silicon-brains/

[2] T. Pfeil, J. Jordan et al., Phys. Rev. X 6, 021023 (2016)

[3] D. Plotnikov et al. LNI P-254, 93 (2016)

[4] J. Senk et al., LNCS 10164, 243 (2017)

[5] M. Denker and S. Grün, LNCS 10087, 58 (2016)

[6] L. Zehl et al., Front. Neuroinform. 10, 26 (2016)


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Stochastic optimization of a function-specific nanoelectronic 1R1S-based binary associative connection structure

A. Heittmann and T.G. Noll

Chair of Integrated Digital Systems and Circuit Design

A neuromorphic architecture based on Binary Associative memories and nanoelectronic resistive switches is proposed for the realization of arbitrary logic/arithmetic functions. The set of associative connections if found by stochastic optimization. With the aid of an example based on a 2-ary 3-bit add-modulo-8 operation it is shown that by incorporation of function-specific structural properties in the optimization process the number of amplifiers used to encode function-specific correlations can be reduced by 36%.

Binary neural associative memories (BiNAMs) have been extensively studied in the context of information retrieval, pattern recognition, and brain modelling [1]. Typically, the input/output patterns of the associative memory are considered to be binary vectors. Therefore, BiNAMs are potential candidates for the implementation of logic and arithmetic functions: the r-ary combination of r independent operands is considered as a generalized address used to look up a desired functional output. With the advent of emerging nanoelectronic devices such as resistive switches (RS) [2] the associative memory concept becomes interesting for the dedicated hardware implementation of logic functions. Due to the regular matrix architecture of AMs, simple and area-efficient memory structures can be used for circuit implementation.

The proposed associative memory comprises q word lines (WLs) and p bit lines (BLs), and simple threshold elements [3] (comparators) which transform a bit line signal into a binary output signal (see Fig.1 and Fig.2). For a given mapping of binary vectors X(h)Y(h), h=1..NM so-called connections can be established between WL and BL according to the clipped Hebbian synaptic rule [1]

qjpiYXc hi

hj

N

h

ij

M

..1..1)()(

1

. (1)

In (1) Xj(h) denotes the (binary) state of the j-th

component in the binary vector X(h), Yi(h) the state

of the i-th component in the binary vector Y(h), NM the total number of pairs (X(h),Y(h)), and “” the logical OR function. A connection between a

particular word line j and a bit line i exists if cij = 1 holds.

FIG. 1: Mapping architecture comprising three associative memories M0-M2. Sparsely encoded operands A and B are combined by M0. M1 and M2 map any D to a predefined sparsely encoded output vector Y=F(AB), a. non-trivial code set used to encode A,B, and Y (z = 8 symbols, n = 8 bits).

The mapping of a given input pattern (or symbol) X to an output pattern (or symbol) Y is realized by determining a sum Si for any bit line i=1...p. Si is compared with a given threshold which results in a derived state for the output bit Yi

11 jX ij

q

j ijji ccXS ii SY 1 (2)

FIG. 2: a) exemplary I/V characteristics of a selector device with parameters N=14, V0=2.214V, aP=1.07x10-

10A, aN =0.96x10-10A, bP=8.809V-1, bN=8.545 V-1. Cell composed of a selector S connected to a word line WL and an RS connected to a bit line BL. A HRS of RS isolates WL and BL while a LRS establishes a unidirectional connection between WL and BL. b) section of a BL. The assumed parameters are VR=1.6V,

RL=RLRS=100kW, RHRS=10MW.

S

S

S

S

RL

Y

ATH

D

D

D

D…

VBL

BL

X0=1

X1=1

X2=0

X3=0

I0

I1

I2 ~ 0

I3 ~ 0

IBL= Ij

VR

VR

0V

0V

b.

+

+

+

+

01

010

SV

N

SV

P

N

S

S

Ve

VeV

VI

SN

SP

S

IS

a.

10‐10

0 1 2‐1‐2

10‐8

10‐6

10‐4

10‐2

VS[V]

|IS[A]|

VRS RS

BL

WL

VonVoff

Ioff

Ion

VSIon/Ioff ~ 10

4

WLBL

WL

LRS

HRS

BL


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For many emerging non-volatile memory architectures based on nanoscaled RSs a so-called selector device is connected in series to the RS which effectively suppresses the sneak-path effect during a read operation [4] and allows for the implementation of so-called unidirectional connections [5] (Fig.2). A particular word line WL is considered to be connected to the BL in the case that the RS in a LRS. In turn, a high resistive state (HRS) electrically isolates the WL from the BL. If states of individual bits of an input vector X are assigned to voltage levels VR (representing a logical ‘1’) and ground (representing a logical ‘0’) respectively, the sum (2) is presented by VBL [5].

MAPPING OF OPERANDS BY BINAM A layered architecture comprising 3 BiNAMS connected in series is sufficient to realize any function Y=F(AB) which depends on two independent operands A and B, see Fig.1. In order to find the appropriate connections for M0, M1, and M2 a set of construction principles have to be applied. The following list explains the used fundamentals:

Encoding of states using sparse 2-out-of-n-codes.

Uniformly, set =2 for all amplifiers.

Usage of a thinned out code set C for encoding operands A, B, and Y (“” denotes the bit-wise EXOR function):

CccCccji jiji ,:, (3)

Sparse active connections: for any input pattern and for any BL: at any time a maximum of two connections exist which deliver current to a BL.

Straightforward construction of so-called raw-matrices M0-M2 based on (1) and code sets used for encoding operands A, B, Y=F(AB), and D, see [5].

Optimization (compaction) of raw-matrices M0-M2 by re-structuring using join and disjoint-operations (see [5]) based on (1) while the emerging code set for R has to be strictly a 2-out-of-n-code.

Guidance of optimization by simulated annealing using a dedicated costs function, see Fig.3.

In order to show the effectiveness of the proposed principles a 2-nary 3-bit add-modulo-8 operation was optimized using two different costs functions, see Fig.3.

The first step of optimization relies on the determination of the mixing matrix M0 in order to obtain the code set D. While option 1 targets at a reduction of amplifiers ND blindly, option 2 accepts an improvement only if the mapping-specific set of activated word lines is effectively reduced after a re-structuring action. While |D|=64 holds in both cased, the optimization of M1/M2 clearly benefits from option 2: both, the number of amplifier and the size of the code set |R| (Fig.4a,d) is

significantly lower for option 2 (36% reduction of amplifiers, 48% reduction of code set size).

FIG. 3: Pseudocode for the optimization of M0 and M1/M2.

While the connection densities in M2 remain comparable (upper limit around 50%, which is close to the efficiency limit reported in [1], Fig.4f), M1 is by 25% denser for option 2. Fig.1 shows the optimized connections found by the algorithm.

FIG. 4: Optimization results for M1/M2 targeting ND=22.

The results clearly show that the structure of the code sets (which are either pre-defined or emerge after matrix re-structuring) has an essential impact on the required hardware resources used to represent a function via an associative memory. However, the results also show that it is possible to partly incorporate the function-specific mapping structure in order to guide the optimization towards a more efficient hardware implementation. It is especially noteworthy that it was possible to significantly reduce the size of the code set R. Partly, specific correlations between particular input pattern and output patterns were captured by M1. It is expected that also M0 is able to capture specific correlations which could results again in an improvement of the efficiency.

[1] G. Palm, Biol. Cybernetics 36 (1), 19 (1980)

[2] R. Waser et al., Adv. Mater. 21, 2632 (2009)

[3] A. Heittmann and T.G. Noll, IEEE Silicon Nanoelectronics Workshop 2016, IEEE Electron Devices Society, P2-7 (2016)

[4] E. Cha, J. Woo, D. Lee, S. Lee, and H. Hwang, ISCAS 2014, 428 (2014)

[5] A.Heittmann and T.G. Noll, IEEE/ACM Nanoarch, accepted for publication (2017)

select randomly two active rows rx, ryif (joinable(rx,ry))

rx join(rx,ry) ; ry inactive

if (postcost > precost)recover(rx,ry)

elseif (log(RAND() > -7.0/T)

disjoint(rx)disjoint(ry)

set all rows to active, T = 1repeat until (exit_condition())

T=T+1output active rows

precost=COSTS()

postcost=COSTS()

Option 1:

COSTS() =

Option 2:

COSTS() =

iYBAFi

MiWL

)(:

)( 1#

ND

NR

NR

: M0

: M1/M2

: M0

: M1/M2

M1

M1

pon[x 100 %] pon[x 100 %]

# active connections # active connections

NY = 8

NR [# amplifiers]

|R| [# of different codes]

60

50

40

30

20

ND = 22

120

100

80

60

40

20

0.5

0.4

0.3

0.2

0.110

240

220

200

180

160

140

200

150

100

30

58

23

15992

36

176 140

32.4%

24.1%

54.5%

55.4 %64

250 256256

102 104 106100

102 104 106100 102 104 106100 102 104 106100

102 104 106100 102 104 106100

0.5

0.4

0.3

0.2

0.1

M1

M2

M2

a. b. c.

d. e. f.

T

T T T

TT

option 1

option 2

|D| = 64

rawmatrix

rawmatrix

rawmatrix


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Publications


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Alexeev P., Asadchikov V., Bessas D., Butashin A., Deryabin A., Dill F. U., Ehnes A., Herlitschke M., Hermann R. P., Jafari A., Prokhorov I., Roshchin B., Rohlsberger R., Schlage K., Sergueev I., Siemens A., and Wille H. C. The sapphire backscattering monochromator at the Dynamics beamline P01 of PETRA III Hyperfine Interactions 237, 59 (2016) Appenzeller J., Zhang F., Das S., and Knoch J. Transition Metal Dichalcogenide Schottky Barrier Transistors in 2D Materials for Nanoelectronics, eds. Michel Houssa A. D., Alessandro Molle (CRC Press), 207 (2016) Arango Y. C., Huang L. B., Chen C. Y., Avila J., Asensio M. C., Grutzmacher D., Luth H., Lu J. G., and Schapers T. Quantum transport and nano angle-resolved photoemission spectroscopy on the topological surface states of single Sb2Te3 nanowires Scientific Reports 6, 29493 (2016) Baca S. G., van Leusen J., Speldrich M., and Kogerler P. Understanding the magnetism of Fe2Ln dimers, step-by-step Inorganic Chemistry Frontiers 3, 1071 (2016) Bäumer C., Raab N., Menke T., Schmitz C., Rosezin R., Muller P., Andre M., Feyer V., Bruchhaus R., Gunkel F., Schneider C. M., Waser R., and Dittmann R. Verification of redox-processes as switching and retention failure mechanisms in Nb:SrTiO3/metal devices Nanoscale 8, 13967 (2016) Bäumer C., Schmitz C., Marchewka A., Mueller D. N., Valenta R., Hackl J., Raab N., Rogers S. P., Khan M. I., Nemsak S., Shim M., Menzel S., Schneider C. M., Waser R., and Dittmann R. Quantifying redox-induced Schottky barrier variations in memristive devices via in operando spectromicroscopy with graphene electrodes Nature Communications 7, 12398 (2016) Bagschik K., Fromter R., Bach J., Beyersdorff B., Muller L., Schleitzer S., Berntsen M. H., Weier C., Adam R., Viefhaus J., Schneider C. M., Grubel G., and Oepen H. P. Employing soft x-ray resonant magnetic scattering to study domain sizes and anisotropy in Co/Pd multilayers Physical Review B 94, 134413 (2016) Bagschik K., Fromter R., Muller L., Roseker W., Bach J., Staeck P., Thonnissen C., Schleitzer S., Berntsen M. H., Weier C., Adam R., Viefhaus J., Schneider C. M., Grubel G., and Oepen H. P. Spatial coherence determination from the Fourier analysis of a resonant soft X-ray magnetic speckle pattern Optics Express 24, 23162 (2016) Baksh P. D., Odstrcil M., Kim H. S., Boden S. A., Frey J. G., and Brocklesby W. S. Wide-field broadband extreme ultraviolet transmission ptychography using a high-harmonic source Optics Letters 41, 1317 (2016) Banszerus L., Schmitz M., Engels S., Goldsche M., Watanabe K., Taniguch T., Beschoten B., and Stampfer C. Ballistic transport exceeding 28 μm in CVD grown graphene Nano Letters 16, 1387 (2016) Barzanjeh S. and Vitali D. Phonon Josephson junction with nanomechanical resonators Physical Review A 93, 033846 (2016) Bauer O., Schmitz C. H., Ikonomov J., Willenbockel M., Soubatch S., Tautz F. S., and Sokolowski M. Au enrichment and vertical relaxation of the Cu3Au(111) surface studied by normal-incidence x-ray standing waves Physical Review B 93, 235429 (2016)


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Belabbes A., Bihlmayer G., Bechstedt F., Blügel S., and Manchon A. Hund's rule-driven Dzyaloshinskii-Moriya interaction at 3d-5d interfaces Physical Review Letters 117, 247202 (2016) Belabbes A., Bihlmayer G., Blügel S., and Manchon A. Oxygen-enabled control of Dzyaloshinskii-Moriya interaction in ultra-thin magnetic films Scientific Reports 6, 24634 (2016) Belu A., Schnitker J., Bertazzo S., Neumann E., Mayer D., Offenhäusser A., and Santoro F. Ultra-thin resin embedding method for scanning electron microscopy of individual cells on high and low aspect ratio 3D nanostructures Journal of Microscopy 263, 78 (2016) Berger C. M., Mahmoud A., Hermann R. P., Braun W., Yazhenskikh E., Sohn Y. J., Menzler N. H., Guillon O., and Bram M. Calcium-iron oxide as energy storage medium in rechargeable oxide batteries Journal of the American Ceramic Society 99, 4083 (2016) Bessas D., Winkler M., Sergueev I., Konig J. D., Bottner H., and Hermann R. P. Lattice dynamics in elemental modulated Sb2Te3 films Physica Status Solidi a-Applications and Materials Science 213, 694 (2016) Beyene G. A., Tobin I., Juschkin L., Hayden P., O'Sullivan G., Sokell E., Zakharov V. S., Zakharov S. V., and O'Reilly F. Laser-assisted vacuum arc extreme ultraviolet source: a comparison of picosecond and nanosecond laser triggering Journal of Physics D-Applied Physics 49, 225201 (2016) Bick D. S., Griesche J. D., Schneller T., Staikov G., Waser R., and Valov I. PrxBa1-xCoO3 oxide electrodes for oxygen evolution reaction in alkaline solutions by chemical solution deposition Journal of the Electrochemical Society 163, F166 (2016) Bick D. S., Kindsmuller A., Staikov G., Gunkel F., Muller D., Schneller T., Waser R., and Valov I. Stability and degradation of perovskite electrocatalysts for oxygen evolution reaction Electrochimica Acta 218, 156 (2016) Bindel J. R., Pezzotta M., Ulrich J., Liebmann M., Sherman E. Y., and Morgenstern M. Probing variations of the Rashba spin-orbit coupling at the nanometre scale Nature Physics 12, 920 (2016) Blaeser S., Glass S., Schulte-Braucks C., Narimani K., von den Driesch N., Wirths S., Tiedemann A. T., Trellenkamp S., Buca D., Mantl S., and Zhao Q. T. Line tunneling dominating charge transport in SiGe/Si heterostructure TFETs IEEE Transactions on Electron Devices 63, 4173 (2016) Botzem T., McNeil R. P. G., Mol J. M., Schuh D., Bougeard D., and Bluhm H. Quadrupolar and anisotropy effects on dephasing in two-electron spin qubits in GaAs Nature Communications 7, 11170 (2016) Bouaziz J., Lounis S., Blügel S., and Ishida H. Microscopic theory of the residual surface resistivity of Rashba electrons Physical Review B 94, 045433 (2016) Bouhassoune M., Dias M. D., Zimmermann B., Dederichs P. H., and Lounis S. RKKY-like contributions to the magnetic anisotropy energy: 3d adatoms on Pt(111) surface Physical Review B 94, 125402 (2016) Bourone S. D. M., Kaulen C., Homberger M., and Simon U. Directed self-assembly and infrared reflection absorption spectroscopy analysis of amphiphilic and zwitterionic Janus gold nanoparticles Langmuir 32, 954 (2016)


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Braatz C. R., Esat T., Wagner C., Temirov R., Tautz F. S., and Jakob P. Switching orientation of adsorbed molecules: Reverse domino on a metal surface Surface Science 643, 98 (2016) Bragaglia V., Arciprete F., Zhang W., Mio A. M., Zallo E., Perumal K., Giussani A., Cecchi S., Boschker J. E., Riechert H., Privitera S., Rimini E., Mazzarello R., and Calarco R. Metal - Insulator Transition driven by vacancy ordering in GeSbTe phase change materials Scientific Reports 6, 23843 (2016) Braun L., Mussler G., Hruban A., Konczykowski M., Schumann T., Wolf M., Munzenberg M., Perfetti L., and Kampfrath T. Ultrafast photocurrents at the surface of the three-dimensional topological insulator Bi2Se3 Nature Communications 7, 13259 (2016) Breuckmann N. P. and Terhal B. M. Constructions and noise threshold of hyperbolic surface codes IEEE Transactions on Information Theory 62, 3731 (2016) Breuer T., Nielen L., Roesgen B., Waser R., Rana V., and Linn E. Realization of minimum and maximum gate function in Ta2O5-based memristive devices Scientific Reports 6, 23967 (2016) Brisbois M., Caes S., Sougrati M. T., Vertruyen B., Schrijnemakers A., Cloots R., Eshraghi N., Hermann R. P., Mahmoud A., and Boschini F. Na2FePO4F/multi-walled carbon nanotubes for lithium-ion batteries: Operando Mossbauer study of spray-dried composites Solar Energy Materials and Solar Cells 148, 67 (2016) Broda J., Kuster A., Westhues S., Fahrenkamp D., Vogg A.T.J., Steitz J., Mottaghy F. M., Muller-Newen G., and Simon U. Assessing the intracellular integrity of phosphine-stabilized ultrasmall cytotoxic gold nanoparticles enabled by fluorescence labeling Advanced Healthcare Materials 5, 3118 (2016) Brose S., Danylyuk S., Tempeler J., Kim H. S., Loosen P., and Juschkin L. Enabling laboratory EUV research with a compact exposure tool in Extreme Ultraviolet (EUV) Lithography VII 9776, 97760r (2016) Brose S., Tempeler J., Danylyuk S., Loosen P., and Juschkin L. Achromatic Talbot lithography with partially coherent extreme ultraviolet radiation: process window analysis Journal of Micro-Nanolithography Mems and Moems 15, 043502 (2016) Buca D., von den Driesch N., Stange D., Wirths S., Geiger R., Braucks C. S., Mantl S., Hartmann J. M., Ikonic Z., Witzens J., Sigg H., and Grutzmacher D. GeSn lasers for CMOS integration in IEEE International Electron Devices Meeting (Iedm) (2016) Burgler D. E., Hess V., Esat T., Fahrendorf S., Matthes F., Schneider C. M., Belson C., Monakhov K. Y., Kogerler P., Ghisolfi A., Braunstein P., Atodiresei N., Caciuc V., and Blügel S. Spin-hybrids: a single-molecule approach to spintronics E-Journal of Surface Science and Nanotechnology 14, 17 (2016) Bussmann J., Odstrcil M., Bresenitz R., Rudolf D., Miao J., Brocklesby W. S., and Juschkin L. Coherent diffractive imaging with a laboratory-scale, gas-discharge plasma extreme ultraviolet light source in Springer Proceedings in Physics, 14th Int. Conference on X-Ray Lasers 169, 275 (2016) Cai B. Y., Schwarzkopf J., Hollmann E., Braun D., Schmidbauer M., Grellmann T., and Wordenweber R. Electronic characterization of polar nanoregions in relaxor-type ferroelectric NaNbO3 films Physical Review B 93, 224107 (2016)


130

Carbone C., Moras P., Sheverdyaeva P. M., Pacile D., Papagno M., Ferrari L., Topwal D., Vescovo E., Bihlmayer G., Freimuth F., Mokrousov Y., and Blügel S. Asymmetric band gaps in a Rashba film system Physical Review B 93, 125409 (2016) Caretta A., Casarin B., Di Pietro P., Perucchi A., Lupi S., Bragaglia V., Calarco R., Lange F. R. L., Wuttig M., Parmigiani F., and Malvestuto M. Interband characterization and electronic transport control of nanoscaled GeTe/Sb2Te3 superlattices Physical Review B 94, 045319 (2016) Casarin B., Caretta A., Momand J., Kooi B. J., Verheijen M. A., Bragaglia V., Calarco R., Chukalina M., Yu X. M., Robertson J., Lange F. R. L., Wuttig M., Redaelli A., Varesi E., Parmigiani F., and Malvestuto M. Revisiting the local structure in Ge-Sb-Te based chalcogenide superlattices Scientific Reports 6, 22353 (2016) Caspers C., Gloskovskii A., Gorgoi M., Besson C., Luysberg M., Rushchanskii K.Z., Lezaic M., Fadley C.S., Drube W., and Muller M. Interface engineering to create a strong spin filter contact to silicon Scientific Reports 6, 22912 (2016) Casu E. A., Vitale W. A., Oliva N., Rosca T., Biswas A., Alper C., Krammer A., Luong G. V., Zhao Q. T., Mantl S., Schuler A., Seabaugh A., and Ionescu A. M. Hybrid phase-change - tunnel FET (PC-TFET) switch with subthreshold swing < 10mV/decade and sub-0.1 body factor: digital and analog benchmarking In IEEE International Electron Devices Meeting (Iedm) (2016) Chen L., Li W. F., Maybeck V., Offenhäusser A., and Krause H. J. Statistical study of biomechanics of living brain cells during growth and maturation on artificial substrates Biomaterials 106, 240 (2016) Chen L., Maybeck V., Offenhäusser A., and Krause H. J. Implementation and application of a novel 2D magnetic twisting cytometry based on multi-pole electromagnet Review of Scientific Instruments 87, 064301 (2016) Chen L., Omenzetter A., Schnakenberg U., Maybeck V., Offenhäusser A., and Krause H. J. Passivation of magnetic material used in cell culture environment Sensors and Actuators B-Chemical 236, 85 (2016) Chiesa A., Carretta S., Santini P., Amoretti G., and Pavarini E. Many-body ab initio study of antiferromagnetic Cr7M molecular rings Physical Review B 94, 224422 (2016) Ciccarelli C., Anderson L., Tshitoyan V., Ferguson A. J., Gerhard F., Gould C., Molenkamp L. W., Gayles J., Zelezny J., Smejkal L., Yuan Z., Sinova J., Freimuth F., and Jungwirth T. Room-temperature spin-orbit torque in NiMnSb Nature Physics 12, 855 (2016) Claudio T., Stein N., Petermann N., Stroppa D. G., Koza M. M., Wiggers H., Klobes B., Schierning G., and Hermann R. P. Lattice dynamics and thermoelectric properties of nanocrystalline silicon-germanium alloys Physica Status Solidi a-Applications and Materials Science 213, 515 (2016) Cobanera E., Ulrich J., and Hassler F. Changing anyonic ground degeneracy with engineered gauge fields Physical Review B 94, 125434 (2016) Criger B. and Terhal B. Noise thresholds for the [4,2,2]-concatenated toric code Quantum Information & Computation 16, 1261 (2016)


131

Dai Y., Schubert J., Hollmann E., and Wordenweber R. Electric field induced relaxor behavior in anisotropically strained SrTiO3 films Physica B-Condensed Matter 485, 78 (2016) Dang H. T., Dias M. D., Liebsch A., and Lounis S. Strong correlation effects in theoretical STM studies of magnetic adatoms Physical Review B 93, 115123 (2016) Denker M. and Grün S. Designing workflows for the reproducible analysis of electrophysiological data in Brain-Inspired Computing: Second International Workshop, BrainComp 2015, eds. Amunts K., Grandinetti L., Lippert T. and Petkov N. (Springer International Publishing, Cham), 58 (2016) Deringer V. L. and Dronskowski R. Stabilities and reconstructions of clean PbS and PbSe surfaces: DFT results and the role of dispersion forces Journal of Physical Chemistry C 120, 8813 (2016) di Mario L., Turchini S., Zamborlini G., Feyer V., Tian L., Schneider C. M., Rubini S., and Martelli F. Schottky barrier measurements on individual GaAs nanowires by X-ray photoemission microscopy Applied Surface Science 386, 72 (2016) Dias M. D., Bouaziz J., Bouhassoune M., Blügel S., and Lounis S. Chirality-driven orbital magnetic moments as a new probe for topological magnetic structures Nature Communications 7, 13613 (2016) Dittmann N., Splettstoesser J., and Giazotto F. Clocked single-spin source based on a spin-split superconductor New Journal of Physics 18, 083019 (2016) Doring L., Hengst C., Otto F., and Schafer R. Interacting tails of asymmetric domain walls: Theory and experiments Physical Review B 93, 024414 (2016) Drögeler M., Franzen C., Volmer F., Pohlmann T., Banszerus L., Wolter M., Watanabe K., Taniguchi T., Stampfer C., and Beschoten B. Spin Lifetimes Exceeding 12 ns in Graphene Nonlocal Spin Valve Devices Nano Letters 16, 3533 (2016) Du H. C., Jia C. J., and Mayer J. Surface atomic structure and growth mechanism of monodisperse 100-faceted strontium titanate zirconate nanocubes Chemistry of Materials 28, 650 (2016) Dupe B., Bihlmayer G., Bottcher M., Blügel S., and Heinze S. Engineering skyrmions in transition-metal multilayers for spintronics Nature Communications 7, 11779 (2016) Eissing A. K., Meden V., and Kennes D. M. Functional renormalization group in Floquet space Physical Review B 94, 245116 (2016) Eissing A. K., Meden V., and Kennes D. M. Renormalization in periodically driven quantum dots Physical Review Letters 116, 026801 (2016) Elmers H. J., Wallauer R., Liebmann M., Kellner J., Morgenstern M., Wang R. N., Boschker J. E., Calarco R., Sanchez-Barriga J., Rader O., Kutnyakhov D., Chernov S. V., Medjanik K., Tusche C., Ellguth M., Volfova H., Borek S., Braun J., Minar J., Ebert H., and Schonhense G. Spin mapping of surface and bulk Rashba states in ferroelectric alpha-GeTe(111) films Physical Review B 94, 201403 (2016)


132

Esat T., Lechtenberg B., Deilmann T., Wagner C., Kruger P., Temirov R., Rohlfing M., Anders F. B., and Tautz F. S. A chemically driven quantum phase transition in a two-molecule Kondo system Nature Physics 12, 867 (2016) Feng L. Y., Lyu Z. Z., Offenhäusser A., and Mayer D. Electrochemically triggered aptamer immobilization via click reaction for vascular endothelial growth factor detection Engineering in Life Sciences 16, 550 (2016) Fischer I. A., Augel L., Kropp T., Jitpakdeebodin S., Franz N., Oliveira F., Rolseth E., Mass T., Taubner T., and Schulze J. Ge-on-Si PIN-photodetectors with Al nanoantennas: The effect of nanoantenna size on light scattering into waveguide modes Applied Physics Letters 108, 071108 (2016) Fischer S., Kremer H. I., Berghoff B., Mass T., Taubner T., and Knoch J. Dopant-free complementary metal oxide silicon field effect transistors Physica Status Solidi a-Applications and Materials Science 213, 1494 (2016) Fleck K., Aslam N., Hoffmann-Eifert S., Longo V., Roozeboom F., Kessels W. M. M., Bottger U., Waser R., and Menzel S. The influence of non-stoichiometry on the switching kinetics of strontium-titanate ReRAM devices Journal of Applied Physics 120, 244502 (2016) Fleck K., Bottger U., Waser R., Aslam N., Hoffmann-Eifert S., and Menzel S. Energy dissipation during pulsed switching of strontium-titanate based resistive switching memory devices in Proceedings of the 46th European Solid-State Device Research Conference (Essderc), 160 (2016) Fleck K., La Torre C., Aslam N., Hoffmann-Eifert S., Bottger U., and Menzel S. Uniting Gradual and Abrupt SET Processes in Resistive Switching Oxides Physical Review Applied 6, 064015 (2016) Fox A., Mikulics M., Marso M., Kocan M., Sofer Z., Kordos P., Luth H., Schubert J., Grutzmacher D., and Hardtdegen H. Highly reliable long-term operation of AlGaN/GaN/AlN HFETs grown on silver substrate in 11th Int. Conference on Advanced Semiconductor Devices & Microsystems (Asdam), 65 (2016) Freimuth F., Blügel S., and Mokrousov Y. Laser-induced torques in metallic ferromagnets Physical Review B 94, 144432 (2016) Freimuth F., Blügel S., and Mokrousov Y. The inverse thermal spin-orbit torque and the relation of the Dzyaloshinskii-Moriya interaction to ground-state energy currents Journal of Physics-Condensed Matter 28, 316001 (2016) Freitag N. M., Chizhova L. A., Nemes-Incze P., Woods C. R., Gorbachev R. V., Cao Y., Geim A. K., Novoselov K. S., Burgdorfer J., Libisch F., and Morgenstern M. Electrostatically confined monolayer graphene quantum dots with orbital and valley splittings Nano Letters 16, 5798 (2016) Friedrich R., Caciuc V., Atodiresei N., and Blügel S. Exchange interactions of magnetic surfaces below two-dimensional materials Physical Review B 93, 220406 (2016) Fu W. Y., Feng L. Y., Mayer D., Panaitov G., Kireev D., Offenhäusser A., and Krause H. J. Electrolyte-gated graphene ambipolar frequency multipliers for biochemical sensing Nano Letters 16, 2295 (2016) Fu Z. D., Nair H. S., Xiao Y. G., Senyshyn A., Pomjakushin V. Y., Feng E. X., Su Y. X., Jin W. T., and Bruckel T. Magnetic structures and magnetoelastic coupling of Fe-doped hexagonal manganites LuMn1-xFexO3

(0 <= x <= 0.3) Physical Review B 94, 125150 (2016)


133

Fu Z. D., Xiao Y. G., Feoktystov A., Pipich V., Appavou M.S., Su Y.X., Feng E.X., Jin W.T., and Bruckel T. Field-induced self-assembly of iron oxide nanoparticles investigated using small-angle neutron scattering Nanoscale 8, 18541 (2016) Funari R., Della Ventura B., Altucci C., Offenhäusser A., Mayer D., and Velotta R. Single molecule characterization of UV-activated antibodies on gold by atomic force microscopy Langmuir 32, 8084 (2016) Funck C., Hoffmann-Eifert S., Waser R., and Menzel S. Simulation of threshold switching based on an electric field induced thermal runaway in International Conference on Simulation of Semiconductor Processes and Devices (Sispad), 319 (2016) Funck C., Menzel S., Aslam N., Zhang H. H., Hardtdegen A., Waser R., and Hoffmann-Eifert S. Multidimensional simulation of threshold switching in NbO2 based on an electric field triggered thermal runaway model Advanced Electronic Materials 2, 1600169 (2016) Gaillardon P. E., Amaru L., Siemon A., Linn E., Waser R., Chattopadhyay A., and De Micheli G. The Programmable Logic-in-Memory (PLiM) computer in Proceedings of the 2016 Design, Automation & Test in Europe Conference & Exhibition (Date), 427 (2016) Galanakis I., Ozdogan K., and Sasioglu E. Spin-filter and spin-gapless semiconductors: The case of Heusler compounds AIP Advances 6, 055606 (2016) Galanakis I., Tirpanci S., Ozdogan K., and Sasioglu E. Itinerant G-type antiferromagnetism in D03-type V3Z (Z = Al, Ga, In) compounds: A first-principles study Physical Review B 94, 064401 (2016) Gehlmann M., Aguilera I., Bihlmayer G., Mlynczak E., Eschbach M., Doring S., Gospodaric P., Cramm S., Kardynal B., Plucinski L., Blügel S., and Schneider C. M. Quasi 2D electronic states with high spin-polarization in centrosymmetric MoS2 bulk crystals Scientific Reports 6, 26197 (2016) Genreith-Schriever A. R. and De Souza R. A. Field-enhanced ion transport in solids: Reexamination with molecular dynamics simulations Physical Review B 94, 224304 (2016) Georgi A., Nemes-Incze P., Szafranek B., Neumaier D., Geringer V., Liebmann M., and Morgenstern M. Apparent rippling with honeycomb symmetry and tunable periodicity observed by scanning tunneling microscopy on suspended graphene Physical Review B 94, 184302 (2016) Geranton G., Zimmermann B., Long N. H., Mavropoulos P., Blügel S., Freimuth F., and Mokrousov Y. Spin-orbit torques and spin accumulation in FePt/Pt and Co/Cu thin films from first principles: The role of impurities Physical Review B 93, 224420 (2016) Grutzmacher D., Mikulics M., and Hardtdegen H. Low-energy consumption nano-opto-electronics based on III-nitride-LED mesoscopic structures in 46th European Solid-State Device Research Conference (Essderc), 327 (2016) Grutzmacher D., Stange D., von den Driesch N., Wirths S., Mantl S., Buca D. M., Hartmann J. M., Geiger R., and Sigg H. On the track towards an electrically pumped group IV laser in IEEE Silicon Nanoelectronics Workshop (Snw), 124 (2016) Gunkel F., Bell C., Inoue H., Kim B., Swartz A. G., Merz T. A., Hikita Y., Harashima S., Sato H. K., Minohara M., Hoffmann-Eifert S., Dittmann R., and Hwang H. Y. Defect control of conventional and anomalous electron transport at complex oxide interfaces Physical Review X 6, 031035 (2016)


134

Gunkel F., Waser R., Ramadan A. H. H., De Souza R. A., Hoffmann-Eifert S., and Dittmann R. Space charges and defect concentration profiles at complex oxide interfaces Physical Review B 93, 245431 (2016) Gustavsson S., Yan F., Catelani G., Bylander J., Kamal A., Birenbaum J., Hover D., Rosenberg D., Samach G., Sears A. P., Weber S. J., Yoder J. L., Clarke J., Kerman A. J., Yoshihara F., Nakamura Y., Orlando T. P., and Oliver W. D. Suppressing relaxation in superconducting qubits by quasiparticle pumping Science 354, 1573 (2016) Gutrath B. S., Schiefer F., Homberger M., Englert U., Serb M. D., Bettray W., Beljakov I., Meded V., Wenzel W., and Simon U. Molecular and electronic structure of the cluster [Au8(PPh3)8](NO3)2 European Journal of Inorganic Chemistry 2016, 975 (2016) Haas F., Wenz T., Zellekens P., Demarina N., Rieger T., Lepsa M., Grutzmacher D., Luth H., and Schapers T. Angle-dependent magnetotransport in GaAs/InAs core/shell nanowires Scientific Reports 6, 24573 (2016) Hagen E., Dahmen D., Stavrinou M. L., Linden H., Tetzlaff T., van Albada S. J., Grun S., Diesmann M., and Einevoll G. T. Hybrid Scheme for Modeling Local Field Potentials from Point-Neuron Networks Cerebral Cortex 26, 4461 (2016) Hahn H., Funck C., Geipel S., Kalisch H., and Vescan A. The III-Nitride Double Heterostructure Revisited: Benefits for Threshold Voltage Engineering of MIS Devices IEEE Transactions on Electron Devices 63, 606 (2016) Hajiheidari F., Zhang W., and Mazzarello R. Effects of a magnetic Fe monolayer on the structural and surface electronic properties of Sb2Te3 Physical Review B 94, 125421 (2016) Hamad E. M., Bilatto S. E. R., Adly N. Y., Correa D. S., Wolfrum B., Schnoing M. J., Offenhäusser A., and Yakushenko A. Inkjet printing of UV-curable adhesive and dielectric inks for microfluidic devices Lab on a Chip 16, 70 (2016) Han J. H., Muralidhar R., Waser R., and Bazant M. Z. Resistive Switching in Aqueous Nanopores by Shock Electrodeposition Electrochimica Acta 222, 370 (2016) Han Q., Dang H. T., and Millis A. J. Ferromagnetism and correlation strength in cubic barium ruthenate in comparison to strontium and calcium ruthenate: A dynamical mean-field study Physical Review B 93, 155103 (2016) Hanke J. P., Freimuth F., Nandy A. K., Zhang H., Blügel S., and Mokrousov Y. Role of Berry phase theory for describing orbital magnetism: From magnetic heterostructures to topological orbital ferromagnets Physical Review B 94, 121114 (2016) Hardtdegen A., La Torre C., Zhang H., Funck C., Menzel S., Waser R., and Hoffmann-Eifert S. Internal Cell Resistance as the Origin of Abrupt Reset Behavior in HfO2-based Devices determined from Current Compliance Series in IEEE 8th International Memory Workshop (Imw) (2016) Hardtdegen H. and Mikulics M. Towards III-nitride nano-LED based single photon emitters: technology and applications in 11th International Conference on Advanced Semiconductor Devices & Microsystems, eds. Hascik S., Dzuba J., and Vanko G., 27 (2016)


135

Hardtdegen H., Riess S., Schuck M., Keller K., Jost P., Du H., Bornhofft M., Schwedt A., Mussler G., von der Ahe M., Mayer J., Roth G., Grutzmacher D., and Mikulics M. A model structure for interfacial phase change memories: Epitaxial trigonal Ge1Sb2Te4 Journal of Alloys and Compounds 679, 285 (2016) Havel V., Fleck K., Rosgen B., Rana V., Menzel S., Bottger U., and Waser R. Ultrafast Switching in Ta2O5-based Resistive Memories in IEEE Silicon Nanoelectronics Workshop (Snw), 82 (2016) Heedt S., Manolescu A., Nemnes G. A., Prost W., Schubert J., Grutzmacher D., and Schapers T. Adiabatic Edge Channel Transport in a Nanowire Quantum Point Contact Register Nano Letters 16, 4569 (2016) Heedt S., Prost W., Schubert J., Grutzmacher D., and Schapers T. Ballistic Transport and Exchange Interaction in InAs Nanowire Quantum Point Contacts Nano Letters 16, 3116 (2016) Heedt S., Vakulov D., Rieger T., Rosenbach D., Trellenkamp S., Grutzmacher D., Lepsa M. I., and Schapers T. Electronic Properties of Complex Self-Assembled InAs Nanowire Networks Advanced Electronic Materials 2, 1500460 (2016) Hell M., Wegewijs M. R., and DiVincenzo D. P. Qubit quantum-dot sensors: Noise cancellation by coherent backaction, initial slips, and elliptical precession Physical Review B 93, 045418 (2016) Hentschel R., Wachowiak A., Grosser A., Jahn A., Merkel U., Wille A., Kalisch H., Vescan A., Schmult S., and Mikolajick T. Pseudo-vertical GaN-based trench gate metal oxide semiconductor field effect transistor in 11th International Conference on Advanced Semiconductor Devices & Microsystems (Asdam), 5 (2016) Heo C., Kiselev N. S., Nandy A. K., Blügel S., and Rasing T. Switching of chiral magnetic skyrmions by picosecond magnetic field pulses via transient topological states Scientific Reports 6, 27146 (2016) Herlitschke M., Disch S., Sergueev I., Schlage K., Wetterskog E., Bergstrom L., and Hermann R. P. Spin disorder in maghemite nanoparticles investigated using polarized neutrons and nuclear resonant scattering in International Conference on Polarised Neutrons for Condensed Matter Investigations, eds. Lee W.T.H., McIntyre G. and Rule K., 711, 012002 (2016) Herlitschke M., Klobes B., Sergueev I., Hering P., Peron J., and Hermann R. P. Elasticity and magnetocaloric effect in MnFe4Si3 Physical Review B 93, 094304 (2016) Hermanova S., Zarevucka M., Bousa D., Mikulics M., and Sofer Z. Lipase enzymes on graphene oxide support for high-efficiency biocatalysis Applied Materials Today 5, 200 (2016) Herzog F., Heedt S., Goerke S., Ibrahim A., Rupprecht B., Heyn C., Hardtdegen H., Schapers T., Wilde M. A., and Grundler D. Confinement and inhomogeneous broadening effects in the quantum oscillatory magnetization of quantum dot ensembles Journal of Physics-Condensed Matter 28, 045301 (2016) Hesselmann S. and Wessel S. Thermal Ising transitions in the vicinity of two-dimensional quantum critical points Physical Review B 93, 155157 (2016)


136

Honecker A., Wessel S., Kerkdyk R., Pruschke T., Mila F., and Normand B. Thermodynamic properties of highly frustrated quantum spin ladders: Influence of many-particle bound states Physical Review B 93, 054408 (2016) Hong T., Wang H., Zhang Y., Krause H. J., Braginski A. I., Xie X. M., Offenhäusser A., and Jiang M. H. Flux modulation scheme for direct current SQUID readout revisited Applied Physics Letters 108, 062601 (2016) Hooda S., Khan S. A., Satpati B., Stange D., Buca D., Bala M., Pannu C., Kanjilal D., and Kabiraj D. Effect of ion beam parameters on engineering of nanoscale voids and their stability under post-growth annealing Applied Physics a-Materials Science & Processing 122, 227 (2016) Horakova K., Chab V., Heczko O., Drchal V., Fekete L., Honolka J., Kopecek J., Kudrnovsky J., Polyak Y., Sajdl P., Vondracek M., Lancok J., Feyer V., Wiemann C., and Schneider C. M. Surface analysis of the Heusler Ni49.7Mn29.1Ga21.2 Alloy: The composition, phase transition, and twinned microstructure of martensite Journal of Applied Physics 120, 113905 (2016) Horst F., Graef M., Hosenfeld F., Farokhnejad A., Hain F., Luong G. V., Zhao Q. T., Iniguez B., Kloes A., and IEEE Implementation of a DC Compact Model for Double-Gate Tunnel-FET Based on 2D Calculations and Application in Circuit Simulation in 46th European Solid-State Device Research Conference, 456 (2016) Huisman T. J., Mikhaylovskiy R. V., Costa J. D., Freimuth F., Paz E., Ventura J., Freitas P. P., Blügel S., Mokrousov Y., Rasing T., and Kimel A. V. Femtosecond control of electric currents in metallic ferromagnetic heterostructures Nature Nanotechnology 11, 455 (2016) Ibanez-Azpiroz J., Dias M. D., Blügel S., and Lounis S. Zero-Point Spin-Fluctuations of Single Adatoms Nano Letters 16, 4305 (2016) Izarova N. V., Santiago-Schubel B., Willbold S., Hess V., and Kogerler P. Classical/Non-classical Polyoxometalate Hybrids Chemistry-a European Journal 22, 16052 (2016) Jin J.H., Stoica T., Trellenkamp S., Chen Y., Anttu N., Migunov V., Kawabata R.M.S., Buenconsejo P.J.S., Lam Y.M., Haas F., Hardtdegen H., Grutzmacher D., and Kardynal B.E. Dense, Regular GaAs Nanowire Arrays by Catalyst-Free Vapor Phase Epitaxy for Light Harvesting ACS Applied Materials & Interfaces 8, 22484 (2016) Jin W. T., Xiao Y., Bukowski Z., Su Y., Nandi S., Sazonov A. P., Meven M., Zaharko O., Demirdis S., Nemkovski K., Schmalzl K., Tran L. M., Guguchia Z., Feng E., Fu Z., and Bruckel T. Phase diagram of Eu magnetic ordering in Sn-flux-grown Eu(Fe1-xCox)2As2 single crystals Physical Review B 94, 184513 (2016) Jin W. T., Xiao Y., Su Y., Nandi S., Jiao W. H., Nisbet G., Demirdis S., Cao G. H., and Bruckel T. Magnetic polarization of Ir in underdoped nonsuperconducting Eu(Fe0.94Ir0.06)2As2 Physical Review B 93, 024517 (2016) Johnen S., Meissner F., Krug M., Baltz T., Endler I., Mokwa W., and Walter P. Properties of Retinal Precursor Cells Grown on Vertically Aligned Multiwalled Carbon Nanotubes Generated for the Modification of Retinal Implant-Embedded Microelectrode Arrays Journal of Ophthalmology 2016, 2371021 (2016) Jung L. N., Hauer B., Li P. N., Bornhofft M., Mayer J., and Taubner T. Exploring the detection limits of infrared near-field microscopy regarding small buried structures and pushing them by exploiting superlens-related effects Optics Express 24, 4431 (2016)


137

Kaes M. and Salinga M. Impact of defect occupation on conduction in amorphous Ge2Sb2Te5 Scientific Reports 6, 31699 (2016) Kammermeier M., Wenk P., Schliemann J., Heedt S., and Schapers T. Weak (anti)localization in tubular semiconductor nanowires with spin-orbit coupling Physical Review B 93, 205306 (2016) Kampmeier J., Weyrich C., Lanius M., Schall M., Neumann E., Mussler G., Schapers T., and Grutzmacher D. Selective area growth of Bi2Te3 and Sb2Te3 topological insulator thin films Journal of Crystal Growth 443, 38 (2016) Kao Y. J., Huang C. Y., Su C. Y., Pithan C., Hennings D. F., and Waser R. Hydroxyl defect effect on the resistance degradation behavior in Y-doped (Ba,Ca)(Ti,Zr)O3 bulk ceramics Journal of the European Ceramic Society 36, 3147 (2016) Kao Y. J., Su C. Y., Pithan C., Hennings D. F., Huang C. Y., and Waser R. Hydroxyl Defect Effect on Reoxidation of Sc-Doped (Ba,Ca)(Ti,Zr)O3 Fired in Reducing Atmospheres Journal of the American Ceramic Society 99, 1311 (2016) Karrasch C., Kennes D. M., and Heidrich-Meisner F. Thermal Conductivity of the One-Dimensional Fermi-Hubbard Model Physical Review Letters 117, 116401 (2016) Kennes D. M. and Karrasch C. Entanglement scaling of excited states in large one-dimensional many-body localized systems Physical Review B 93, 245129 (2016) Kennes D. M. and Karrasch C. Extending the range of real time density matrix renormalization group simulations Computer Physics Communications 200, 37 (2016) Khajetoorians A. A., Steinbrecher M., Ternes M., Bouhassoune M., Dias M. D., Lounis S., Wiebe J., and Wiesendanger R. Tailoring the chiral magnetic interaction between two individual atoms Nature Communications 7, 10620 (2016) Khanbabaee B., Bussone G., Knutsson J. V., Geijselaers I., Pryor C. E., Rieger T., Demarina N., Grutzmacher D., Lepsa M. I., Timm R., and Pietsch U. Band bending at the heterointerface of GaAs/InAs core/shell nanowires monitored by synchrotron X-ray photoelectron spectroscopy Journal of Applied Physics 120, 145703 (2016) Kiani A. and Pavarini E. Electronic correlation and magnetic frustration in Li2VOSiO4 and VOMoO4 Physical Review B 94, 075112 (2016) Kieslich G., Cerretti G., Veremchuk I., Hermann R. P., Panthofer M., Grin J., and Tremel W. A chemists view: Metal oxides with adaptive structures for thermoelectric applications Physica Status Solidi a-Applications and Materials Science 213, 808 (2016) Kim H. S., Baksh P., Odstrcil M., Miszczak M., Frey J. G., Juschkin L., and Brocklesby W. S. Lloyd's mirror interference lithography with EUV radiation from a high-harmonic source Applied Physics Express 9, 076701 (2016) Kim H. S., Danylyuk S., Brocklesby W. S., and Juschkin L. Single Exposure Imaging of Talbot Carpets and Resolution Characterization of Detectors for Micro- and Nano- Patterns Journal of the Optical Society of Korea 20, 245 (2016) Kim H. S., Danylyuk S., Brose S., Loosen P., Bergmann K., Brocklesby W. S., and Juschkin L. Lensless Proximity EUV Lithography with a Xenon Gas Discharge Plasma Radiation in Springer Proceedings in Physics, 14th Int. Conference on X-Ray Lasers 169, 313 (2016)


138

Kim H. S., Li W., Marconi M. C., Brocklesby W. S., and Juschkin L. Restorative Self-Image of Rough-Line Grids: Application to Coherent EUV Talbot Lithography IEEE Photonics Journal 8, 2600209 (2016) Kim W., Chattopadhyay A., Siemon A., Linn E., Waser R., and Rana V. Multistate Memristive Tantalum Oxide Devices for Ternary Arithmetic Scientific Reports 6, 36652 (2016) Kim W., Hardtdegen A., Rodenbucher C., Menzel S., Wouters D. J., Hoffmann-Eifert S., Buca D., Waser R., and Rana V. Forming-Free Metal-Oxide ReRAM by Oxygen Ion Implantation Process in IEEE International Electron Devices Meeting (Iedm) (2016) Kim W., Menzel S., Wouters D. J., Guo Y. Z., Robertson J., Roesgen B., Waser R., and Rana V. Impact of oxygen exchange reaction at the ohmic interface in Ta2O5-based ReRAM devices Nanoscale 8, 17774 (2016) Kim W., Menzel S., Wouters D. J., Waser R., and Rana V. 3-Bit Multilevel Switching by Deep Reset Phenomenon in Pt/W/TaOX/Pt-ReRAM Devices IEEE Electron Device Letters 37, 564 (2016) Kim W., Rosgen B., Breuer T., Menzel S., Wouters D., Waser R., and Rana V. Nonlinearity analysis of TaOx redox-based RRAM Microelectronic Engineering 154, 38 (2016) Kim W., Wouters D. J., Menzel S., Rodenbucher C., Waser R., and Rana V. Lowering Forming Voltage and Forming-Free Behavior of Ta2O5 ReRAM Devices in 46th European Solid-State Device Research Conference (Essderc), 164 (2016) Kireev D., Sarik D., Wu T., Xie X., Wolfrum B., and Offenhäusser A. High throughput transfer technique: Save your graphene Carbon 107, 319 (2016) Klobes B., Arinicheva Y., Neumeier S., Simon R. E., Jafari A., Bosbach D., and Hermann R. P. Quadrupole splitting and Eu partial lattice dynamics in europium orthophosphate EuPO4 Hyperfine Interactions 237, 31 (2016) Klobes B., Hu M. Y., Beekman M., Johnson D. C., and Hermann R. P. Confined lattice dynamics of single and quadruple SnSe bilayers in [(SnSe)1.04]m[MoSe2]n ferecrystals Nanoscale 8, 856 (2016) Knoch J. Nanowire Tunneling Field-Effect Transistors in Semiconductors and Semimetals, Vol 94: Semiconductor Nanowires II: Properties and Applications 94, 273 (2016) Koelmans W. W., Bachmann T., Zipoli F., Ott A. K., Dou C., Ferrari A. C., Cojocaru-Miredin O., Zhang S., Scheu C., Wuttig M., Nagareddy V. K., Craciun M. F., Alexeev A. M., Wright C. D., Jonnalagadda V. P., Curioni A., Sebastian A., and Eleftheriou E. Carbon-based resistive memories in IEEE 8th International Memory Workshop (Imw) (2016) Konschelle F., Tokatly I. V., and Bergeret F. S. Ballistic Josephson junctions in the presence of generic spin dependent fields Physical Review B 94, 014515 (2016) Konze P. M., Deringer V. L., and Dronskowski R. Understanding the Shape of GeTe Nanocrystals from First Principles Chemistry of Materials 28, 6682 (2016) Koposova E., Liu X., Pendin A., Thiele B., Shumilova G., Ermolenko Y., Offenhäusser A., and Mourzina Y. Influence of Meso-Substitution of the Porphyrin Ring on Enhanced Hydrogen Evolution in a Photochemical System Journal of Physical Chemistry C 120, 13873 (2016)


139

Kovacik R., Murthy S. S., Quiroga C. E., Ederer C., and Franchini C. Combined first-principles and model Hamiltonian study of the perovskite series RMnO3 (R = La,Pr,Nd,Sm,Eu, and Gd) Physical Review B 93, 075139 (2016) Krasovskii E. E., Friedrich C., Schattke W., and Echenique P. M. Rapid propagation of a Bloch wave packet excited by a femtosecond ultraviolet pulse Physical Review B 94, 195434 (2016) Krause K. J., Adly N., Yakushenko A., Schnitker J., Mayer D., Offenhäusser A., and Wolfrum B. Influence of Self-Assembled Alkanethiol Monolayers on Stochastic Amperometric On-Chip Detection of Silver Nanoparticles Analytical Chemistry 88, 3632 (2016) Kroger I., Stadtmuller B., and Kumpf C. Submonolayer and multilayer growth of titaniumoxide-phthalocyanine on Ag(111) New Journal of Physics 18, 113022 (2016) Krug I.P., Doganay H., Nickel F., Gottlob D. M., Schneider C.M., Morelli A., Preziosi D., Lindfors-Vrejoiu I., Laskowski R., and Barrett N. Interface-mediated ferroelectric patterning and Mn valency in nano-structured PbTiO3/La0.7Sr0.3MnO3 Journal of Applied Physics 120, 095304 (2016) Ksh P. B., Odstreil M., Kim H., Boden S. A., Card R., Chad J., Frey J. G., and Brucklesby W. S. High Resolution, Wide Field of View, Ptychographic Imaging of a Biological Sample using a High Harmonic Generation Source in Conference on Lasers and Electro-Optics (Cleo) (2016) Kumar C. M. N., Xiao Y., Nair H. S., Voigt J., Schmitz B., Chatterji T., Jalarvo N. H., and Bruckel T. Hyperfine and crystal field interactions in multiferroic HoCrO3 Journal of Physics-Condensed Matter 28, 476001 (2016) Kundu P., Belu A., Neumann E., Mayer D., and Offenhäusser A. 3D Au-SiO2 nanohybrids as a potential scaffold coating material for neuroengineering RSC Advances 6, 47948 (2016) Kura C., Aoki Y., Tsuji E., Habazaki H., and Martin M. Fabrication of a resistive switching gallium oxide thin film with a tailored gallium valence state and oxygen deficiency by rf cosputtering process RSC Advances 6, 8964 (2016) La Torre C., Fleck K., Starschich S., Linn E., Waser R., and Menzel S. Dependence of the SET switching variability on the initial state in HfOx-based ReRAM Physica Status Solidi a-Applications and Materials Science 213, 316 (2016) Lanius M., Kampmeier J., Kolling S., Mussler G., Koenraad P. M., and Grutzmacher D. Topography and structure of ultrathin topological insulator Sb2Te3 films on Si(111) grown by means of molecular beam epitaxy Journal of Crystal Growth 453, 158 (2016) Lanius M., Kampmeier J., Weyrich C., Kolling S., Schall M., Schuffelgen P., Neumann E., Luysberg M., Mussler G., Koenraad P. M., Schapers T., and Grutzmacher D. P-N Junctions in Ultrathin Topological Insulator Sb2Te3/Bi2Te3 Heterostructures Grown by Molecular Beam Epitaxy Crystal Growth & Design 16, 2057 (2016) Lasri K., Mahmoud A., Saadoune I., Sougrati M. T., Stievano L., Lippens P. E., Hermann R. P., and Ehrenberg H. Toward understanding the lithiation/delithiation process in Fe0.5TiOPO4/C electrode material for lithium-ion batteries Solar Energy Materials and Solar Cells 148, 11 (2016)


140

Lee W., Yoo S., Yoon K.J., Yeu I.W., Chang H.J., Choi J.H., Hoffmann-Eifert S., Waser R., and Hwang C.S. Resistance switching behavior of atomic layer deposited SrTiO3 film through possible formation of Sr2Ti6O13 or Sr1Ti11O20 phases Scientific Reports 6, 20550 (2016) Leinen P., Green M. F. B., Esat T., Wagner C., Tautz F. S., and Temirov R. Hand Controlled Manipulation of Single Molecules via a Scanning Probe Microscope with a 3D Virtual Reality Interface Jove-Journal of Visualized Experiments, e54506 (2016) Lejaeghere K., Bihlmayer G., Bjorkman T., Blaha P., Blügel S., Blum V., Caliste D., Castelli I. E., Clark S.J., Dal Corso A., de Gironcoli S., Deutsch T., Dewhurst J.K., Di Marco I., Draxl C., Dulak M., Eriksson O., Flores-Livas J. A., Garrity K. F., Genovese L., Giannozzi P., Giantomassi M., Goedecker S., Gonze X., Granas O., Gross E. K.U., Gulans A., Gygi F., Hamann D. R., Hasnip P. J., Holzwarth N.A.W., Iusan D., Jochym D. B., Jollet F., Jones D., Kresse G., Koepernik K., Kucukbenli E., Kvashnin Y. O., Locht I. L.M., Lubeck S., Marsman M., Marzari N., Nitzsche U., Nordstrom L., Ozaki T., Paulatto L., Pickard C.J., Poelmans W., Probert M. I. J., Refson K., Richter M., Rignanese G. M., Saha S., Scheffler M., Schlipf M., Schwarz K., Sharma S., Tavazza F., Thunstrom P., Tkatchenko A., Torrent M., Vanderbilt D., van Setten M.J., Van Speybroeck V., Wills J. M., Yates J. R., Zhang G. X., and Cottenier S. Reproducibility in density functional theory calculations of solids Science 351, aad3000 (2016) Lepsa M. I., Rieger T., Zellekens P., Hackemuller F. J., Schapers T., and Grutzmacher D. Structural and Electrical Properties of GaAs/InSb Core-Shell Nanowires in Compound Semiconductor Week (Csw) Includes 28th International Conference on Indium Phosphide & Related Materials (Iprm) & 43rd International Symposium on Compound Semiconductors (Iscs) (2016) Li P. N., Yang X. S., Mass T. W. W., Hanss J., Lewin M., Michel A. K. U., Wuttig M., and Taubner T. Reversible optical switching of highly confined phonon-polaritons with an ultrathin phase-change material Nature Materials 15, 870 (2016) Li Y. Q., Liu X. H., and Dronskowski R. Synthesis and Structure Determination of the Quaternary Zinc Nitride Halides Zn2NX1-yX'y (X, X' = Cl, Br, I; 0 < y < 1) Inorganics 4, 29 (2016) Liebmann M., Rinaldi C., Di Sante D., Kellner J., Pauly C., Wang R. N., Boschker J. E., Giussani A., Bertoli S., Cantoni M., Baldrati L., Asa M., Vobornik I., Panaccione G., Marchenko D., Sanchez-Barriga J., Rader O., Calarco R., Picozzi S., Bertacco R., and Morgenstern M. Giant Rashba-Type Spin Splitting in Ferroelectric GeTe(111) Advanced Materials 28, 560 (2016) Liu C., Han Q., Luong G. V., Narimani K., Glass S., Tiedemann A. T., Trellenkamp S., Yu W. J., Wang X., Mantl S., and Zhao Q. T. Si n-TFETs on ultra thin body with suppressed ambipolarity in 46th European Solid-State Device Research Conference (Essderc), 408 (2016) Liu C., Han Q. H., Glass S., Luong G. V., Narimani K., Tiedemann A. T., Fox A., Yu W. J., Wang X., Mantl S., and Zhao Q. T. Experimental I-V(T) and C-V Analysis of Si Planar p-TFETs on Ultrathin Body IEEE Transactions on Electron Devices 63, 5036 (2016) Liu M., Klobes B., and Banhart J. Positron lifetime study of the formation of vacancy clusters and dislocations in quenched Al, Al-Mg and Al-Si alloys Journal of Materials Science 51, 7754 (2016) Lloyd S. and Terhal B. M. Adiabatic and Hamiltonian computing on a 2D lattice with simple two-qubit interactions New Journal of Physics 18, 023042 (2016)


141

Long N. H., Mavropoulos P., Bauer D. S. G., Zimmermann B., Mokrousov Y., and Blügel S. Strong spin-orbit fields and Dyakonov-Perel spin dephasing in supported metallic films Physical Review B 94, 180406 (2016) Long N. H., Mavropoulos P., Zimmermann B., Blügel S., and Mokrousov Y. Giant spin Nernst effect induced by resonant scattering at surfaces of metallic films Physical Review B 93, 180406 (2016) Lukens G., Yacoub H., Kalisch H., and Vescan A. Limitations of threshold voltage engineering of AlGaN/GaN heterostructures by dielectric interface charge density and manipulation by oxygen plasma surface treatments Journal of Applied Physics 119, 205705 (2016) Lukens W. W., Speldrich M., Yang P., Duignan T. J., Autschbach J., and Kogerler P. The roles of 4f-and 5f-orbitals in bonding: a magnetochemical, crystal field, density functional theory, and multi-reference wavefunction study Dalton Transactions 45, 11508 (2016) Luong G. V., Narimani K., Tiedemann A. T., Bernardy P., Trellenkamp S., Zhao Q. T., and Mantl S. Complementary Strained Si GAA Nanowire TFET Inverter With Suppressed Ambipolarity IEEE Electron Device Letters 37, 950 (2016) Luong G. V., Strangio S., Tiedemannn A., Lenk S., Trellenkamp S., Bourdelle K. K., Zhao Q. T., and Mantl S. Experimental demonstration of strained Si nanowire GAA n-TFETs and inverter operation with complementary TFET logic at low supply voltages Solid-State Electronics 115, 152 (2016) Luth H., Mikulics M., Winden A., Trellenkamp S., Sofer Z., Marso M., Grutzmacher D., and Hardtdegen H. InGaN mesoscopic structures for low energy consumption nano-opto-electronics in 11th International Conference on Advanced Semiconductor Devices & Microsystems (Asdam), 69 (2016) Mahy J. G., Tasseroul L., Herlitschke M., Hermann R. P., and Lambert S. D. Fe3+/iron oxide/SiO2 xerogel catalysts for p-nitrophenol degradation by photo-Fenton effects: Influence of thermal treatment on catalysts texture Materials Today-Proceedings 3, 464 (2016) Maintz S., Deringer V. L., Tchougreeff A. L., and Dronskowski R. LOBSTER: A Tool to Extract Chemical Bonding from Plane-Wave Based DFT Journal of Computational Chemistry 37, 1030 (2016) Maiworm M., Wagner C., Tautz S., and Findeisen R. Manipulation on a Molecular Level: Towards Controlled Molecular 3D Printing in American Control Conference (Acc), 3094 (2016) Marchewka A., Roesgen B., Skaja K., Du H. C., Jia C. L., Mayer J., Rana V., Waser R., and Menzel S. Nanoionic Resistive Switching Memories: On the Physical Nature of the Dynamic Reset Process Advanced Electronic Materials 2, 1500233 (2016) Marchewka A., Waser R., and Menzel S. A 2D Axisymmetric Dynamic Drift-Diffusion Model for Numerical Simulation of Resistive Switching Phenomena in Metal Oxides in International Conference on Simulation of Semiconductor Processes and Devices (Sispad), 145 (2016) Markhof L. and Meden V. Spectral function of the Tomonaga-Luttinger model revisited: Power laws and universality Physical Review B 93, 085108 (2016) Markov A., Greben K., Mayer D., Offenhäusser A., and Wordenweber R. In Situ Analysis of the Growth and Dielectric Properties of Organic Self-Assembled Monolayers: A Way To Tailor Organic Layers for Electronic Applications ACS Applied Materials & Interfaces 8, 16451 (2016)


142

Marso M., Mikulics M., Luth H., Sofer Z., Kordos P., and Hardtdegen H. Hybrid Optoelectronics Based on a Nanocrystal/III-N Nano-LED Platform in 11th International Conference on Advanced Semiconductor Devices & Microsystems, eds. Hascik S., Dzuba J. and Vanko G., 77 (2016) Martinez-Galera A. J., Schroder U. A., Huttmann F., Jolie W., Craes F., Busse C., Caciuc V., Atodiresei N., Blügel S., and Michely T. Oxygen orders differently under graphene: new superstructures on Ir(111) Nanoscale 8, 1932 (2016) Menditto R., Sickinger H., Weides M., Kohlstedt H., Zonda M., Novotny T., Koelle D., Kleiner R., and Goldobin E. Phase retrapping in phi Josephson junction: Onset of the butterfly effect Physical Review B 93, 174506 (2016) Metlenko V., Jung W., Bishop S. R., Tuller H. L., and De Souza R. A. Oxygen diffusion and surface exchange in the mixed conducting oxides SrTi1-yFeyO3-delta Physical Chemistry Chemical Physics 18, 29495 (2016) Meyer D., Schafer T., Schulz P., Jung S., Rittich J., Mokros D., Segger I., Maercks F., Effertz C., Mazzarello R., and Wuttig M. Dithiocarbamate Self-Assembled Monolayers as Efficient Surface Modifiers for Low Work Function Noble Metals Langmuir 32, 8812 (2016) Meyer R., Zurhelle A. F., De Souza R. A., Waser R., and Gunkel F. Dynamics of the metal-insulator transition of donor-doped SrTiO3 Physical Review B 94, 115408 (2016) Michel E., Ibach H., and Schneider C. M. High resolution electron energy loss spectroscopy of spin waves in ultra-thin cobalt films Surface and Interface Analysis 48, 1104 (2016) Michel E., Ibach H., Schneider C. M., Santos D. L. R., and Costa A. T. Lifetime and mean free path of spin waves in ultrathin cobalt films Physical Review B 94, 014420 (2016) Mikulics M., Arango Y. C., Winden A., Adam R., Hardtdegen A., Grutzmacher D., Plinski E., Gregusova D., Novak J., Kordos P., Moonshiram A., Marso M., Sofer Z., Luth H., and Hardtdegen H. Direct electro-optical pumping for hybrid CdSe nanocrystal/III-nitride based nano-light-emitting diodes Applied Physics Letters 108, 061107 (2016) Mikulics M., Marso M., Adam R., Schuck M., Fox A., Sobolewski R., Kordos P., Luth H., Grutzmacher D., and Hardtdegen H. Electrical and optical characterization of freestanding Ge1Sb2Te4 nano-membranes integrated in coplanar strip lines in 11th International Conference on Advanced Semiconductor Devices & Microsystems (Asdam), 73 (2016) Mikulics M., Winden A., Marso M., Moonshiram A., Luth H., Grutzmacher D., and Hardtdegen H. Nano-light-emitting-diodes based on InGaN mesoscopic structures for energy saving optoelectronics Applied Physics Letters 109, 041103 (2016) Mlynczak E., Eschbach M., Borek S., Minar J., Braun J., Aguilera I., Bihlmayer G., Doring S., Gehlmann M., Gospodaric P., Suga S., Plucinski L., Blügel S., Ebert H., and Schneider C. M. Fermi Surface Manipulation by External Magnetic Field Demonstrated for a Prototypical Ferromagnet Physical Review X 6, 041048 (2016) Modugno M., Ibanez-Azpiroz J., and Pettini G. Tight-binding models for ultracold atoms in optical lattices: general formulation and applications Science China-Physics Mechanics & Astronomy 59, 660001 (2016)


143

Moers J., Mikulics M., Marso M., Trellenkamp S., Sofer Z., Grutzmacher D., and Hardtdegen H. Fabrication of UV sources for novel lithographical techniques: development of nano-LED etching procedures in 11th Int. Conference on Advanced Semiconductor Devices & Microsystems (Asdam), 81 (2016) Momand J., Lange F.R.L., Wang R. N., Boschker J.E., Verheijen M.A., Calarco R., Wuttig M., and Kooi B.J. Atomic stacking and van-der-Waals bonding in GeTe-Sb2Te3 superlattices Journal of Materials Research 31, 3115 (2016) Moors M., Adepalli K. K., Lu Q. Y., Wedig A., Baumer C., Skaja K., Arndt B., Tuller H. L., Dittmann R., Waser R., Yildiz B., and Valov I. Resistive Switching Mechanisms on TaOx and SrRuO3 Thin-Film Surfaces Probed by Scanning Tunneling Microscopy ACS Nano 10, 1481 (2016) Morgan C., Misiorny M., Metten D., Heedt S., Schapers T., Schneider C. M., and Meyer C. Impact of Tunnel-Barrier Strength on Magnetoresistance in Carbon Nanotubes Physical Review Applied 5, 054010 (2016) Morgenstern M., Freitag N., Vaid A., Pratzer M., and Liebmann M. Graphene quantum dots: wave function mapping by scanning tunneling spectroscopy and transport spectroscopy of quantum dots prepared by local anodic oxidation Physica Status Solidi-Rapid Research Letters 10, 24 (2016) Morimae T., Nagaj D., and Schuch N. Quantum proofs can be verified using only single-qubit measurements Physical Review A 93, 022326 (2016) Muller M., Nemsak S., Plucinski L., and Schneider C. M. Functional materials for information and energy technology: Insights by photoelectron spectroscopy Journal of Electron Spectroscopy and Related Phenomena 208, 24 (2016) Muller M. C. T. D., Friedrich C., and Blügel S. Acoustic magnons in the long-wavelength limit: Investigating the Goldstone violation in many-body perturbation theory Physical Review B 94, 064433 (2016) Muller M. R., Salazar R., Fathipour S., Xu H., Kallis K., Kunzelmann U., Seabaugh A., Appenzeller J., and Knoch J. Gate-Controlled WSe2 Transistors Using a Buried Triple-Gate Structure Nanoscale Research Letters 11, 512 (2016) Muratova I. S., Mikhelson K. N., Ermolenko Y., Offenhäusser A., and Mourzina Y. On "resistance overpotential" caused by a potential drop along the ultrathin high aspect ratio gold nanowire electrodes in cyclic voltammetry Journal of Solid State Electrochemistry 20, 3359 (2016) Muratova I. S., Mikhelson K. N., Ermolenko Y. E., Offenhäusser A., and Mourzina Y. Chemiresistors based on ultrathin gold nanowires for sensing halides, pyridine and dopamine Sensors and Actuators B-Chemical 232, 420 (2016) N W., U G., W M., and Schneider M. Nanomaterial-modified Flexible Micro-electrode Array by Electrophoretic Journal of Biochips & Tissue Chips 6, 1000115 (2016) Nandi S., Xiao Y., Qureshi N., Paramanik U. B., Jin W. T., Su Y., Ouladdiaf B., Hossain Z., and Bruckel T. Magnetic structures of the Eu and Cr moments in EuCr2As2: Neutron diffraction study Physical Review B 94, 094411 (2016) Nandy A. K., Kiselev N. S., and Blügel S. Interlayer Exchange Coupling: A General Scheme Turning Chiral Magnets into Magnetic Multilayers Carrying Atomic-Scale Skyrmions Physical Review Letters 116, 177202 (2016)


144

Narimani K., Luong G. V., Schulte-Braucks C., Trellenkamp S., Zhao Q. T., Mantl S., and Chowdhury M. F. Current Mirrors with Strained Si Single Nanowire Gate All Around Schottky Barrier MOSFETs in Joint International Eurosoi Workshop and International Conference on Ultimate Integration on Silicon (Eurosoi-Ulis 2016), 178 (2016) Nemkovski K. S., Kozlenko D. P., Alekseev P. A., Mignot J. M., Menushenkov A. P., Yaroslavtsev A. A., Clementyev E. S., Ivanov A. S., Rols S., Klobes B., Hermann R. P., and Gribanov A. V. Europium mixed-valence, long-range magnetic order, and dynamic magnetic response in EuCu2(SixGe1-x)2 Physical Review B 94, 195101 (2016) Neumann C., Banszerus L., Schmitz M., Reichardt S., Sonntag J., Taniguchi T., Watanabe K., Beschoten B., and Stampfer C. Line shape of the Raman 2D peak of graphene in van der Waals heterostructures Physica Status Solidi B-Basic Solid State Physics 253, 2326 (2016) Neumann C., Rizzi L., Reichardt S., Terres B., Khodkov T., Watanabe K., Taniguchi T., Beschoten B., and Stampfer C. Spatial Control of Laser-Induced Doping Profiles in Graphene on Hexagonal Boron Nitride ACS Applied Materials & Interfaces 8, 9377 (2016) Nghiem H. T. M., Kennes D. M., Klockner C., Meden V., and Costi T. A. Ohmic two-state system from the perspective of the interacting resonant level model: Thermodynamics and transient dynamics Physical Review B 93, 165130 (2016) Nguyen T. A., Backes D., Singh A., Mansell R., Barnes C., Ritchie D. A., Mussler G., Lanius M., Grutzmacher D., and Narayan V. Topological states and phase transitions in Sb2Te3-GeTe multilayers Scientific Reports 6, 27716 (2016) Nielen L., Ohm S., Such O., Klimo M., Waser R., and Linn E. Memristive Sorting Networks Enabled by Electrochemical Metallization Cells International Journal of Unconventional Computing 12, 303 (2016) Niu C. W., Buhl P. M., Bihlmayer G., Wortmann D., Blügel S., and Mokrousov Y. Two-dimensional topological crystalline insulator phase in quantum wells of trivial insulators 2d Materials 3, 025037 (2016) Nordmann S., Berghoff B., Hessel A., Wilck N., Osullivan B., Debucquoy M., John J., Starschich S., and Knoch J. A monolithic all-silicon multi-junction solar device for direct water splitting Renewable Energy 94, 90 (2016) Odstrcil M., Baksh P., Boden S. A., Card R., Chad J. E., Frey J. G., and Brocklesby W. S. Ptychographic coherent diffractive imaging with orthogonal probe relaxation Optics Express 24, 8360 (2016) Odstrcil M., Baksh P., Gawith C., Vrcelj R., Frey J. G., and Brocklesby W. S. Nonlinear ptychographic coherent diffractive imaging Optics Express 24, 20245 (2016) Ojwang D. O., Grins J., Wardecki D., Valvo M., Renman V., Haggstrom L., Ericsson T., Gustafsson T., Mahmoud A., Hermann R. P., and Svensson G. Structure Characterization and Properties of K-Containing Copper Hexacyanoferrate Inorganic Chemistry 55, 5924 (2016) Ono T. and Tsukamoto S. Real-space method for first-principles electron transport calculations: Self-energy terms of electrodes for large systems Physical Review B 93, 045421 (2016)


145

Otsuka Y., Pithan C., Dornseiffer J., Takada T., Konoike T., and Waser R. Synthesis of nitrogen and lanthanum codoped barium titanate with a novel thermal ammonolysis reactor Journal of the European Ceramic Society 36, 2719 (2016) Otten D., Rubbert S., Ulrich J., and Hassler F. Universal power-law decay of electron-electron interactions due to nonlinear screening in a Josephson junction array Physical Review B 94, 115403 (2016) Oyarzun S., Nandy A. K., Rortais F., Rojas-Sanchez J. C., Dau M. T., Noel P., Laczkowski P., Pouget S., Okuno H., Vila L., Vergnaud C., Beigne C., Marty A., Attane J. P., Gambarelli S., George J. M., Jaffres H., Blügel S., and Jamet M. Evidence for spin-to-charge conversion by Rashba coupling in metallic states at the Fe/Ge(111) interface Nature Communications 7, 13857 (2016) Pandey R., Schulte-Braucks C., Sajjad R. N., Barth M., Ghosh R. K., Grisafe B., Sharma P., von den Driesch N., Vohra A., Rayner B., Loo R., Mantl S., Buca D., Yeh C. C., Wu C. H., Tsai W., Antoniadis D., and Datta S. Performance Benchmarking of p-type In0.65Ga0.35As/GaAs0.4Sb0.6 and Ge/Ge0.93Sn0.07 Hetero-junction Tunnel FETs in IEEE International Electron Devices Meeting (Iedm), UNSP 19.6 (2016) Parshin P. P., Chumakov A. I., Alekseev P. A., Nemkovski K. S., Dubrovinskii L., Kantor A., Persson J., and Ruffer R. Peculiarities of FeSi Phonon Spectrum Induced by a Change of Atomic Volume Journal of Experimental and Theoretical Physics 123, 1073 (2016) Parshin P. P., Chumakov A. I., Alekseev P. A., Nemkovski K. S., Persson J., Dubrovinsky L., Kantor A., and Ruffer R. Experimental observation of phonons as spectators in FeSi electronic gap formation Physical Review B 93, 081102 (2016) Paschinger W., Rogl G., Grytsiv A., Michor H., Heinrich P. R., Muller H., Puchegger S., Klobes B., Hermann R. P., Reinecker M., Eisenmenger-Sitter C., Broz P., Bauer E., Giester G., Zehetbauer M., and Rogl P. F. Ba-filled Ni-Sb-Sn based skutterudites with anomalously high lattice thermal conductivity Dalton Transactions 45, 11071 (2016) Passens M., Caciuc V., Atodiresei N., Moors M., Blügel S., Waser R., and Karthauser S. Tuning the surface electronic structure of a Pt3Ti(111) electro catalyst Nanoscale 8, 13924 (2016) Pauly C., Rasche B., Koepernik K., Richter M., Borisenko S., Liebmann M., Ruck M., van den Brink J., and Morgenstern M. Electronic Structure of the Dark Surface of the Weak Topological Insulator Bi14Rh3I9 ACS Nano 10, 3995 (2016) Peranio N., Eibl O., Bassler S., Nielsch K., Klobes B., Hermann R. P., Daniel M., Albrecht M., Gorlitz H., Pacheco V., Bedoya-Martinez N., Hashibon A., and Elsasser C. From thermoelectric bulk to nanomaterials: Current progress for Bi2Te3 and CoSb3 Physica Status Solidi a-Applications and Materials Science 213, 739 (2016) Peters L., Di Marco I., Granas O., Sasioglu E., Altun A., Rossen S., Friedrich C., Blügel S., Katsnelson M. I., Kirilyuk A., and Eriksson O. Correlation effects and orbital magnetism of Co clusters Physical Review B 93, 224428 (2016) Pfeil T., Jordan J., Tetzlaff T., Grubl A., Schemmel J., Diesmann M., and Meier K. Effect of Heterogeneity on Decorrelation Mechanisms in Spiking Neural Networks: A Neuromorphic-Hardware Study Physical Review X 6, 021023 (2016)


146

Plank H., Danilov S. N., Bel'kov V. V., Shalygin V. A., Kampmeier J., Lanius M., Mussler G., Grutzmacher D., and Ganichev S. D. Opto-electronic characterization of three dimensional topological insulators Journal of Applied Physics 120, 165301 (2016) Plank H., Golub L. E., Bauer S., Bel'kov V. V., Herrmann T., Olbrich P., Eschbach M., Plucinski L., Schneider C. M., Kampmeier J., Lanius M., Mussler G., Grutzmacher D., and Ganichev S. D. Photon drag effect in (Bi1-xSbx)2Te3 three-dimensional topological insulators Physical Review B 93, 125434 (2016) Plotnikov D., Blundell I., Ippen T., Eppler J. M., Rumpe B., and Morrison A., NESTML: a modeling language for spiking neurons in Lecture Notes in Informatics (LNI) (Gesellschaft für Informatik e.V. (GI)) P-254, 93 (2016) Potapkin V., Dubrovinsky L., Sergueev I., Ekholm M., Kantor I., Bessas D., Bykova E., Prakapenka V., Hermann R. P., Ruffer R., Cerantola V., Jonsson H. J. M., Olovsson W., Mankovsky S., Ebert H., and Abrikosov I. A. Magnetic interactions in NiO at ultrahigh pressure Physical Review B 93, 201110 (2016) Privitera S. M. S., Mio A. M., Smecca E., Alberti A., Zhang W., Mazzarello R., Benke J., Persch C., La Via F., and Rimini E. Structural and electronic transitions in Ge2Sb2Te5 induced by ion irradiation damage Physical Review B 94, 094103 (2016) Rahmanizadeh K., Bihlmayer G., and Blügel S. Spin-orbit and exchange effects in the 2DEG of BiAlO3-based oxide heterostructures EPL 115, 17006 (2016) Rainko D., Stange D., von den Driesch N., Schulte-Braucks C., Mussler G., Ikonic Z., Hartmann J. M., Luysberg M., Mantl S., Grutzmacher D., and Buca D., (Si)GeSn nanostructures for Light Emitters in Silicon Photonics and Photonic Integrated Circuits V 9891, 98910w.(2016) Ramadan A. H. H. and De Souza R. A. Atomistic simulations of symmetrical low-angle 100 (01l) tilt boundaries in SrTiO3 Acta Materialia 118, 286 (2016) Rauch D., Dietrich M., Simons T., Simon U., Porch A., and Moos R. Microwave Cavity Perturbation Studies on H-form and Cu Ion-Exchanged SCR Catalyst Materials: Correlation of Ammonia Storage and Dielectric Properties Topics in Catalysis 60, 243 (2017) Reichardt S. and Stampfer C. Modeling charge relaxation in graphene quantum dots induced by electron-phonon interaction Physical Review B 93, 245423 (2016) Renault O., Zborowski C., Risterucci P., Wiemann C., Grenet G., Schneider C. M., and Tougaard S. Quantitative spectromicroscopy from inelastically scattered photoelectrons in the hard X-ray range Applied Physics Letters 109, 011602 (2016) Rentrop J. F., Meden V., and Jakobs S. G. Renormalization group flow of the Luttinger-Ward functional: Conserving approximations and application to the Anderson impurity model Physical Review B 93, 195160 (2016) Richer S. and DiVincenzo D. Circuit design implementing longitudinal coupling: A scalable scheme for superconducting qubits Physical Review B 93, 134501 (2016) Richter M. C., Mariot J. M., Gafoor M. A., Nicolai L., Heckmann O., Djukic U., Ndiaye W., Vobornik I., Fujii J., Barrett N., Feyer V., Schneider C. M., and Hricovini K. Bi atoms mobility-driven circular domains at the Bi/InAs(111) interface Surface Science 651, 147 (2016)


147

Rieger T., Rosenbach D., Vakulov D., Heedt S., Schapers T., Grutzmacher D., and Lepsa M. I. Crystal Phase Transformation in Self-Assembled InAs Nanowire Junctions on Patterned Si Substrates Nano Letters 16, 1933 (2016) Riwar R. P., Hosseinkhani A., Burkhart L. D., Gao Y. Y., Schoelkopf R. J., Glazman L. I., and Catelani G. Normal-metal quasiparticle traps for superconducting qubits Physical Review B 94, 104516 (2016) Riwar R. P., Roche B., Jehl X., and Splettstoesser J. Readout of relaxation rates by nonadiabatic pumping spectroscopy Physical Review B 93, 235401 (2016) Rodenbucher C., Hildebrandt E., Szot K., Sharath S. U., Kurian J., Komissinskiy P., Breuer U., Waser R., and Alff L. Hafnium carbide formation in oxygen deficient hafnium oxide thin films Applied Physics Letters 108, 252903 (2016) Rodenbucher C., Luysberg M., Schwedt A., Havel V., Gunkel F., Mayer J., and Waser R. Homogeneity and variation of donor doping in Verneuil-grown SrTiO3:Nb single crystals Scientific Reports 6, 32250 (2016) Rupp J. A. J., Waser R., and Wouters D. J. Threshold Switching in Amorphous Cr-doped Vanadium Oxide for New Crossbar Selector in IEEE 8th International Memory Workshop (Imw) (2016) Rushchanskii K. Z., Bilanych R. M., Molnar A. A., Yevych R. M., Kohutych A. A., Perechinskii S. I., Samulionis V., Banys J., and Vysochanskii Y. M. Ferroelectricity in (PbySn1-y)2P2S6 mixed crystals and random field BEG model Physica Status Solidi B-Basic Solid State Physics 253, 384 (2016) Rushchanskii K. Z., Molnar A., Bilanych R., Yevych R., Kohutych A., Vysochanskii Y. M., Samulionis V., and Banys J. Observation of nonequilibrium behavior near the Lifshitz point in ferroelectrics with incommensurate phase Physical Review B 93, 014101 (2016) Salamonczyk M., Zhang J., Portale G., Zhu C. H., Kentzinger E., Gleeson J. T., Jakli A., De Michele C., Dhont J. K. G., Sprunt S., and Stiakakis E. Smectic phase in suspensions of gapped DNA duplexes Nature Communications 7, 13358 (2016) Samkharadze N., Bruno A., Scarlino P., Zheng G., DiVincenzo D. P., DiCarlo L., and Vandersypen L.M.K. High-Kinetic-Inductance Superconducting Nanowire Resonators for Circuit QED in a Magnetic Field Physical Review Applied 5, 044004 (2016) Sanchez-Barriga J., Vergniory M. G., Evtushinsky D., Aguilera I., Varykhalov A., Bluel S., and Rader O. Surface Fermi arc connectivity in the type-II Weyl semimetal candidate WTe2 Physical Review B 94, 161401 (2016) Sazonov A., Hutanu V., Meven M., Roth G., Kezsmarki I., Murakawa H., Tokura Y., and Nafradi B. The low-temperature crystal structure of the multiferroic melilite Ca2CoSi2O7 Acta Crystallographica Section B-Structural Science Crystal Engineering and Materials 72, 126 (2016) Sazonov A., Meven M., Roth G., Georgii R., Kezsmarki I., Kocsis V., Tokunaga Y., Taguchi Y., Tokura Y., and Hutanu V. Origin of forbidden reflections in multiferroic Ba2CoGe2O7 by neutron diffraction: symmetry lowering or Renninger effect? Journal of Applied Crystallography 49, 556 (2016) Schapers T., Heedt S., Bringer A., Hardtdegen H., Schubert J., Grutzmacher D., Kammermeier M., Wenk P., Schliemann J., and Prost W. Ballistic and Spin Transport in InAs Nanowires in IEEE Nanotechnology Materials and Devices Conference (Nmdc) (2016)


148

Schapers T., Heedt S., Bringer A., Otto I., Sladek K., Hardtdegen H., Grutzmacher D., and Prost W. Spintronics with semiconductor nanowires in Compound Semiconductor Week (Csw) Includes 28th International Conference on Indium Phosphide & Related Materials (Iprm) & 43rd International Symposium on Compound Semiconductors (Iscs) (2016) Schmalbuch K., Gobbels S., Schafers P., Rodenbucher C., Schlammes P., Schapers T., Lepsa M., Guntherodt G., and Beschoten B. Comment on "Two-Dimensional Optical Control of Electron Spin Orientation by Linearly Polarized Light in InGaAs" Reply Physical Review Letters 117, 139702 (2016) Schmalzl K., Schmidt W., Raymond S., Feilbach H., Mounier C., Vettard B., and Bruckel T. The upgrade of the cold neutron three-axis spectrometer IN12 at the ILL Nuclear Instruments & Methods in Physics Research Section a-Accelerators Spectrometers Detectors and Associated Equipment 819, 89 (2016) Schmitz C., Wilson D., Rudolf D., Wiemann C., Plucinski L., Riess S., Schuck M., Hardtdegen H., Schneider C. M., Tautz F. S., and Juschkin L. Compact extreme ultraviolet source for laboratory-based photoemission spectromicroscopy Applied Physics Letters 108, 234101 (2016) Schmitz S., Monakhov K. Y., van Leusen J., Izarova N. V., Hess V., and Kogerler P. Co5

II/ III horseshoe and Ni4II lacunary cubane coordination clusters: the isobutyrate/N-

butyldiethanolamine reaction system RSC Advances 6, 100664 (2016) Schmitz S., van Leusen J., Ellern A., Kogerler P., and Monakhov K. Y. Thioether-terminated nickel(II) coordination clusters with Ni6 horseshoe- and Ni8 rollercoaster-shaped cores Inorganic Chemistry Frontiers 3, 523 (2016) Schmitz S., van Leusen J., Izarova N. V., Lan Y., Wernsdorfer W., Koegerler P., and Monakhov K. Y. Supramolecular 3d-4f single-molecule magnet architectures Dalton Transactions 45, 16148 (2016) Schnee M., Besson C., Frielinghaus R., Lurz C., Kogerler P., Schneider C. M., and Meyer C. Quantum transport in carbon nanotubes covalently functionalized with magnetic molecules Physica Status Solidi B-Basic Solid State Physics 253, 2424 (2016) Schober G. A. H., Giering K. U., Scherer M. M., Honerkamp C., and Salmhofer M. Functional renormalization and mean-field approach to multiband systems with spin-orbit coupling: Application to the Rashba model with attractive interaction Physical Review B 93, 115111 (2016) Schonauer K., Weiss S., Feyer V., Luftner D., Stadtmuller B., Schwarz D., Sueyoshi T., Kumpf C., Puschnig P., Ramsey M. G., Tautz F. S., and Soubatch S. Charge transfer and symmetry reduction at the CuPc/Ag(110) interface studied by photoemission tomography Physical Review B 94, 205144 (2016) Schoneberg J., Otte F., Neel N., Weismann A., Mokrousov Y., Kroger J., Berndt R., and Heinze S. Ballistic Anisotropic Magnetoresistance of Single-Atom Contacts Nano Letters 16, 1450 (2016) Schraknepper H., Baumer C., Gunkel F., Dittmann R., and De Souza R. A. Pulsed laser deposition of SrRuO3 thin-films: The role of the pulse repetition rate APL Materials 4, 126109 (2016) Schraknepper H. and De Souza R. A. Competing descriptions of diffusion profiles with two features: Surface space-charge layer versus fast grain-boundary diffusion Journal of Applied Physics 119, 064903 (2016)


149

Schulenborg J., Saptsov R. B., Haupt F., Splettstoesser J., and Wegewijs M. R. Fermion-parity duality and energy relaxation in interacting open systems Physical Review B 93, 081411 (2016) Schulte-Braucks C., Glass S., Hofmann E., Stange D., von den Driesch N., Zhao Q. T., Buca D., Mantl S., Hartmann J. M., and Ikonic Z. Process Modules for GeSn Nanoelectronics with high Sn-contents in Joint International Eurosoi Workshop and International Conference on Ultimate Integration on Silicon (Eurosoi-Ulis 2016), 24 (2016) Schulte-Braucks C., von den Driesch N., Glass S., Tiedemann A.T., Breuer U., Besmehn A., Hartmann J.M., Ikonic Z., Zhao Q. T., Mantl S., and Buca D. Low Temperature Deposition of High-k/Metal Gate Stacks on High-Sn Content (Si)GeSn-Alloys ACS Applied Materials & Interfaces 8, 13133 (2016) Schumacher M., Weber H., Jovari P., Tsuchiya Y., Youngs T. G. A., Kaban I., and Mazzarello R. Structural, electronic and kinetic properties of the phase-change material Ge2Sb2Te5 in the liquid state Scientific Reports 6, 27434 (2016) Schwarz D., Henneke C., and Kumpf C. Towards functionalization of graphene: in situ study of the nucleation of copper-phtalocyanine on graphene New Journal of Physics 18, 023034 (2016) Schweflinghaus B., Dias M. D., and Lounis S. Observing spin excitations in 3d transition-metal adatoms on Pt(111) with inelastic scanning tunneling spectroscopy: A first-principles perspective Physical Review B 93, 035451 (2016) Schweflinghaus B., Zimmermann B., Heide M., Bihlmayer G., and Blügel S. Role of Dzyaloshinskii-Moriya interaction for magnetism in transition-metal chains at Pt step edges Physical Review B 94, 024403 (2016) Seifert T., Jaiswal S., Martens U., Hannegan J., Braun L., Maldonado P., Freimuth F., Kronenberg A., Henrizi J., Radu I., Beaurepaire E., Mokrousov Y., Oppeneer P. M., Jourdan M., Jakob G., Turchinovich D., Hayden L. M., Wolf M., Munzenberg M., Klaui M., and Kampfrath T. Efficient metallic spintronic emitters of ultrabroadband terahertz radiation Nature Photonics 10, 483 (2016) Sertsu M. G., Giglia A., Brose S., Park D., Wang Z. S., Mayer J., Juschkin L., and Nicolosi P. Deposition and characterization of B4C/CeO2 multilayers at 6.x nm extreme ultraviolet wavelengths Journal of Applied Physics 119, 095301 (2016) Sesselmann A., Klobes B., Dasgupta T., Gourdon O., Hermann R., and Mueller E. Neutron diffraction and thermoelectric properties of indium filled InxCo4Sb12 (x=0.05, 0.2) and indium cerium filled Ce0.05In0.1Co4Sb12 skutterudites Physica Status Solidi a-Applications and Materials Science 213, 766 (2016) Sessi P., Russmann P., Bathon T., Barla A., Kokh K. A., Tereshchenko O. E., Fauth K., Mahatha S. K., Valbuena M. A., Godey S., Glott F., Mugarza A., Gargiani P., Valvidares M., Long N. H., Carbone C., Mavropoulos P., Blügel S., and Bode M. Superparamagnetism-induced mesoscopic electron focusing in topological insulators Physical Review B 94, 075137 (2016) Sessi P., Storz O., Bathon T., Wilfert S., Kokh K. A., Tereshchenko O. E., Bihlmayer G., and Bode M. Scattering properties of the three-dimensional topological insulator Sb2Te3: Coexistence of topologically trivial and nontrivial surface states with opposite spin-momentum helicity Physical Review B 93, 035110 (2016) Sforzini J., Hapala P., Franke M., van Straaten G., Stohr A., Link S., Soubatch S., Jelinek P., Lee T. L., Starke U., Svec M., Bocquet F. C., and Tautz F. S. Structural and Electronic Properties of Nitrogen-Doped Graphene Physical Review Letters 116, 126805 (2016)


150

Sforzini J., Telychko M., Krejci O., Vondracek M., Svec M., Bocquet F. C., and Tautz F. S. Transformation of metallic boron into substitutional dopants in graphene on 6H-SiC(0001) Physical Review B 93, 041302 (2016) Shekhter P., Uzan-Saguy C., Schubert J., Amouyal Y., and Eizenberg M. Experimental and computational study of zero dimensional metallic behavior at the LaLuO3/SrTiO3 interface Journal of Vacuum Science & Technology B 34, 021204 (2016) Sougrati M. T., Darwiche A., Liu X. H., Mahmoud A., Hermann R. P., Jouen S., Monconduit L., Dronskowski R., and Stievano L. Transition-Metal Carbodiimides as Molecular Negative Electrode Materials for Lithium- and Sodium-Ion Batteries with Excellent Cycling Properties Angewandte Chemie-International Edition 55, 5090 (2016) Stadtmuller B., Haag N., Seidel J., van Straaten G., Franke M., Kumpf C., Cinchetti M., and Aeschlimann M. Adsorption heights and bonding strength of organic molecules on a Pb-Ag surface alloy Physical Review B 94, 235436 (2016) Stadtmuller B., Seidel J., Haag N., Grad L., Tusche C., van Straaten G., Franke M., Kirschner J., Kumpf C., Cinchetti M., and Aeschlimann M. Modifying the Surface of a Rashba-Split Pb-Ag Alloy Using Tailored Metal-Organic Bonds Physical Review Letters 117, 096805 (2016) Stampfer C. and Reichardt S. Correspondence: On the nature of strong piezoelectricity in graphene on SiO2 Nature Communications 7, 11570 (2016) Stange D., von den Driesch N., Rainko D., Schulte-Braucks C., Wirths S., Mussler G., Tiedemann A. T., Stoica T., Hartmann J. M., Ikonic Z., Mantl S., Grutzmacher D., and Buca D. Study of GeSn based heterostructures: towards optimized group IV MQW LEDs Optics Express 24, 1358 (2016) Stange D., Wirths S., Geiger R., Schulte-Braucks C., Marzban B., von den Driesch N., Mussler G., Zabel T., Stoica T., Hartmann J. M., Mantl S., Ikonic Z., Gruetzmacher D., Sigg H., Witzens J., and Buca D. Optically Pumped GeSn Microdisk Lasers on Si ACS Photonics 3, 1279 (2016) Starschich S., Menzel S., and Bottger U. Evidence for oxygen vacancies movement during wake-up in ferroelectric hafnium oxide Applied Physics Letters 108, 032903 (2016) Steinberg S., Stoffel R. P., and Dronskowski R. Search for the Mysterious SiTe - An Examination of the Binary Si-Te System Using First-Principles-Based Methods Crystal Growth & Design 16, 6152 (2016) Steinbrecher M., Sonntag A., Dias M. D., Bouhassoune M., Lounis S., Wiebe J., Wiesendanger R., and Khajetoorians A. A. Absence of a spin-signature from a single Ho adatom as probed by spin-sensitive tunneling Nature Communications 7, 10454 (2016) Stoica T., Stoica M., Duchamp M., Tiedemann A., Mantl S., Grutzmacher D., Buca D., and Kardynal B. E. Vapor transport growth of MoS2 nucleated on SiO2 patterns and graphene flakes Nano Research 9, 3504 (2016) Suzuki T. J., Kennes D. M., and Meden V. Current noise of the interacting resonant level model Physical Review B 93, 085306 (2016)


151

Taoka N., Capellini G., von den Driesch N., Buca D., Zaumseil P., Schubert M. A., Klesse W. M., Montanari M., and Schroeder T. Sn migration control at high temperature due to high deposition speed for forming high-quality GeSn layer Applied Physics Express 9, 031201 (2016) Tas M., Sasioglu E., Galanakis I., Friedrich C., and Blügel S. Quasiparticle band structure of the almost-gAPLess transition-metal-based Heusler semiconductors Physical Review B 93, 195155 (2016) Terhal B. M. and Weigand D. Encoding a qubit into a cavity mode in circuit QED using phase estimation Physical Review A 93, 012315 (2016) Terres B., Chizhova L.A., Libisch F., Peiro J., Jorger D., Engels S., Girschik A., Watanabe K., Taniguchi T., Rotkin S. V., Burgdorfer J., and Stampfer C. Size quantization of Dirac fermions in graphene constrictions Nature Communications 7, 11528 (2016) Tietz F., Raj I. A., Ma Q., Baumann S., Mahmoud A., and Hermann R. P. Material properties of perovskites in the quasi-ternary system LaFeO3-LaCoO3-LaNiO3 Journal of Solid State Chemistry 237, 183 (2016) Tonkikh A. A., Voloshina E. N., Werner P., Blumtritt H., Senkovskiy B., Guntherodt G., Parkin S. S. P., and Dedkov Y. S. Structural and electronic properties of epitaxial multilayer h-BN on Ni(111) for spintronics applications Scientific Reports 6, 23547 (2016) Torre E., Canova C., Denker M., Gerstein G., Helias M., and Grun S. ASSET: Analysis of Sequences of Synchronous Events in Massively Parallel Spike Trains Plos Computational Biology 12, e1004939 (2016) Tran D. P., Winter M. A., Wolfrum B., Stockmann R., Yang C. T., Pourhassan-Moghaddam M., Offenhäusser A., and Thierry B. Toward Intraoperative Detection of Disseminated Tumor Cells in Lymph Nodes with Silicon Nanowire Field Effect Transistors ACS Nano 10, 2357 (2016) Tran F., Blaha P., Betzinger M., and Blügel S. Approximations to the exact exchange potential: KLI versus semilocal Physical Review B 94, 165149 (2016) Trushin E., Betzinger M., Blügel S., and Gorling A. Band gaps, ionization potentials, and electron affinities of periodic electron systems via the adiabatic-connection fluctuation-dissipation theorem Physical Review B 94, 075123 (2016) Tsuruoka T., Valov I., Mannequin C., Hasegawa T., Waser R., and Aono M. Humidity effects on the redox reactions and ionic transport in a Cu/Ta2O5/Pt atomic switch structure Japanese Journal of Applied Physics 55, 06gj09 (2016) Tveten E. G., Muller T., Linder J., and Brataas A. Intrinsic magnetization of antiferromagnetic textures Physical Review B 93, 104408 (2016) Ulrich J. and Hassler F. Dual approach to circuit quantization using loop charges Physical Review B 94, 094505 (2016) Valov I. and Lu W. D. Nanoscale electrochemistry using dielectric thin films as solid electrolytes Nanoscale 8, 13828 (2016)


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Verbiest G. J. and Rost M. J. Resonance frequencies of AFM cantilevers in contact with a surface Ultramicroscopy 171, 70 (2016) Verbiest G. J., Stampfer C., Huber S. E., Andersen M., and Reuter K. Interplay between nanometer-scale strain variations and externally applied strain in graphene Physical Review B 93, 195438 (2016)

Verbiest G. J., Xu D., Goldsche M., Khodkov T., Barzanjeh S., von den Driesch N., Buca D., and Stampfer C. Tunable mechanical coupling between driven microelectromechanical resonators Applied Physics Letters 109, 143507 (2016) Vodungbo B., Tudu B., Perron J., Delaunay R., Muller L., Berntsen M. H., Grubel G., Malinowski G., Weier C., Gautier J., Lambert G., Zeitoun P., Gutt C., Jal E., Reid A. H., Granitzka P. W., Jaouen N., Dakovski G. L., Moeller S., Minitti M. P., Mitra A., Carron S., Pfau B., Schmising C. V. K., Schneider M., Eisebitt S., and Luning J. Indirect excitation of ultrafast demagnetization Scientific Reports 6, 18970 (2016) von den Driesch N., Stange D., Wirths S., Rainko D., Mussler G., Stoica T., Ikonic Z., Hartmann J. M., Grutzmacher D., Mantl S., and Buca D. Direct bandgap GeSn light emitting diodes for short-wave infrared applications grown on Si Silicon Photonics XI 9752, 97520c (2016) Vrejoiu I., Himcinschi C., Jin L., Jia C. L., Raab N., Engelmayer J., Waser R., Dittmann R., and van Loosdrecht P. H. M. Probing orbital ordering in LaVO3 epitaxial films by Raman scattering APL Materials 4, 046103 (2016) Wadley P., Howells B., Zelezny J., Andrews C., Hills V., Campion R. P., Novak V., Olejnik K., Maccherozzi F., Dhesi S.S., Martin S. Y., Wagner T., Wunderlich J., Freimuth F., Mokrousov Y., Kunes J., Chauhan J. S., Grzybowski M. J., Rushforth A. W., Edmonds K. W., Gallagher B. L., and Jungwirth T. Electrical switching of an antiferromagnet Science 351, 587 (2016) Waldow S. P. and De Souza R. A. Computational Study of Oxygen Diffusion along a[100] Dislocations in the Perovskite Oxide SrTiO3 ACS Applied Materials & Interfaces 8, 12246 (2016) Wang R. N., Campi D., Bernasconi M., Momand J., Kooi B. J., Verheijen M. A., Wuttig M., and Calarco R. Ordered Peierls distortion prevented at growth onset of GeTe ultra-thin films Scientific Reports 6, 32895 (2016) Wang W., Ma P. X., Dong H., Krause H. J., Zhang Y., Willbold D., Offenhaeusser A., and Gu Z. W. A magnetic nanoparticles relaxation sensor for protein-protein interaction detection at ultra-low magnetic field Biosensors & Bioelectronics 80, 661 (2016) Wang Z. X., Banszerus L., Otto M., Watanabe K., Taniguchi T., Stampfer C., and Neumaier D. Encapsulated graphene-based Hall sensors on foil with increased sensitivity Physica Status Solidi B-Basic Solid State Physics 253, 2316 (2016) Warner B., El Hallak F., Atodiresei N., Seibt P., Pruser H., Caciuc V., Waters M., Fisher A. J., Blügel S., van Slageren J., and Hirjibehedin C. F. Sub-molecular modulation of a 4f driven Kondo resonance by surface-induced asymmetry Nature Communications 7, 12785 (2016) Waser R., Dittmann R., Linn E., and Menzel S. Energy efficient computing by redox-based memristive oxide elements in IEEE Silicon Nanoelectronics Workshop (Snw), 46 (2016)


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Wedig A., Luebben M., Cho D. Y., Moors M., Skaja K., Rana V., Hasegawa T., Adepalli K. K., Yildiz B., Waser R., and Valov I. Nanoscale cation motion in TaOx, HfOx and TiOx memristive systems Nature Nanotechnology 11, 67 (2016) Weingarten M., Sanders S., Stummler D., Pfeiffer P., Vescan A., and Kalisch H. Optimization of hybrid organic/inorganic poly(3-hexylthiophene-2,5-diyl)/silicon solar cells Journal of Photonics for Energy 6, 025504 (2016) Weingarten M., Zweipfennig T., Sanders S., Stummler D., Pfeiffer P., Vescan A., and Kalisch H. Investigation of organic semiconductor interlayers in hybrid PEDOT:PSS/silicon solar cells Journal of Photonics for Energy 6, 045503 (2016) Wen J., Liu Q., Liu C., Wang Y. Z., Zhang B., Xue Z. Y., Di Z. F., Min J. H., Yu W. J., Liu X. K., Wang X., and Zhao Q. T. Ion-sensitive field-effect transistor with sSi/Si0.5Ge0.5/sSOI quantum-well for high voltage sensitivity Microelectronic Engineering 163, 115 (2016) Wen J., Liu Q., Liu C., Wang Y. Z., Zhang B., Xue Z. Y., Di Z. F., Yu W. J., and Zhao Q. T. Investigation of Coulomb scattering on sSi/Si0.5Ge0.5/sSOI quantum-well p-MOSFETs Journal of Semiconductors 37, 094002 (2016) Wendav T., Fischer I. A., Montanari M., Zoellner M. H., Klesse W., Capellini G., von den Driesch N., Oehme M., Buca D., Busch K., and Schulze J. Compositional dependence of the band-gap of Ge1-x-ySixSny alloys Applied Physics Letters 108, 242104 (2016) Wentzell N., Florens S., Meng T., Meden V., and Andergassen S. Magnetoelectric spectroscopy of Andreev bound states in Josephson quantum dots Physical Review B 94, 085151 (2016) Wetterskog E., Klapper A., Disch S., Josten E., Hermann R. P., Rucker U., Bruckel T., Bergstrom L., and Salazar-Alvarez G. Tuning the structure and habit of iron oxide mesocrystals Nanoscale 8, 15571 (2016) Weyrich C., Drögeler M., Kampmeier J., Eschbach M., Mussler G., Merzenich T., Stoica T., Batov I. E., Schubert J., Plucinski L., Beschoten B., Schneider C. M., Stampfer C., Grutzmacher D., and Schapers T. Growth, characterization, and transport properties of ternary (Bi1-xSbx)2Te3 topological insulator layers Journal of Physics-Condensed Matter 28, 495501 (2016) Whitney R. S., Sanchez R., Haupt F., and Splettstoesser J. Thermoelectricity without absorbing energy from the heat sources Physica E-Low-Dimensional Systems & Nanostructures 75, 257 (2016) Wirths S., Buca D., and Mantl S. Si-Ge-Sn alloys: From growth to applications Progress in Crystal Growth and Characterization of Materials 62, 1 (2016) Woolfson R. J., Timco G. A., Chiesa A., Vitorica-Yrezabal I. J., Tuna F., Guidi T., Pavarini E., Santini P., Carretta S., and Winpenny R. E. P. [CrF(O2C

tBu)2]9: Synthesis and Characterization of a Regular Homometallic Ring with an Odd Number of Metal Centers and Electrons Angewandte Chemie-International Edition 55, 8856 (2016) Wunnemann P., Noyong M., Kreuels K., Brux R., Gordiichuk P., van Rijn P., Plamper F. A., Simon U., and Boker A. Microstructured Hydrogel Templates for the Formation of Conductive Gold Nanowire Arrays Macromolecular Rapid Communications 37, 1446 (2016) Xi L. F., Cho D. Y., Besmehn A., Duchamp M., Grutzmacher D., Lam Y. M., and Kardynal B. E. Effect of Zinc Incorporation on the Performance of Red Light Emitting InP Core Nanocrystals Inorganic Chemistry 55, 8381 (2016)


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Xiao Y., Kumar C. M. N., Nandi S., Su Y., Jin W. T., Fu Z., Faulhaber E., Schneidewind A., and Bruckel T. Spin-wave and electromagnon dispersions in multiferroic MnWO4 as observed by neutron spectroscopy: Isotropic Heisenberg exchange versus anisotropic Dzyaloshinskii-Moriya interaction Physical Review B 93, 214428 (2016) Xu C. C., Baumer C., Heinen R. A., Hoffmann-Eifert S., Gunkel F., and Dittmann R. Disentanglement of growth dynamic and thermodynamic effects in LaAlO3/SrTiO3 heterostructures Scientific Reports 6, 22410 (2016) Xu C. C., Du H. C., van der Torren A. J. H., Aarts J., Jia C. L., and Dittmann R. Formation mechanism of Ruddlesden-Popper-type antiphase boundaries during the kinetically limited growth of Sr rich SrTiO3 thin films Scientific Reports 6, 38296 (2016) Xu X. Y., Wessel S., and Meng Z. Y. Competing pairing channels in the doped honeycomb lattice Hubbard model Physical Review B 94, 115105 (2016) Yacoub H., Fahle D., Eickelkamp M., Wille A., Mauder C., Heuken M., Kalisch H., and Vescan A. Effect of stress voltage on the dynamic buffer response of GaN-on-silicon transistors Journal of Applied Physics 119, 135704 (2016) Yang S., Homberger M., Noyong M., and Simon U. Polyol mediated synthesis and electrochemical performance of nanostructured LiMn2O4 cathodes International Journal of Electrochemical Science 11, 10847 (2016) Yegenoglu A., Quaglio P., Torre E., Grun S., and Endres D. Exploring the Usefulness of Formal Concept Analysis for Robust Detection of Spatio-temporal Spike Patterns in Massively Parallel Spike Trains in Graph-Based Representation and Reasoning (Iccs 2016) 9717, 3 (2016) Zakutna D., Matulkova I., Kentzinger E., Medlin R., Su Y., Nemkovski K., Disch S., Vejpravova J., and Niznansky D. Dispersible cobalt chromite nanoparticles: facile synthesis and size driven collapse of magnetism RSC Advances 6, 107659 (2016) Zalden P., Shu M. J., Chen F., Wu X. X., Zhu Y., Wen H. D., Johnston S., Shen Z. X., Landreman P., Brongersma M., Fong S. W., Wong H. S. P., Sher M. J., Jost P., Kaes M., Salinga M., von Hoegen A., Wuttig M., and Lindenberg A. M. Picosecond Electric-Field-Induced Threshold Switching in Phase-Change Materials Physical Review Letters 117, 067601 (2016) Zanolli Z. Graphene-multiferroic interfaces for spintronics applications Scientific Reports 6, 31346 (2016) Zehl L., Jaillet F., Stoewer A., Grewe J., Sobolev A., Wachtler T., Brochier T. G., Riehle A., Denker M., and Grun S. Handling Metadata in a Neurophysiology Laboratory Frontiers in Neuroinformatics 10, 26 (2016) Zhang B., Zhang W., Shen Z. J., Chen Y. J., Li J. X., Zhang S. B., Zhang Z., Wuttig M., Mazzarello R., Ma E., and Han X. D. Element-resolved atomic structure imaging of rocksalt Ge2Sb2Te5 phase-change material Applied Physics Letters 108, 191902 (2016) Zhang G. R., Gorelov E., Sarvestani E., and Pavarini E. Fermi Surface of Sr2RuO4: Spin-Orbit and Anisotropic Coulomb Interaction Effects Physical Review Letters 116, 106402 (2016) Zhang W., Hajiheidari F., Li Y., and Mazzarello R. Electronic and magnetic properties of H-terminated graphene nanoribbons deposited on the topological insulator Sb2Te3 Scientific Reports 6, 29009 (2016)


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Zhao J. Z., Fan W., Verstraete M. J., Zanolli Z., Fan J., Yang X. B., Xu H., and Tong S. Y. Quasi-One-Dimensional Metal-Insulator Transitions in Compound Semiconductor Surfaces Physical Review Letters 117, 116101 (2016) Zhou X. Z., Matthes F., Burgler D. E., and Schneider C. M. Magnetic surface domain imaging of uncapped epitaxial FeRh(001) thin films across the temperature-induced metamagnetic transition AIP Advances 6, 015211 (2016) Zhu G., Du L. P., Jin L., and Offenhäusser A. Effects of Morphology Constraint on Electrophysiological Properties of Cortical Neurons Scientific Reports 6, 23086 (2016) Zimmermann B., Long N. H., Mavropoulos P., Blügel S., and Mokrousov Y. Influence of complex disorder on skew-scattering Hall effects in L1(0)-ordered FePt alloy Physical Review B 94, 060406 (2016) Zimmermann B., Mavropoulos P., Long N. H., Gerhorst C. R., Blügel S., and Mokrousov Y. Fermi surfaces, spin-mixing parameter, and colossal anisotropy of spin relaxation in transition metals from ab initio theory Physical Review B 93, 144403 (2016) Zurhelle A. F., Deringer V. L., Stoffel R. P., and Dronskowski R. Ab initio lattice dynamics and thermochemistry of layered bismuth telluride (Bi2Te3) Journal of Physics-Condensed Matter 28, 115401 (2016)

Publication Details

JARA-FITJülich Aachen Research Alliancefor Fundamentals of Future Information TechnologyAnnual Report 2016

Published by:Forschungszentrum Jülich GmbH, 52425 JülichRWTH Aachen University, 52062 Aachen

Publication supported by the Excellence Initiative of the German federal and state governments.

Editors:Dr. Wolfgang SpeierManaging Director JARA-FITForschungszentrum Jülich GmbH52425 JülichGermanyPhone: ++49-2461-61-3107

Prof. Dr. Stefan TautzScientific Director JARA-FITPeter Grünberg Institute – Functional Nanostructures at Surfaces Forschungszentrum Jülich GmbH52425 JülichGermanyPhone: ++49-2461-61-4561

Prof. Dr. Matthias Wuttig Scientific Director JARA-FIT I. Institute of Physics A RWTH Aachen University 52074 Aachen Germany Phone: ++49-241-8027155

Contact:Dr. Wolfgang [email protected]

Layout:Ulrike Adomeit Silke Schilling

Year of publication:

Cover pictures refer to the following selected research reports (From left to right)E. Mlynczak et al., Fermi surface manipulation by external magnetic field demonstrated for a prototypical ferro-magnet, p. 119 | M. dos Santos Dias et al., Orbital magnetism as a fingerprint of topological magnetic structures, p. 49 | S. Heedt et al., Ballistic transport in InAs nanowire quantum point contacts, p. 33 | Y. Xiao et al., Spin-wave and electromagnon dispersions in multiferroic MnWO4 as observed by neutron spectroscopy, p. 87

2017

Annual Report 2016

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AN INITIATIVE OFAN INITIATIVE OF

JARA-FITJülich Aachen Research Alliancefor Fundamentals ofFuture Information Technology

OfficeForschungszentrum Jülich GmbH52425 JülichGermany

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