and Superconducting Materials” - University of...

The 4th

Indo-Japan Seminar

“Electronic Structure of Novel Magnetic

and Superconducting Materials”

University of Tokyo, February 1 –2, 2011

Supported by

Japan Society for the Promotion of Sciences (JSPS)

Department of Science and Technology, India (DST)

For LAN connections please use the following SSID and Key-

SSID: indojapan

Key: transitionmetaloxide

Program Feb. 1 (Tue) Chair: A. Fujimori 9:20-9:30 Opening 9:30-10:00 Masashi Kawasaki (U. of Tokyo)

“Transport properties of magnetic oxide thin films and interfaces” 10:00-10:30 Krishnakumar S. R. Menon (SINP)

“Surface and interfacial spin-structures in exchange coupled systems probed by magnetic spectromicroscopies”

10:30-11:00 Marcelo J. Rozenberg (Universite Paris-Sud)

“Direct observation of a 2DEG on the surface of SrTiO3 (or How to get the 2DEG on STO for 20$ instead of 2,000,000$)”

11:00-11:15 Kohei Yoshimatsu (U. of Tokyo)

“Metal-insulator transition and two-dimensional electron liquid in SrVO3 ultrathin films”

11:15-11:30 Coffee break

Chair: A. V. Mahajan

11:30-12:00 Harold Y. Hwang(U. of Tokyo)

“Epitaxial manganite-base junction transistors”

12:00-12:30 Hitoshi Tabata (U. of Tokyo)

“Spintronics and plasmonics based on oxide quantum wells”

12:30-12:45 Suvankar Chakraverty (RIKEN)

“Spontaneous ordering of the transition metals and physical properties of the double perovskite made by pulsed laser deposition”

12:45-13:55 Lunch

Chair: H. Katayama-Yoshida

13:55-14:25 Yoshinori Tokura (U. of Tokyo)

“Skyrmion crystals and topological transport phenomena”

14:25-14:55 E.V. Sampathkumaran (TIFR)

“Inverse metamagnetism' and anomalous 'magnetic phase co-existence' phenomena in some Tb-based intermetallic compounds”


Chair: K. Maiti

15:10-15:40 Ajay K. Sood (IISc)

“Time resolved pump probe studies and Raman spectroscopy of iron pnictides”

15:40-16:10 Parasharam M. Shirage ( AIST)

“Search for novel superconducting materials: high-pressure synthesis as a key technique”

16:10-16:40 Zenji Hiroi (U. of Tokyo)

“Rattling-induced superconductivity”


16:50-18:00 Poster session

18:20-20:20 Banquet (Kaien-Tei ) Feb. 2 (Wed) Chair: A. K. Sood 9:30-10.00 Atsushi Fujimori (U. of Tokyo)

“Three-dimensional Fermi surfaces of Fe pnictides” 10:00-10:30 Ashok K. Ganguli (IIT Delhi)

“Correlation of structural and transport properties of new Fe containing superconductor”

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10:30-11:00 Ryotaro Arita (U. of Tokyo)

“Dichotomy between large local and small ordered magnetic moment in iron-based superconductors”

11:00-11:15 Coffee break Chair: H. Tanaka 11:15-11:45 Hiroshi Katayama-Yoshida (Osaka U.)

“Electronic structure of high-Tc superconductors and dilute magnetic semiconductors: Beyond-LDA calculation vs. experiment”

11:45-12:00 Vijay Raj Singh (U. of Tokyo)

“Carrier-induced ferromagnetism of cobalt-doped anatase TiO2 thin films studied by soft x-ray magnetic circular dichroism”

12:00- 12:15 Ashish Chainani (RIKEN)

“Electronic structure of the titanates Ti4O7 and Co: TiO2” 12:15-13:30 Lunch Chair: R. Arita 13:30-14:00 Avinash V. Mahajan (IIT Bombay)

“NMR investigations of a few strongly correlated systems” 14:00-14:30 Hidekazu Tanaka (Osaka U.)

“Electronic structure of W-doped VO2 correlated oxide semiconductor and their nanoscopic physical property”

14:30-14:45 Coffee break Chair: D. D. Sarma 14:45-15:15 Kalobaran Maiti (TIFR)

“Kondo resonance in bulk non-Kondo systems” 15:15-15:45 Hidenori Takagi (U. of Tokyo)

“Superconductivity at a critical point in Rup and RuAs” 15:35-15:50 Coffee break
























Chair: E. V. Sampathkumaran 16:00-16:30

Masaki Azuma (TIT)

“Giant negative thermal expansion in BiNiO3 induced by intermetallic charge transfer”

16:30-17:00 D. D. Sarma (IISc)

“Colossal magneto-capacitance and multi-glass state in La2NiMnO6”







Feb. 1 (Tue)

Poster session

16:50-18:00

Poster No.

1. P. A. Bhobe (ISSP & RIKEN)

“Antiferromagnetic metal to correlated insulator transition in CrN”

2. V. R. Singh (U. of Tokyo)

“X-ray absorption spectroscopy and x-ray magnetic circular dichroism investigations of Co-doped BiFeO3 films”

3. A. Chainani (RIKEN)

“Electronic structure of the titanates Ti4O7 and Co:TiO2”

4. I. Nishi (U. of Tokyo)

“Composition dependence of Fermi surfaces in BaFe2(As1-xPx)2”

5. S. Pandey (Nagoya U.)

“Investigation of the role of spin-orbit coupling on transport properties of iron pnictide materials”

6. V. K. Verma (U. of Tokyo )

“Study of valence state and magnetic property of Fe in Fe-doped ZnO thin films”

7. G. Shibata (U. of Tokyo )

“Thickness dependence of the magnetic properties of La0.6Sr0.4MnO3 studied by soft X-ray magnetic circular dichroism”

8. H. Watanabe (RIKEN & CREST)

“Theoretical study of a novel spin-orbit-induced Mott insulator in Ir oxides”

9. M. Uchida (U. of Tokyo)

“Oxygen hole states in layered perovskite nickelate R2-xSrxNiO4”

10. S. Ideta (U. of Tokyo)

“Electronic structure of the electron-doped iron-based superconductors Ba(Fe1-xTMx)2As2 (TM = Ni, Cu) observed by angle-resolved photoemission spectroscopy”

11 Y. Yamazaki (U. of Tokyo)

“Soft X-ray magnetic circular dichroism study of the diluted magnetic semiconductor Zn1-xCrxTe”

12. Y. Shiomi (U. of Tokyo) “Anomalous Hall effect induced by spin-texture in a triangular-lattice

magnet Fe1+dSb”

13. S. Ishiwata (U. of Tokyo)

“Versatile helimagnetic phases under high magnetic fields in the cubic perovskite SrFeO3”

14. T. Kadono (U. of Tokyo, Hiroshima U.) “The origin of spin-split band at Sb(111) surface”

15. W. Uemura (U. of Tokyo) “Angle-resolved photoemission spectroscopy study of hole-doped and electron-doped cuprate Y1-zLaz(Ba1-xLax)2Cu3Oy”

16. G. Ahmad (Jamia Millia Islamia University) “Microwave power dependence of microwave absorption in Bi2212 crystals”

17. Y. Nomura (U. of Tokyo)

“Ab itinio derivation of low-energy model for alkali-doped C60

compounds”

18. K. Yoshimatsu (U. of Tokyo)

“Dimensional-crossover-driven metal-insulator transition in SrVO3 ultrathin films”

19. H. Kumigashira (U. of Tokyo)

“Fermi surface of SrRuO3 thin films studied by in-situ soft x-ray angle-resolved photoemission spectroscopy”

20. Munetoshi Seki (U. of Tokyo)

“Magnetic and electronic properties of two-dimensional triangular antiferromagnet RFe2O4 thin films grown by pulsed laser deposition”

21. Keisuke Ishigami (U. of Tokyo)

“Soft x-ray photoemission study of La1-xSrxTiO3 thin films”

22. Takashi Kurumaji (U. of Tokyo)

“Magnetic-field induced competition of two multiferroic orders in a triangular-lattice helimagnet MnI2”

23 Hiroshi Murakawa (ERATO, JST)

“Ferroelectricity induced by spin-dependent metal-ligand hybridization in Ba2CoGe2O7”

24. Kengo Oka (Tokyo Inst. of Technology)

“Pressure-induced spin state transition in BiCoO3”

25. Shin Miyahara (ERATO)

“Theory of magnetoelectric resonance in two-dimensional S=3/2 antiferromagnet Ba2CoGe2O7”

Transport properties of magnetic oxide thin films and interfaces

M. Kawasaki1, 2, 3

1

Quantum-Phase Electronics Center (QPEC) and Department of Applied Physics, University of Tokyo, Tokyo

113-8656, Japan 2

WPI Advanced Institute for Materials Research (AIMR) and Institute for Materials Research (IMR), Tohoku

University, Sendai 980-8577 3 Cross-correlated Materials Research Group (CMRG) and Correlated Electron Research Group (CERG),

RIKEN Advanced Science Institute, Wako 351-0198, Japan 4 CREST, Japan Science and Technology Agency, Tokyo 102-0075, Japan

We present anomalous Hall effect (AHE) in three magnetic oxide compounds, (TiCo)O2,

EuO, and EuTiO3, as a function of electron density. Common features are that they are

semiconductors that can be easily doped into n-type, they can be long-range ordered

ferromagnetic or antiferromagnetic, and all shows AHE. (TiCo)O2 is a diluted magnetic

semiconductor that shows carrier-induced ferromagnetism with a TC of 600 K. Anomalous

Hall conductivity scales with conductivity as AH = xx1.6

with changing electron density and

temperature [1]. We now show that ferromagnetism can be switched on and off by an electric

field [2]. EuTiO3 is known to be antiferromagnetic (TN = 5 K) in insulating state but electron

doping into Ti-O conducting band induces ferromagnetic order of Eu spins. In this sense,

EuTiO3 could be regarded as a magnetic semiconductor. We found EuTiO3 shows AHE where

its sign can be tuned with electron doping. This could be explained by the crossover of Fermi

level through band crossing point [3]. EuO is also a magnetic semiconductor but magnetic

ions are not diluted but fully occupy the cationic site. They show exchange-induced

ferromagnetism (TC = 70 K) that can be enhanced with injecting electrons in a crystal up to

120 K. We found for the first time that they also show AHE. More interestingly, AH changes

its sign depending on electron density and temperature (or conductivity). We also observe

peculiar peak structures in its AH verses magnetic field curves at a field where magnetization

is about to be saturated. This may something to do with a topological effect due to a spin

texture in space [4].

[1] H. Toyosaki, et. al. Nature Materials, 3, 221 (2004)

[2] Y. Yamada, et. al., to be published

[3] K. S. Takahashi, et. al., Phys. Rev. Lett., 103, 057204 (2009)

[4] T. Yamasaki, et. al., Appl. Phys. Lett., in press.

Surface and interfacial spin-structures in exchange coupled systems probed by magnetic

spectromicroscopies

Suman Mandal1, Krishnakumar S. R. Menon

2 and Rachid Belkhou

2

1Surface Physics Division, Saha Institute of Nuclear Physics, 1/AF, BidhanNagar, Kolkata 700 064

2Synchrotron SOLEIL, L'Orme des Merisiers Saint-Aubin, 91192 Gif-sur-Yvette, France

Knowledge of magnetism at surfaces and interfaces is crucial for their technological

applications in key areas such as data storage and other spintronic devices [1, 2].

Spectromicroscopic methods such as X-ray Magnetic Circular Dichroism and X-ray Magnetic

Linear Dichroism contrast employed in a Photoemission Electron Microscope (XMCD-PEEM

and XMLD-PEEM respectively) are suitable to probe the magnetism at the surfaces and

interfaces of exchange coupled systems with high lateral resolutions. Here, I will present

some of our recent studies [3, 4] of microscopic magnetic domain characterizations of bare

NiO(100) surfaces and Fe/NiO(100) exchange coupled systems at the surface and interfaces

using these spectromicroscopic methods. Microscopic modifications and evolution of

antiferromagnetic domain structure in view of exchange bias effect will be discussed along

with the different magnetic anisotropies observed on the ferromagnetic (Fe) layers. The

interfacial uncompensated (Ni) moments are observed to have a collinear coupling with the

Fe moments while the coupling is non-collinear with the antiferromagnetic spin-axis of NiO.

[1] S. A. Wolf et al., Science 294, 1488 (2001).

[2] I. Zutic et al., Rev. Mod. Phys. 76, 323 (2004).

[3] S. Mandal et al., Phys. Rev. B 80, 184408 (2009).

[4] S. Mandal et al.,, (Unpublished).

Direct observation of a 2DEG on the surface of SrTiO3

(or How to get the 2DEG on STO for 20$ instead of 2,000,000$)

M. J. Rozenberg

Laboratoire de Physique des Solides, Universite Paris-Sud, Orsay 91405, France.

As silicon is the basis of conventional electronics, so strontium titanate (SrTiO3) is the

foundation of the emerging field of oxide electronics. SrTiO3 is the preferred template for the

creation of exotic, two-dimensional (2D) phases of electron matter at oxide interfaces that

have metal–insulator transitions, superconductivity or large negative magneto-resistance. A

widely investigated system is the LAO/STO hetero-structure, however, the physical nature of

the electronic structure underlying the 2D electron gas (2DEG), which is crucial to

understanding the remarkable properties, remains elusive. In this talk, I shall describe our

recent discovery of a novel 2DEG at the vacuum-cleaved surface of SrTiO3 [1]. We obtain a

direct observation of the electronic structure of the 2DEG using angle-resolved photoemission

spectroscopy. We find that this 2DEG is confined within a region of about five unit cells and

has a sheet carrier density of ~0.33 electrons per square lattice parameter. Remarkably, the

electronic structure consists of multiple sub-bands of heavy and light electrons. The similarity

of this 2DEG to those reported in SrTiO3-based hetero-structures and field-effect transistors

suggests that different forms of electron confinement at the surface of SrTiO3 lead to

essentially the same 2DEG.

[1] A. Santander-Syro et al., Nature (in press).

Metal-insulator transition and two-dimensional electron liquid

in SrVO3 ultrathin films

K. Yoshimatsu1, K. Horiba

1-3, H. Kumigashira

1, 3, 4

T. Yoshida5, A. Fujimori

5, and M. Oshima

1-3

1 Department of Applied Chemistry, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

2 CREST, Japan Science and Technology Agency, Tokyo 113-8656, Japan

3 SRRO, The Univ. of Tokyo, Tokyo 113-8656, Japan

4 PRESTO, Japan Science and Technology Agency, Saitama 332-0012, Japan

5 Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

Metal-insulator transition (MIT) is one of the most interesting phenomena in condensed

matter physics. According to Mott-Hubbard theory, MIT can be controlled by the relative

magnitude of on-site Coulomb repulsion U and bandwidth W. Thus, in bulk materials, MIT is

studied by the chemical substitution of constituent ions for ones with smaller ion radius,

where W is controlled by resultant changes in bond angle and bond length between transition

metal and oxygen ions.

Another approach to bandwidth control is to use dimensional crossover in a thin film form.

Since the decrease of layer thickness causes the reduction of effective coordination number in

constituent ions, the resultant reduction of effective W may drive MIT in the conductive

transition metal oxides. In this study, we have adopted this approach to a typical 3d1

perovskite material SrVO3 (SVO) that shows metallic behavior, and investigated how spectral

function changes as a function of dimensionality using in situ photoemission spectroscopy

(PES). In the PES spectra, thickness-dependent MIT is clearly observed with decreasing film

thickness. This thickness dependent spectral behavior is in good agreement with the

calculation results based on dynamical mean field theory, indicating that the observed MIT is

caused by the reduction in magnitude of W due to the dimensional crossover [1].

These results suggest the possible creation of two-dimensional electron liquid (2DEL)

states in the ultrathin film of SVO. Recently, we have performed angle-resolved

photoemission spectroscopy (ARPES) on the SVO ultrathin film and obtained some

preliminary results about the confinement of strongly correlated electron in the SVO ultrathin

film. The possible creation of 2DEL states using oxide artificial structures will be discussed.

[1] K. Yoshimatsu et al., Phys. Rev. Lett. 104, 147601 (2010).

Epitaxial manganite-base junction transistors

Takeaki Yajima1, Yasuyuki Hikita

1,2, and Harold Y. Hwang

1,2,3

1Dept. of Advanced Materials Science, University of Tokyo, Kashiwa, Chiba 277-8561, Japan

2Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan

3Dept. of Applied Physics and SLAC Photon Science, Stanford University, Stanford, CA 94305, USA

We have been examining various aspects of perovskite manganite heterostructures. These

materials show strong coupling between charge, spin, and lattice degrees of freedom as

exemplified by „colossal magnetoresistance‟. The recent advances in thin film growth

techniques have enabled the generation of novel phases at oxide heterointerfaces, the atomic

control of their interface electronic structure, and their incorporation in novel device platforms.

We apply these techniques to manganite thin films, first emphasizing the subtleties in

optimizing the growth kinetics and stoichiometry [1,2], which has enabled us to create

atomically precise heterostructures exhibiting room temperature metallic ferromagnetism in

superlattices composed of just 5 unit cell layers [3]. The interface electronic structure was

examined using Schottky junctions formed between La0.7Sr0.3MnO3 and Nb-doped SrTiO3,

where the band offset (Schottky barrier height) can be controlled by the termination layer at

the interface [4]. This band engineering technique was applied in making a metal-base

transistor [5], which takes advantage of the strong internal electric field at interfaces. An

analysis of many devices enables the quantitative understanding of the evolution from a hot-

electron transistor to a permeable base transistor. This structure provides a platform for

developing devices incorporating the exotic ground states of perovskite oxides and their

interfaces.

[1] J. H. Song et al., Advanced Materials 20, 2528 (2008).

[2] D. A. Muller et al., Science 319, 1073 (2008).

[3] L. Fitting Kourkoutis et al., Proceedings of the National Academy of Sciences 107, 11682

(2010).

[4] Y. Hikita et al., Physical Review B 79, 073101 (2009).

[5] T. Yajima et al., Nature Materials (in press).

Spintronics and plasmonics based on oxide quantum wells

H. Tabata1,2

, H.Matsui1, M. Seki

1

1 Department of Electronic engineering, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

2 Department of Bioengineering, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

Diluted magnetic semiconductors (DMSs) are key materials in spintronics devices, where

the intention is to manipulate both the spin and charge degree of freedom by coupling

magnetic ions and charge carriers. In our research group, investigations concerning Zn1-xCoxO

have been extended over a long period of time from the viewpoints of layer growth and

magneto-optics.

Ferromagnetic ordering in Zn1-xCoxO, which is closely correlated with the electron density

of the host. Ferromagnetic (FM) signals were observed from SQUID and Hall measurements

1-3. Furthermore, XAS and XMCD measurements revealed that the origin of FM ordering was

derived from Co2+

ions placed in a tetrahedral coordination about the host, indicating an

intrinsic behavior 4

. Recently, we confirmed that paramagnetic Co2+

ions which did not

contribute to FM ordering were strongly coupled antiferromagnetically with each order,

suggesting inhomogeneity of the Co2+

ions of the host 5. Magnetic correlation between

localized Co2+

ions and excitons, which was investigated using photoluminescent (PL)

spectroscopy in a magnetic field 6

. Zn1-xCoxO shows large magneto-optical properties at the

excitonic edge due to the s,p-d exchange interaction, as observed from MCD measurements.

This suggests the presence of a magnetic-dependent PL property. We clarified that excitonic

PL and Co PL emissions were altered under strong magnetic fields, which was closely related

to the spin selection rule between excitonic and Co transitions. Band and g-factor engineering

based on the use of Zn1-xCoxO as an alloy material 7-9

. We achieved layer-by-layer growth of

Zn1-xCoxO 10

and grew superlattices of Zn1-xCoxO/ZnO with quantum geometry 11

. These

studies introduce new developments in the area of nanophotonics pertaining to ZnO DMSs. In

future, we propose that studies concerning magnetism in oxide DMSs should focus on low-

dimensional systems such as quantum structures.

[1]K. Ueda et al., Appl. Phys. Lett. 75, 4088 (2000).

[2]H. Saeki et al., J. Phys. Condens. Matter 16, S 5533 (2004).

[3]H. Matsui et al., Phys. Rev. B 75, 014438 (2007).

[4]M. Kobayashi et al., Phys. Rev. B 72, 201201R (2005).

[5]M. Kobayashi et al., Phys. Rev. B 81, 075204 (2010).

[6] Z.Y. Xiao et al., J. Appl. Phys. 108, 013502 (2010).

[7] Z.Y. Xiao et al., J. Appl. Phys. 103, 043504 (2008).

[8] H. Matsui et al., J. Appl. Phys. 99, 024902 (2006).

[9] H. Matsui et al., Appl. Phys. Lett. 94, 161907 (2009).

[10]H. Matsui et al., Physica Status Solidi C, 3, 4106 (2006).

Spontaneous ordering of the transition metals and physical properties of the double

perovskite oxides made by pulsed laser deposition technique

Suvankar Chakraverty1, Akira Ohtomo

2 and Masashi Kawasaki

134

1 Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

2 Department of Applied Chemistry, Tokyo Institute of Technology, Tokyo 152-8552, Japan

3 WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

4 CREST, Japan Science and Technology Agency, Tokyo 102-0075, Japan

To search new materials for future spinelectronic applications, double perovskites

A2BB‟O6 are one of the most important classes of oxide materials, where the magnetic as well

as electronic ground states can be designed and manipulated with choosing different

combinations of B and B‟ cations. After the discovery of a high-Tc half-metallic bulk

ferromagnet Sr2FeMoO6 (SFMoO),[1]

synthesis of double perovskite oxides has been

intensively investigated. However, realization of perfect ordering is restricted and an

empirical rule suggests that formal valence difference between B and B‟ (e.g. two for Fe3+

and

Mo5+

) play more important role than ionic radii difference.[2]

Here we report the synthesis of highly ordered Sr2FeTaO6 (SFTaO) and La2FeCrO6 films

on (111) SrTiO3 substrates by using pulsed laser deposition technique, starting from a single

disordered ceramic target. Both of the materials have been found to exhibit surprisingly high

degree of ordering. Structural properties of these materials have been studied in detail to

identify the phase, degree of ordering as well as the surface morphology. Magnetic

measurements have been carried out in SQUID magnetometer to identify the magnetic ground

state of these materials.

We also report a new protocol, we have developed to increase the degree of ordering of a

double perovskite called “pulsed laser interval deposition”. Where, a halt has been made

between two deposition periods to allow two B site atoms to rearrange themselves. We have

seen a drastic effect of interval time on the crystallinity as well as degree of ordering.[3]

Sr2CrReO6 has been used as a model system for this study.

[1] K.-I. Kobayashi et al. Nature 395, 677 (1998).

[2] M. T. Anderson et al. Prog. Solid State Chem. 22, 197 (1993).

[3] Suvankar Chakraverty et al. Appl. Phys. Lett. 97, 243107 (2010).

Skyrmion crystals and topological transport phenomena

Yoshinori Tokura1,2,3

1 University of Tokyo, Tokyo, Japan

2RIKEN ASI, Wako, Japan

3ERATO Multiferroics Project, JST, Tokyo

A class of helimagnet is derived from the Dzyaloshinskii-Moriya(DM) interaction on a

non-centrosymmetric crystal; prototypical examples are the B20 type (FeSi type) transition-

metal silicide and germanide families. Recently, the Skrymion lattice was confirmed to form

in a narrow temperature(T) -magnetic field(B) region near the hlimagnetic to paramagnetic

transition boundary. By contrast, thin films of B20 type MSi (M=Mn or Fe1-xCox ) or MGe

(M=Mn, Fe), whose thickness is smaller than the helical spin modulation period (=10-

100nm), ubiquitously form the two-dimensional (2D) Skyrmion crystal with magnetic fields

(B) applied normal to the film plane over a wide T-B region. The implication of such a 2D

Skyrmion crystal in the magneto-transport properties is discussed, such as the spin-chirality-

induced topological Hall effect.

This work was done in collaboration with X.Z. Yu, N. Kanazawa, Y. Onose, Y. Shiomi, Y. Matsui, N. Nagaosa, J.H. Park, J.H. Han, K. Kimoto, W.Z. Zhang, T. Arima, S. Wakimoto, K. Ohoyama, and K. Kakurai

„Inverse metamagnetism‟ and anomalous „magnetic phase co-existence‟ phenomena in

some Tb-based intermetallic compounds

E. V. Sampathkumaran

Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005

It is a well-recognized fact that the sign of magnetoresistance (MR) is negative at the field-

induced magnetic transitions. In contrast to this traditional belief, in a rare-earth-based

intermetallic compound, Tb5Si3, crystallizing in Mn5Si3 – type hexagonal structure, MR is

found to be interestingly positive, irrespective of whether the transition is first-order or

second-order (with varying temperature). We invoke the concept of „inverse metamagnetism‟

(in which magnetic fluctuations are introduced beyond the transition-field) to explain this.

There are observations attributable to an unusual magnetic-phase-coexistence phenomenon, in

which the transport interestingly is apparently dominated by high-resistive phase. We

observed similar anomalies in a few other Tb alloys as well.

• Sudden increase in positive MR at the “meta”magnetic transition in Tb5Si3 in the

entire T-range below magnetic ordering (<69 K), irrespective of whether the

transition is first order or second order

Mn5Si3 – type hexagonal structure

Raman spectroscopy and ultrafast pump-probe study of carrier dynamics in

iron pnictide superconductors

A.K. Sood

Department of Physics, Indian Institute of Science, Bangalore – 560 012, India

The talk will cover our recent Raman studies of iron pnictide superconductors as a function

of temperature. We will also present differential reflectivity pump-probe experiments carried

out using 40fs laser pulses on parent and doped 122 iron pnictide single crystals. In FeSe0.82,

a phonon mode near 100 cm-1

exhibits a sharp increase by ~5% in the frequency below a

temperature Ts (~100 K) attributed to strong spin-phonon coupling and onset of short-range

antiferromagnetic order. In addition, two high frequency modes are observed at 1350 cm-1

and 1600 cm-1

, attributed to electronic Raman scattering from (x2-y

2) to xz/yz d-orbitals of Fe

[1]. In CeFeAsO0.9F0.1, we find evidence for strong coupling between the phonons and crystal

field excitations [2]. Below the superconducting transition temperature, the phonon mode

near 280 cm-1

shows softening, signals its coupling with the superconducting gap. In

Ca(Fe0.95Co0.05) 2As2 also, the phonon mode near 260 cm-1

shows coupling with the

superconducting gap [3]. First-principles density-functional-theory-based calculations have

also been done to determine the effects of the strength of on-site electron correlation,

magnetic ordering, and spin-phonon coupling [4]. Temperature dependence of photo-excited

carriers in undoped and doped 122 systems showing interesting trends will be discussed.

I thank all my coauthors mentioned in the references below for their valuable contributions

to the ongoing collaboration.

[1] Pradeep Kumar et.al, Solid State Comm. (Fast Track) 150, 557 (2010).

[2]. Pradeep Kumar et.al, J. of Phys. Cond. Matt. 22, 255402 (2010).

[3]. Pradeep Kumar, et al., to be published

[4]. Anil Kumar et.al, J. Phys. Cond. Matt. 22, 385701 (2010).

Search for novel superconducting materials: high-pressure synthesis as a key technique

P. M. Shirage1, K. Kihou

1,2, C. H. Lee

1,2, H. Kito

1,2, Y. Tanaka

1, H. Eisaki

1,2, A. Iyo

1,2

1Nanoelectronics Research Institute (NeRI), National Institute of Advanced Industrial Science and Technology

(AIST), 1-1-1 Central 2, Umezono, Tsukuba, Ibaraki, 305-8568, Japan.

2 JST, Transformative Research-Project on Iron Pnictides (TRIP), 5, Sanbancho, Tokyo 102-0075, Japan

High-pressure synthesis technique (HPST) is a powerful technique for searching novel

superconducting materials. For synthesizing any material, it is critically important to sort out

adequate synthesis process, such as the choice of starting materials, reaction temperature,

atmosphere, etc. HPST provides us with a number of advantages over conventional solid state

reaction. First of all, pressure modifies the phase diagrams of matters towards the direction

which favors smaller volume. Second, owing to the enhanced reactivity under HP, it is a high-

throughput process, particularly effective for hunting new materials. Thirdly, by adopting HP

technique, one can prevent the evaporation of toxic and/or volatile elements, such as Tl, Hg,

As, K, and F.

Recent discovery of iron-based superconductors by Kamihara et al. [1], which are

composed of the alternating stacking of Ln2O2 layers and T2Pn2 layers (Ln: Lanthanides; T: Fe,

Co, Ni, Ru; Pn : As, P, etc), have been identified as novel high-superconducting transition

temperature (Tc) materials. Although, these materials known to exhibit high-Tc, the

preparation methods are very limited either due to toxicity and strong reactivity or materials

are unstable when prepared by a conventional method. We demonstrate the ability of HPST

for preparation of new iron-based superconductors and for searching new materials. By using

HPST, indeed, we succeeded in discovering several new materials. Inducing

superconductivity by creating an oxygen deficiency rather than doping fluorine at oxygen

site[2,3]; hydrogen doping for enhancing Tc by contracting lattice parameters[4]; stabilization

of the heavy lanthanide series like ErFeAsO1-y by hydrogen doping method (hydrogen work

as a medium to reduce the lattice mismatch between FeAs and LnO layers) [5]; etc, are few of

the special outcomes of HPST. Systematic studies on variation of the Ln in Ln-1111 series

indicate the existence of the strong correlation between the crystal structure and Tc. We also

succeeded in discovering the perovskite-type blocking layered Ca4Al2O6-yFe2Pn2 [6] and

Ca3Al2O5-yFe2Pn2 [7] superconductors which exhibit superconductivity for both Pn =As and P.

The usefulness of HPST and details of above results will be discussed in details.

[1] Y. Kamihara et al., J. Am. Chem. Soc. 130, 3296 (2008).

[2] K. Miyazawa et al., J. Phys. Soc. Jpn. 78, 034712 (2009).

[3] P. M. Shirage et al., Physica C 469, 355-369 (2009).

[4] K. Miyazawa et al., Appl. Phys. Lett. 96, 072517 (2010).

[5] P. M. Shiarge et al., EPL (2010) in press.

[6] P. M. Shirage et al., Appl. Phys. Lett. 97, 172506 (2010).

[7] P. M. Shirage et al., J. Am. Chem. Soc. communicated.

Rattling-induced superconductivity

Zenji Hiroi, A. Onosaka, Y. Okamoto and J. Yamaura

Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan

β-pyrochlore osmium oxides AOs2O6 with A = Cs, Rb and K exhibit superconducting

transitions at Tc = 3.3, 6.3 and 9.6 K, respectively, much higher than Tc = 1.0 K for the β-

pyrochlore rhenium oxide Cd2Re2O7 [1]. They crystallize in the cubic β -pyrochlore structure

with a ~ 10 Å, where OsO6 octahedra are connected by their vertices to form a 3D network

involving large atomic cages for the alkali metal atoms. Since the electronic structure near the

Fermi energy is dominated by strongly hybridized Os 5d and O 2p states with A states

appearing far above, the superconducting carriers mostly reside in the 3D skeleton. Thus, the

β-pyrochlore oxides are really 3D superconductors from both structural and electronic points

of view. In the three members, KOs2O6 with the highest Tc is distinguished from the others,

exhibiting various unconventional features. For example, a jump in specific heat at Tc is very

large in KOs2O6 compared with the others: C/Tc is approximately 2.9 in KOs2O6, while

close to 1.43 in the others, which means that the former lies in the strong-coupling regime,

while the latters in the weak-coupling regime.

Another interesting finding in the -pyrochlore oxides is that the alkali metal atom lying in

the oversized cage exhibits intensive 'rattling' behavior similar as reported in filled

skutterudite or clathrate compounds. Particularly, the smallest K atom shows the largest

rattling. It is believed that the superconductivity is induced by the rattling [2]. Possibly related

to this, a first-order phase transition at 7.6 K below Tc is found only in KOs2O6.

More recently, we have studied another interesting compound that shows intense rattling as

well as superconductivity [3]. It is an intermetallic compound Al10V or AlxV2Al20 crystalizing

in the CeCr2Al20 structure, where excess Al atoms are rattling inside a cage made of other Al

atoms. It is found that the rattling affects conducting properties and probably induce the

superconductivity, though the electron-rattler interactions are smaller than in the β-pyrochlore

oxides.

[1] Z. Hiroi et al., Phys. Rev. B 76, 014523 (2007).

[2] Y. Nagao et al., J. Phys. Soc. Jpn. 78, 064702 (2009).

[3] A. D. Caplin et al., J. Phys. F 8, 51 (1978).

Three-dimensional electronic structure and superconductuivity in Fe pnictides

Atsushi Fujimori

Department of Physics, University of Tokyo and

JST, Transformative Research-Project on Iron Pnictides (TRIP)

Band dispersions and Fermi surfaces of Fe pnictides are studied by ARPES in three-

dimensional momentum space.

Three-dimensionality is found to become stronger with electron doping in Ba(Fe1-

xCox)2As2. Due to the upward shift of the chemical potential in a rigid-band-like manner, the

cylindrical hole Fermi surfaces are warped and then disconnected into ellipsoids. Three-

dimensionality becomes strong also for isovalent doping, i.e., P substitution in BaFe2(As1-

xPx)2, but in a non-rigid-band-like manner. The simultaneous observations of the increased

three-dimensionality and nodal superconductivity as well as the eventual disappearance of

superconductivity are discussed within the framework of spin-fluctuation-mediate

superconductivity.

This work has been done in collaboration with T. Yoshida, S. Ideta, I. Nishi, M. Kubota,

K. Ono, S. Kasahara, T. Shibauchi, T. Terashima, Y. Matsuda, M. Nakajima, S. Uchida, Y.

Tomioka, T. Ito, K. Kihou, C. H. Lee, A. Iyo, H. Eisaki, H. Ikeda, R. Arita, Y. Nakashima, M.

Matsuo, T. Sasagawa, H. Harima, M. Nakajima, T. Ito, and S. Uchida.

Correlation of structural and transport properties of new Fe containing

superconductors

Ashok K Ganguli

Department of Chemistry, Indian Institute of Technology, Delhi New Delhi 110016, India

The past two years have seen a significant research activity on the new class of Fe based

superconductors (LnOFeAs, AFe2As2, AFeAs and FeSe) with maximum Tc of 55 K [1,2] in

Sm(O/F)FeAs. All these Fe-based superconductors consist of distorted FeAs/FeSe tetrahedra

which form the charge carrier layers [3]. LnOFeAs (Ln = rare earth) and AFe2As2 (A = Ca, Sr,

Ba) are antiferromagnetic semi-metals and exhibit structural transition around 150 K [4].

Superconductivity can be induced by doping of electrons or holes which modifies the

electronic and magnetic structure of these materials [5]. Our studies aim to enhance the Tc and

Hc by optimising the charge carriers using chemical substitution. We have obtained the first

antimony-doped superconductor (which also enhances Tc to 30.1 K from 28.5 K)[6]. Y-

substituted Ce/Y(O/F)FeAs show enhancement in Tc (from 38 K to 49 K) and Hc2 (from 94 T

to 148 T) due to increase in chemical pressure [7,8]. Superconductivity (Tc ~ 9 K) has also

been induced by substitution of isovalent P-substitution (increase in chemical pressure) in

PrOFeAs. Interestingly PrOFe0.9Co0.1As1-xPx, show suppression in Tc with increase in

chemical pressure (P-content) [9]. Cobalt substituted LnOFe/CoAs (Ln = Ce and Pr)

compounds show superconductivity with maximum Tc of ~14 K for PrOFe0.9Co0.1As [10].

Enhancement of Tc from 11.4 K to 12.3 K was achieved for CeOFe0.9Co0.1As with a small

increase of pressure (up to 0.4 GPa) [11]. Yb-doped Ce/YbO0.9F0.1FeAs also show

enhancement in Tc (from 43 K to 49 K) with raising the external pressure. Many of these

superconductors show very high critical fields. The talk focuses on the pnictide

superconductors developed so far, their transport properties and the commonalities and

differences with other superconducting families.

[1] Y. Kamihara et al., J. Amer. Chem. Soc. 130, 3296 (2008).

[2] Z. A. Ren et al., Phys. Lett. 25, 2215 (2008).

[3] C. W. Chu, Nature Physics 5, 787(2009).

[4] C. R. Cruz et al., Nature 453, 899 (2008).

[5] J. Prakash et al., Europhys. Lett. 84, 5700 (2008).

[6] S. J. Singh et al., Supercond. Sci. Technol. 22, 045017 (2009).

[7] J. Prakash et al., Appl. Phys. Letts. 95, 262507(2009).

[8] A. K. Ganguly et al., Eur. Phys. J. B 73, 177 (2010).

[9] S. Sharma et al., Supercond. Sci. Technol. (communicated).

[10] J. Prakash et al., J. Solid State Chem. 183, 338 (2010).

[11] R. S. Kumar et al., Appl. Phys. Lett. 98, 012511 (2011).

Dichotomy between large local and small ordered magnetic moments in iron-based

superconductors

P. Hansmann1, R. Arita

2,3,4, A. Toschi

1, S. Sakai

1, G. Sangiovanni

1, and K. Held

1

1Institut for Solid State Physics, Vienna University of Technology, 1040 Vienna, Austria

2Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan,

3JST, TRIP, Sanbancho, Chiyoda, Tokyo 102-0075, Japan,

4JST, CREST, Hongo, Tokyo 113-8656, Japan

In the recently discovered iron-based superconductors, the role of electronic correlation

is still unclear. When the local magnetic moment is small and the system is metallic, we

usually expect that weak-coupling theories can be applied. On the other hand, if the local

magnetic moment is large and the exchange coupling between neighboring spins is the

dominant interaction, not only the electronic states around the Fermi level but also the higher

energy excitations (on the scale of the local Hubbard interaction U) are expected to play an

important role in the superconducting pairing mechanism. For instance, the latter is definitely

the case for cuprates, which are Mott insulating in the absence of carrier doping. Iron

pnictides instead are metallic and undergo a spin-density wave (SDW) transition below

T~150K whose characteristics are still under debate.

Experimentally, it has been clarified that the different members of the pnictide family have

quite different ordered magnetic moments. On the other hand, the band structures of these

compounds however do not show distinctive differences, and indeed, LSDA always yields an

ordered moment of ~2μB for the experimental crystal structures. These facts suggest that

dynamical quantum fluctuations, not included in LSDA, are crucial for these systems. Namely,

they can explain the presence of large local magnetic moments which form because of local

Coulomb and exchange interaction but only give rise to a much smaller ordered moment at

lower temperatures.

Among various approaches which attempt to go beyond LDA, dynamical mean field theory

(DMFT) is one of the most promising methods, particularly when combined with ab initio

calculations. Recently, we have performed LDA+DMFT calculation for iron-based

superconductors, focusing on the dynamics of the local magnetic moment [1]. We found that

correlation effects are hardly visible in the single-particle spectral function, while, at the same

time, the (two-particle) spin-spin correlation function reveals the existence of a large local

magnetic moment.

[1] P. Hansmann et al., Phys. Rev. Lett. 104, 197002 (2010).

Electronic structure of high-Tc superconductors and dilute magnetic semiconductors:

beyond-LDA calculation vs. experiment

Yoshitaka Mino1, Tatsuo Kano

1, Masayuki Toyoda

2, Hidetoshi Kizaki

1, Tetsuya Fukushima

1,

Kazunori Sato1 and H. Katayama-Yoshida

1

1Department of Materials Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan

2Research Center for Integrated Science, Japan Advanced Institute of Science and Technology, Ishikawa 923-

1292, Japan

It is well accepted that the local density approximation (LDA) not only explains

experimental results reasonably but also predicts materials properties successfully in dilute

magnetic semiconductors (DMS) [1]. However, at the same time, it becomes apparent that the

LDA has systematic over estimation of exchange-correlation energy in the description of the

electronic structure. Typical example is the systems of transition metal oxides, in particular

high-TC superconductors based on cuprates [2], such as La2-xSrxCuO4 and YBa2Cu3O7, or

DMS [1], such as (Zn,Co)O and (Ti,Co)O2, where Zener‟s double-exchange mechanism

dominated the magnetism. It is known that the LDA cannot reproduce anti-ferromagnetic

charge-transfer insulating phases of them. In this paper, by combining the self-interaction

correction (SIC) method proposed by Filippetti et al. [3] with the Korringa-Kohn-Rostoker

coherent potential method [4], the acceptor doping dependence of electronic structure in

cuprate highTC materials is calculated. It is found that by taking into account the SIC, the anti-

ferromagnetic charge-transfer insulator is correctly predicted and experimental photoemission

spectra are reproduced very reasonably. By comparing the total energies of ferromagnetic

(FM), anti-ferromagnetic (AF) and disordered local magnetic states (DLM), we will discuss

the phase diagram (AF, DLM, and FM state) and doping dependence of the magnetic state

caused by the competition between the super-exchange and Zener‟s p-d exchange interactions

in La2-xSrxCuO4 and YBa2Cu3O6+x.

We also discuss the magnetic mechanism and electronic structure by comparing the SIC-

LDA vs. photoemission experiments on (Zn,Co)O [4] and (Ti,Co)O2 [5]. If time is available,

we will discuss the role of spinodal nano-decompostion in Konbu-Phase and Dairiseki-Phase

[6] in order to increase the Tc of High-Tc DMS and cuprate high-Tc superconductors.

[1] K. Sato et al., Rev. Mod. Phys., 82, 1633 (2010).

[2] W. E. Pickett, Rev. Mod. Phys., 61, 433 (1989).

[3] A. Filippetti et al., Phys. Rev. B 67,125109 (2003).

[4] M. Toyoda et al., Physica B 376, 647 (2006).

[5] H. Kizaki et al., Applied Physics Express, 2, 053004 (2009)

[6] K. Sato et al., to be submitted to Rev. Mod. Phys. (2011).

Carrier-induced ferromagnetism of cobalt-doped anatase TiO2 thin films studied by soft

x-ray magnetic circular dichroism

V. R. Singh1*

, T. Kataoka1, Y. Yamazaki

1, V. K.Verma

1, G. Shibata

1, A. Fujimori

1,

F.-H. Chang2, H.-J. Lin

2, D.-J. Huang

2, C. T. Chen

2, Y. Yamada

3, T. Fukumura

3,4 and

M. Kawasaki3, 5, 6

1Department of Physics, Graduate School of Science, University of Tokyo, Japan.

2National Synchrotron Radiation Research Center (NSRRC), Taiwan.

3Institute for Materials Research, Tohoku University, Japan. 4PRESTO, Japan Science and Technology Agency, Japan.

5WPI Advanced Institute for Materials Research, Tohoku University, Japan.

6CREST, Japan Science and Technology Agency, Japan.

The Co-doped TiO2 system, first reported to be ferromagnetic by Matsumoto et al. [1], has

recently received much attention. In anatase Co-doped TiO2, the magnetization value is large

compared to rutile Co-doped TiO2 probably because of the high mobility. Here, we have

performed x-ray absorption spectroscopy (XAS) and soft x-ray magnetic circular dichroism

(XMCD) studies on anatase Co-doped TiO2 by the surface-sensitive total electron yield

(TEY) mode and the bulk-sensitive total fluorescence yield (TFY) mode.

Epitaxial Ti1-xCoxO2-δ (x =0.05) anatase thin films were grown on LaAlO3 (001) substrates

by the pulsed laser deposition method at 250 K at different oxygen pressures. The samples

fabricated at PO2=8 x 10−7

, 2 x 10−6

and 1 x 10−6

Torr are referred to as metallic, insulating,

and intermediate, respectively. The carrier densities ne for the metallic, intermediate and

insulating samples were found to be 4 x1019

, 1.1 x 1019

and 4.1 x 1018

cm-3

, respectively.

Magnetization measurements of Ti1-xCoxO2-δ (x =0.05) thin films reveal ferromagnetic

behavior in M-H loop at room temperature with the saturation magnetization of 0.63 - 2.14

µB/Co. The XAS and the XMCD spectra taken in the TEY mode for the metallic and

intermediate samples have clear multiplet features characteristic of Co

2+ ions coordinated by

O2–

ions. These spectra show sharp contrast to the featureless XMCD spectrum of Co metal or

metallic clusters. For the insulating sample we observed clear XAS spectra similar to that of

metallic and intermediate samples, however, no XMCD was observed in TEY mode. The

estimated magnetic moment for the metallic and the intermediate samples obtained from

XMCD in the TEY mode was 0.21 µB/Co and 0.26 µB/Co, respectively. On the other hand, in

the bulk region probed by the TFY mode, broad and strong XMCD spectra with similar

spectral line shapes were obtained for all the samples. The magnetization and XMCD

intensity increased with carrier density, strongly suggesting carrier-induced origin of the

ferromagnetism. The magnetic moment of these films obtained in the bulk sensitive TFY

mode was 0.29 - 2.4 µB/Co, which is in the same range as the saturation magnetization. The

broadening of the TFY spectra may be related to the position of Co atoms which might be

frozen in various local structures as suggested by Matsumura et al. [2].

[1] Y. Matsumoto et al., Science 291, 854 (2001).

[2] T. Matsumura et al., Phys. Rev. B 76, 115320 (2007).

Electronic structure of the titanates Ti4O7 and Co:TiO2

M. Taguchi1, T. Ohtsuki

1, A. Chainani

1,2, M. Matsunami

1, R. Eguchi

1, Y. Takata

1,2, M.

Yabashi2, K. Tamasaku

2, Y. Nishino

2, T. Ishikawa

2, Y. Senba

3, H. Ohashi

3, S. Tsuda

4, S.

Watanabe4, C.-T. Chen

5, K. Fujiwara

6, Y. Nakamura

6, H. Takagi

6, and S. Shin

1,4

1 Excitation-order Research Team, RIKEN Spring-8 Center, Sayo, Hyogo 679-5148, Japan

2 Coherent X-ray Optics Laboratory, RIKEN Spring-8 Center, Sayo, Hyogo 679-5148, Japan

3 JASRI/Spring-8, Sayo, Hyogo 679-5148, Japan

4 Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8561, Japan

5 Beijing Center for Crystal R & D, Chinese Academy of Science, Zhongguancun, Beijing

6 Department of Advanced Materials Science, University of Tokyo, Kashiwa, Chiba 277-8561, Japan

We investigate the electronic structure of the titanates : (a) Ti4O7, the Magneli

compound known to exhibit two sharp transitions in electrical resistivity as a function of

temperature[1], and (b) Co:TiO2, a room temperature dilute ferromagnetic oxide[2].

(a) Ti4O7 exhibits two sharp jumps in resistivity at T1 = 142 K and T2 = 154 K. Using a

combination of X-ray absorption and resonant photoemission spectroscopy across the Ti L-

edge, as well as laser- and hard x-ray photoemission, we study the electronic structure across

T1 and T2. The Ti 3d partial density of states in Ti4O7 exhibit a Fermi edge in the high

temperature metal phase which gets pseudo-gapped in the intermediate phase between T1 and

T2, and then becomes a charge ordered insulator below T1. The results suggest that the

intermediate state, sandwiched between a charge ordered Mott-insulator (≡electronic solid,

below T1) and a Fermi liquid metal(≡electronic liquid, above T2), is best described as an

anomalous dual-character „solid+liquid‟ electronic phase[3].

(b) We study the surface and bulk electronic structure of the room-temperature ferromagnet

Co:TiO2 anatase films using soft and hard x-ray photoemission spectroscopy. We obtain direct

evidence of metallic Ti3+

states in the bulk, which get suppressed in surface sensitive

experiments. X-ray absorption and resonant photoemission spectroscopy reveal Ti3+

electrons

at the Fermi level(EF), and high-spin Co2+

electrons occurring away from EF. The results show

the importance of the charge neutrality condition : Co2+

+ VO2-

+ 2Ti4+

Co2+

+ 2Ti3+

(VO is

oxygen vacancy), which gives rise to the elusive Ti 3d carriers mediating ferromagnetism via

the Co 3d-O 2p-Ti 3d exchange interaction pathway of the occupied orbitals[4].

[1] M. Marezio et al., Phys. Rev. Lett. 28, 1390 (1972) ; M. Abbate et al., Phys. Rev. B. 51,

10150 (1995).


[3] M. Taguchi et al., Phys. Rev. Lett. 104, 106401 (2010).

[4] T. Ohtsuki et al., 2011.

NMR Investigations of a few strongly correlated systems

A.V. Mahajan

IIT Bombay, Powai, Mumbai 400076

In this talk, I will present recent results on three systems with diverse emphases; (i)

superconductivity in Ba(Fe1-xRux)2As2, (ii) ferromagnetism in nanoparticles of SnO2, and (iii)

geometrical frustration in the triangular system Ba3YIr2O9.

The variation of 75

As NMR parameters with composition and temperature has been studied

for Ba(Fe1-xRux)2As2 system where Fe is replaced by isovalent Ru. While the Ru-end member

was found to be a conventional Fermi liquid, the composition x=0.5 corresponding to the

highest Tc (~20 K) in this system shows a downturn in the Korringa ratio with decreasing

temperature, evidencing the presence of antiferromagnetic (AF) fluctuations. These results

point to the possible role of AF fluctuations in driving superconductivity.

The appearance of ferromagnetism in nanoparticles of a variety of, otherwise diamagnetic,

oxides such as ZnO, Al2O3, SnO2, etc. has been reported earlier. We present here, 119

Sn NMR

measurements in nanoparticles of SnO2. From our data, we infer that nearly 30% of the Sn

atoms carry a static local moment while the remaining have fluctuating local moments of

varying magnitudes.

The fractional valence of Ir in the geometrically frustrated, edge-shared triangular system

Ba3YIr2O9 presents possibilities of exotic magnetic behaviour. We will present and discuss our

results of an 89

Y NMR study of this compound in addition to magnetization and heat capacity

measurements.

Electronic structure of W-doped VO2 correlated oxide semiconductor and their

nanoscopic physical property

Hidekazu Tanaka1, Hidefumi Takami

1, Kenichi Kawatani

1, Teruo Kanki

1,

1Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka, 567-0047,

Japan

Vanadium dioxide (VO2) has electronically attractive properties, showing orders of

magnitude changes in resistivity at metal-insulator transition temperature, around 340 K. A

Mott–Hubbard or Peierls transition mechanism has been discussed in relation to these

phenomena. Dopant control of VO2 is a promising method for controlling their giant

physical properties around RT. We use hard X-ray core-level photoemission spectroscopy

(HX-PES) to investigate the electronic structure of W-doped VO2 (VWO) thin films

exhibiting a high temperature coefficient of resistance, above −10 %/K at room temperature

[1]. According to the W 4d core-level spectra, the chemical state of doped W takes only a 6+

valence state, which suggests the introduction of V3+

. The satellite structure of the V 2p3/2

main peak, which corresponds to the metallic-coherent screened states, was enhanced for

VWO compared with VO2 indicating that electron doping plays an important role in the

control of metal-insulator transition. We discuss a filling control mechanism of metal-

insulator transition on this material utilizing strongly correlated electronics. Furthermore,

we will discuss their nanoscopic phase separation nature from the detailed analysis of HX-

PES spectra and possibility of enhanced functionality using artificial nano structuring

technique.

[1] H. Takami et al., Appl. Phys. Exp. 3, 063201 (2010).

[2] S. Yamanaka, et al., Nano Lett., in press

Kondo resonance in bulk non-Kondo systems

K.Maiti

Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005, India

Magnetic impurity in a metal often leads to a logarithmic enhancement of electrical

resistivity at low temperatures in contrast to the decrease expected. This phenomena is known

as Kondo effect and occurs due to the antiparallel coupling of the impurity states with the

conduction electronic states. Such coupled electronic states appear as a sharp feature at the

chemical potential, called Kondo resonance feature. Employing high resolution

photoemission spectroscopy, we studied the evolution of Kondo resonance feature in RB6 (R

= La, Ce, Pr, Nd) and Ce2(RhCo)Si3 as a function of temperature that helps to probe this effect

as a function of 4f binding energy and 4f-conduction electron hybridization strength.

Experimental spectra of Kondo systems reveal the growth of multiple Kondo resonance

features with decreasing temperature relative to the uncompensated local moment

contributions that experimentally demonstrates the Kondo effect. Ironically, the features near

Fermi level in the valence band spectra of PrB6, NdB6, Ce2RhSi3 etc also exhibit similar

temperature dependence signalling the presence of Kondo compensation effect although their

bulk physical properties do not exhibit Kondo effect.

Superconductivity at a critical point in RuP and RuAs

D. Hirai1, T. Takayama

1, H. Takagi

1, 2

1 Department of Advanced Materials, University of Tokyo, Kashiwa 277-8561, Japan

2 RIKEN Advanced Science Institute, Wako 351-0198, Japan

RuP and RuAs crystallize in an orthorhombic MnP structure. We found a first order metal -

spin singlet insulator transition around 260 K and 200 K respectively in these two compounds.

The structural analysis indicates the presense of additional phase transition roughly 50 K

above the metal-insulator transition, accompanied with weak anomalies in the resitivity and

the magnetic susceptibility. Rh-doping suppresses those sequential phase transitions and the

system eventually becomes metallic down to the lowest temperature. When the resistivity and

susceptibility anomalies, very likely representing the high temperature phase transition, fade

out upon doping, we found superconductivity with a maximum Tc of 3.7 K and 1.8 K for (Ru,

Rh)P and (Ru, Rh)As, respectively. Interestingly, superconductivity is observed only around

the seemingly critical point and the doping dependence of Tc is dome-shaped, as observed in

the superconductivity at a magnetic critical point in heavy fermions.

Giant negative thermal expansion in BiNiO3 induced by intermetallic charge transfer

Masaki Azuma,1,2

Wei-tin Chen,2,3

Hayato Seki,2 Michal Czapski,

2 Kengo Oka,

1,2 Shintaro

Ishiwata,2,4

Yuichi Shimakawa2 and J. Paul Attfield

3

1Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama, 226-8503,

Japan 2Institute for Chemical Research, Kyoto University, Uji, Kyoto, 611-0011, Japan

3Centre for Science at Extreme Conditions and School of Chemistry, University of Edinburgh, Mayfield Road,

Edinburgh, EH9 3JZ, United Kingdom 4Department of Applied Physics and Quantum-Phase Electronics Center (QPEC), University of Tokyo, 2-11-16,

Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan

The unusual property of negative thermal expansion (NTE) is of fundamental interest and

may be used to fabricate composites with zero or other controlled thermal expansion values.

Many framework materials such as ZrW2O8 and Cd(CN)2 show NTE over a wide temperature

range. NTE can also result from transitions between different electronic or magnetic states

strongly coupled to the lattice, giving large negative expansions down to a previous record

dilatometric value of -25 10-6

K-1

for (Mn0.96Fe0.04)3(Zn0.5Ge0.5)N at 316-386 K. BiNiO3 is

an antiferromagnetic insulator with a unique charge distribution of Bi3+

0.5Bi5+

0.5Ni2+

O3. It

shows a 2.6% volume reduction under pressure due to a Bi/Ni charge transfer accompanied by

an insulator to metal transition. The charge transfer transition is shifted to ambient pressure

through lanthanum substitution for Bi. Because the low-temperature and high-temperature

phases coexist changing their fractions each other in a wide temperature range above room

temperature, the weighted volume smoothly decreases on heating. The crystallographic linear

expansion coefficient for Bi0.95La0.05NiO3 is -137 10-6

K-1

and a value of -82 10-6

K-1

is

observed between 320 and 380 K from a dilatometric measurement on a ceramic pellet.

[1] S. Ishiwata et al., J. Mater. Chem. 12, 3733 (2002).

[2] M. Azuma et al., J. Am. Chem. Soc. 129, 14433 (2007).

[3] S. Ishiwata et al., Phys. Rev. B 72, 045104 (2005).

[4] M. Azuma et al., submitted.

Colossal magneto-capacitance and multi-glass state in La2NiMnO6

D. D. Sarma,

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, Japan

La2NiMnO6 exists in both monoclinic and rhombohedral forms, usually forming as a

mixture of both phases. Both crystallographic forms behave like re-entrant spin-glass, with

successive phase transitions from paramagnetic to ferromagnetic, and ferromagnetic to spin

glass states. While both crystallographic forms exhibit glassy dielectric behaviour, there are

drastic differences in the magneto-dielectric properties of the two crystallographic pahses. The

rhombohedral phase does not exhibit any magnetocapacitive coupling; in contrast, the

monoclinic La2NiMnO6 shows a substantial magnetocapacitance near the ferromagnetic

transition temperature. We discuss the role of disorder in controlling most of these properties.

This work has been carried out in collaboration with Debraj Choudhury, P. Mandal, R.

Mathieu, Abhijit Hazarika, Somnath Pal, Sundar Rajan, A. Sundaresan, Umesh V. Waghmare,

O. Karis, and P. Nordblad.

Posters

Antiferromagnetic metal to correlated insulator transition in CrN

P. A. Bhobe1, 2

, A. Chainani2, M. Taguchi

2, T. Takeuchi

3, R. Eguchi

1,2, M. Matsunami

1,2, K.

Ishizaka1, Y. Takata

2, M. Oura

2, Y. Senba

3, H. Ohashi

3, Y. Nishino

2, M. Yabashi

2,

K. Tamasaku2, T. Ishikawa

2, K. Takenaka

4,5, H. Takagi

5, and S. Shin

1, 2

1 Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan. 2 RIKEN, SPring-8 Centre, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan

3JASRI/SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan

4Department of Crystalline Materials Science, Nagoya University, Nagoya 464-8603, Japan

5RIKEN, The Institute for Physical and Chemical Research, Wako, Saitama 351-0198, Japan

CrN is a paramagnet at room temperature with NaCl-type crystal structure that undergoes

a magneto-structural transition below TN ~ 280 K to an antiferromagnetically ordered

orthorhombic Pnma phase. Various electrical resistivity measurements present an

inconclusive picture of its electronic state, suggesting a (i) metal to metal, (ii) insulator to

insulator and (iii) insulator to metal transition across TN [1]. We have investigated the

electronic structure of polycrystalline CrN across TN using x-rays and laser photoemission

spectroscopy (PES). Valence band spectrum identifies high intensity Cr 3d dominated peak

close to the Fermi-level (EF). However, only the tail of this peak reaches EF indicating a

highly reduced DOS. Role of electron correlations in the description of electronic structure of

CrN has been suggested by [2]. Using Cr 2p-3d resonant PES, we obtain an on-site Coulomb

energy U~ 4.5 eV that remains unchanged above and below TN. High resolution (6 meV)

bulk-sensitive laser PES reveals a clear Fermi edge indicating a metallic phase below TN. Low

temperature metallic phase is also reflected in the Cr 2p core-level spectra where a well-

screened feature develops due to coherent states at EF as substantiated by cluster model

calculations using the experimentally observed U. Also, our detailed analysis of the electrical

resistivity confirms an insulating behavior above TN with an activation gap of ~ 70 meV that

transforms into a disordered metal below TN. The results thus indicate CrN exhibits a coupled

magneto-structural and insulator to metal transition as a function of temperature and comply

with the recent study that identified softening of bulk CrN under pressure due to a crossover

from a localized to a molecular orbital electronic transition [3].

[1] J. D. Browne et al., Phys. Status Solidi 1, 715 (1970) Y. Tsuchiya et al., Mater. Trans.,

JIM 37, 121 (1996); P. S. Herle et al., J. Solid State Chem. 134, 120 (1997); D. Gall et al., J.

Appl. Phys. 91, 5882 (2002); C. Constantin et al., Appl. Phys. Lett. 85, 6371 (2004).

[2] Aditi Herwadkar and Walter R. L. Lambrecht, Phys. Rev. B 79, 035125 (2009).

[3] Francisco Rivadulla et al., Nature Mater. 8, 947 (2009).

X-Ray Absorption Spectroscopy and X-Ray Magnetic Circular Dichroism Investigations

of Co-doped BiFeO3 Films

V. R. Singh1*

, K. Ishigami1, Y. Yamazaki

1, V. K. Verma

1, A. Fujimori

1, Y. Takeda

2, T. Okane

2,

Y. Saitoh2, H. Yamagami

2, 3, Y. Nakamura

4, M. Azuma

4 and Y. Shimakawa

4

1Department of Physics, Graduate School of Science, University of Tokyo, Japan

2Condensed Matter Science Division, JAEA, Hyogo, 679-5198 Japan

3 Kyoto Sangyo University, Kita-Ku,Kyoto-City 603-8555,Japan

4Institutes for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan

BiFeO3 (BFO)-based epitaxial thin films by partially substituting Co ions for the B-site Fe

ions [1] are found to show enhanced magnetic property. The electronic structures of BFO and

Co-doped BFO have been studied by X-ray absorption spectroscopy (XAS) and soft-X-ray

magnetic circular dichroism (XMCD). The Fe and Co XAS spectra it is clear that both Fe and

Co have the valence of 3+. It has become clear that magnetization increases up-to 20% Co

doping in BFO, which is in good agreement with magnetization measurement.

[1] Y. Nakamura et al., Jpn. J. Appl. Phys. 49 (2010) 051501

Electronic structure of the titanates Ti4O7 and Co:TiO2

M. Taguchi1, T. Ohtsuki

1, A. Chainani

1,2, M. Matsunami

1, R. Eguchi

1, Y. Takata

1,2, M.

Yabashi2, K. Tamasaku

2, Y. Nishino

2, T. Ishikawa

2, Y. Senba

3, H. Ohashi

3, S. Tsuda

4, S.

Watanabe4, C.-T. Chen

5, K. Fujiwara

6, Y. Nakamura

6, H. Takagi

6, and S. Shin

1,4

1 Excitation-order Research Team, RIKEN Spring-8 Center, Sayo, Hyogo 679-5148, Japan

2 Coherent X-ray Optics Laboratory, RIKEN Spring-8 Center, Sayo, Hyogo 679-5148, Japan

3 JASRI/Spring-8, Sayo, Hyogo 679-5148, Japan

4 Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8561, Japan

5 Beijing Center for Crystal R & D, Chinese Academy of Science, Zhongguancun, Beijing

6 Department of Advanced Materials Science, University of Tokyo, Kashiwa, Chiba 277-8561, Japan

We investigate the electronic structure of the titanates : (a) Ti4O7, the Magneli

compound known to exhibit two sharp transitions in electrical resistivity as a function of

temperature[1], and (b) Co:TiO2, a room temperature dilute ferromagnetic oxide[2].

(a) Ti4O7 exhibits two sharp jumps in resistivity at T1 = 142 K and T2 = 154 K. Using a

combination of X-ray absorption and resonant photoemission spectroscopy across the Ti L-

edge, as well as laser- and hard x-ray photoemission, we study the electronic structure across

T1 and T2. The Ti 3d partial density of states in Ti4O7 exhibit a Fermi edge in the high

temperature metal phase which gets pseudo-gapped in the intermediate phase between T1 and

T2, and then becomes a charge ordered insulator below T1. The results suggest that the

intermediate state, sandwiched between a charge ordered Mott-insulator (≡electronic solid,

below T1) and a Fermi liquid metal(≡electronic liquid, above T2), is best described as an

anomalous dual-character „solid+liquid‟ electronic phase[3].

(b) We study the surface and bulk electronic structure of the room-temperature ferromagnet

Co:TiO2 anatase films using soft and hard x-ray photoemission spectroscopy. We obtain direct

evidence of metallic Ti3+

states in the bulk, which get suppressed in surface sensitive

experiments. X-ray absorption and resonant photoemission spectroscopy reveal Ti3+

electrons

at the Fermi level(EF), and high-spin Co2+

electrons occurring away from EF. The results show

the importance of the charge neutrality condition : Co2+

+ VO2-

+ 2Ti4+

Co2+

+ 2Ti3+

(VO is

oxygen vacancy), which gives rise to the elusive Ti 3d carriers mediating ferromagnetism via

the Co 3d-O 2p-Ti 3d exchange interaction pathway of the occupied orbitals[4].

[1] M. Marezio et al., Phys. Rev. Lett. 28, 1390 (1972) ; M. Abbate et al., Phys. Rev. B. 51,

10150 (1995).


[3] M. Taguchi et al., Phys. Rev. Lett. 104, 106401 (2010).

[4] T. Ohtsuki et al., to appear in Phys. Rev. Lett. 28, (2011).

Composition dependence of Fermi surfaces in BaFe2(As1-xPx)2

I. Nishi1, S. Ideta

1, T. Yoshida

1,5, A. Fujimori

1,5,

S. Kasahara2, T. Terashima

2, T. Shibauchi

2, Y. Matsuda

2,

M. Nakajima1, S. Uchida

1,5, Y. Tomioka

3, T. Ito

3, K. Kihou

3, C. Lee

3, A. Iyo

3,

H. Eisaki3,5

, M. Kubota4, K. Ono

4, H. Ikeda

2,5, and R. Arita

1,5

1Department of Physics and Department of Engineering, University of Tokyo, Hongo, Tokyo 113-0033, Japan, 2Department of Physics and Research Center for Low Temperature and Materials Sciences, Kyoto University,

Kyoto 606-8502, Japan 3Nanoelectronic Research Institute, National Institute of Advanced Industrial Science and Technology (AIST),

Tsukuba, Ibaraki 305-8568, Japan, 4Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization

(KEK), Tsukuba, Ibaraki 305-0801, Japan, and 5JST, TRIP, Chiyoda, Tokyo 102-0075, Japan.

Many experimental results on the iron-based superconductors have indicated that

their superconducting gaps have swave symmetry, that is, the superconducting gap opens on the entire Fermi surfaces (FS's) [1,2]. However, in BaFe2(As1-xPx)2 [3], signatures of line node in the superconducting gap have been pointed out by recent penetration depth, thermal conductivity [4], and NMR studies [5]. Early angle-resolved photoemission spectroscopy (ARPES) studies on Ba1-xKxFe2As2 have revealed the importance of FS nesting for the superconductivity based on two-dimensional (2D) electronic structure. However, strong three-dimensionality in the FS's of the family of BaFe2As2 has been identified by band-structure calculation [6] and confirmed by ARPES studies [7,8]. Band-structure calculation predicts that the shapes of hole FS's in BaFe2(As1-xPx)2 become more three-dimensional with P substitution [3,9,10] because P substitution reduces the c-axis length as well as the pnictogen height. Therefore, it is necessary to reveal the three-dimensional electronic structure of BaFe2(As1-xPx)2 in order to clarify the relationship between FS nesting, superconductivity, and gap symmetry. In this work, we have performed angle-resolved photoemission spectroscopy (ARPES) study of BaFe2(As1-xPx)2 (x=0.38, 0.6, 0.9) in order to investigate its three-dimensional electronic structure.

We have observed strong three-dimensionality of FS‟s as predicted by band-structure calculation. We have investigated the P content dependences of FS‟s and their nesting properties. As for x = 0.6, the overall nesting properties are the same as those for x = 0.38 [11]. This is in accordance with the fact that the x = 0.6 samples are still superconducting with Tc = 8 K, while Tc = 28 K for x = 0.38 samples. The difference of Tc may be due to the subtle changes of the size and shape of the FS‟s. As for x = 0.9, we have found that one of the hole FS‟s is disconnected around the points as predicted by band-structure calculation. The disconnection of the FS deteriorates the nesting properties, which may lead to the suppression of superconductivity in the x = 0.9 compound. [1] H. Ding et al.,Europhys. Lett.,83 (2008), p. 47001. [2] K. Terashima et al., Proc. Natl. Acad. Sci. U.S.A., 106 (2009), p. 7330. [3] S. Kasahara et al., Phys. Rev. B, 81 (2010), p. 184519. [4] K. Hashimoto et al., Phys. Rev. B, 81 (2010), p. 220501. [5] Y. Nakai et al., Phys. Rev. B, 81 (2010), p.020503. [6] D. J. Singh et al., Phys. Rev. B, 79 (2008), p.094511. [7] W. Malaeb et al., J. Phys. Soc. Jpn., 78 (2009), p.123706. [8] P. Vilmercati et al., Phys. Rev. B, 79 (2009), p.220503. [9] J. G. Analytis et al., arXiv:1002.1304. [10] H. Shishido et al., Phys. Rev. Lett., 104 (2010), p.057008.

[11] T. Yoshida et al., arXiv:1008.2080, Phys. Rev. Lett. in press.

Investigation of the role of spin-orbit coupling on transport properties of iron pnictide

materials

Sudhakar Pandey1, Hiroshi Kontani

1, Dai Hirashima

1, Ryotaro Arita

2 , Hideo Aoki

2

1 Department of Physics, Nagoya University, Nagoya 464-8602, Japan

2 Department of Physics,

University of Tokyo, Tokyo 113-0033, Japan

A generic feature associated with the electronic structure of iron pnictide and

chalcogenide materials, which are currently under intense investigation for their

superconducting properties, is that the 3d orbitals of Fe make dominant contribution to

density of states near the Fermi level. Therefore, the transport properties of these materials

can naturally be expected to be governed by the 3d electrons. Incorporating this along with

several other realistic band features within a multiband tight-binding model, we investigate

the role of atomic spin-orbit coupling associated with the 3d orbitals on the transport

properties of these materials. Our investigation highlights the importance of some

characteristic features associated with the electronic band, such as accidental degeneracy,

Dirac cone, and orbital hybridization. We find significantly large spin Hall conductivity in the

paramagnetic state that is comparable with its magnitude in Pt. We also find finite anomalous

Hall conductivity in the ferromagnetic state.

Study of valence state and magnetic property of Fe in Fe-doped ZnO thin films

V. K. Verma1, V. R. Singh

1, K. Ishigami

1, Y. Yamazaki

1, G. Shibata

1,

T. Kadono1, A. Fujimori

1, T. Koide

2, Sourav Chattopadhyay

3 and T. K. Nath

3

1Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

2Photon Factory, IMSS, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan

3Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur 721302, W. B., India

Diluted magnetic semiconductors (DMSs) based on ZnO have attracted considerable

attention in the past years, because some transition metal (TM)-doped ZnO DMSs exhibit

ferromagnetism at room temperature (RT) [1]. Recently, Chang et al. [2] observed room

temperature ferromagnetism in Co and Al co-doped ZnO.

In the present work, 5% Fe-doped and 5% Fe, 1% Al co-doped ZnO epitaxial thin films

were fabricated on (0001)-αAl2O3 (sapphire) substrates by pulsed laser deposition technique.

The film thicknesses were about 2000 Å. Here, we report on Fe L2, 3 x-ray absorption (XAS)

and x-ray magnetic circular dichroism (XMCD) experiments of Zn0.95Fe0.05O and

Zn0.94Fe0.05Al0.01O thin films to study the electronic structure and the magnetic properties of

Fe ions embedded in the lattice of ZnO thin films that show ferromagnetism at room

temperature. The X-ray diffraction patterns clearly showed that there was no metallic Fe

cluster. From the line shape of Fe L2, 3-edge XAS in both films, it is confirmed that Fe ions are

in both the 3+ and 2+ states while the Fe2+

/Fe3+

ratio increases in the Zn0.94Fe0.05Al0.01O thin

film compared to Zn0.95Fe0.05O and the ferromagnetism comes from both Fe3+

and Fe2+

ions.

In the Zn0.94Fe0.05Al0.01O thin film, the magnetization decreases compared to Zn0.95Fe0.05O

although the conductivity increases, indicating that ferromagnetism is not carrier induced.

[1] T. Dietl et al., Science 287, 1019 (2000).

[2] G. S. Chang et al., J. Phys.: Condens. Matter 21, 056002 (2009).

Thickness dependence of the magnetic properties of La0.6Sr0.4MnO3 thin films studied by

soft x-ray magnetic circular dichroism

G. Shibata1, V. R. Singh

1, V. K. Verma

1, K. Ishigami

1, A. Fujimori

1,

T. Koide2, K. Yoshimatsu

3, E. Sakai

3, H. Kumigashira

3, M. Oshima

3

1 Department of Physics and Department of Complexity Science and Engineering, University of Tokyo, Bunkyo-

ku, Tokyo 113-0033, Japan 2 Photon Factory, Institute for Materials Structure Science, High Energy Accelerator Research Organization,

Tsukuba, Ibaraki 305-0801, Japan 3 Department of Applied Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

La0.6Sr0.4MnO3 (LSMO) shows giant magnetoresistance and half-metallicity and one of

the most promising materials for spin electronics. However, it has been reported that when

one fabricates a tunneling magnetoresistance device with LSMO/SrTiO3 (STO) interfaces, the

performance of the device becomes worse than what is expected from the bulk magnetic

properties [1]. This has been attributed to the loss of the half-metallicity at the interfaces

between LSMO and STO [2]. To understand this phenomenon, it is necessary to clarify the

electronic structure and the magnetic properties of the LSMO/STO interfaces using a

microscopic probe.

In this study, we have performed x-ray magnetic circular dichroism (XMCD)

measurements on STO/LSMO/STO heterostructures and have studied the dependence of the

magnetic properties on the LSMO thickness. The intrinsic XAS and XMCD spectra show

thickness-independent line shapes. With decreasing LSMO thickness, the decrease in the

XMCD intensity is observed, consistent with the previous magnetization measurement by

SQUID [2]. From the magnetic-field dependence of the XMCD intensity, we find that the

magnetic state gradually changes from ferromagnetic to paramagnetic state with decreasing

film thickness.

[1] F. Pailloux et al., Phys. Rev. B 66, 014407 (2002)

[2] K. Yoshimatsu et al., Appl. Phys. Lett. 94, 071901 (2009)

Theoretical study of a novel spin-orbit-induced Mott insulator in Ir oxides

Hiroshi Watanabe1,2

, Tomonori Shirakawa1,2

, and Seiji Yunoki1,2,3

1 Computational Condensed Matter Physics Laboratory, RIKEN ASI, Wako, Saitama 351-0198, Japan

2 CREST, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan

3 Computational Material Science Research Team, RIKEN AICS, Kobe, Hyogo 650-0047, Japan

Recently, the layered 5d transition metal oxide Sr2IrO4 has attracted much attention as a

novel Mott insulator. In this system, the large crystal field separates the t2g and eg orbitals and

the lower t2g orbital is filled with five electrons, (t2g)5. Several experiments shows that the

novel Jeff=|S-L|=1/2 Mott insulating state, which is different from the usual 3d Mott insulating

state, is realized in Sr2IrO4 due to the large spin-orbit coupling [1].

To investigate this novel Mott insulating state, we have studied the ground state properties

of 3-orbital Hubbard Model with the variational Monte Carlo method and variational cluster

approximation [2]. We have confirmed that the large spin-orbit coupling, which is

characteristic of 5d electron systems, greatly reduces the metal-insulator transition point and

the Jeff=1/2 Mott insulating state is stabilized in a realistic parameter region. In our

presentation, we will show the ground state phase diagram, one-particle excitation spectrum,

and momentum distribution function and discuss the detailed properties of this novel Mott

insulating state.

[1] B. J. Kim et al., Science 323, 1329 (2009).

[2] H. Watanabe et al., Phys. Rev. Lett. 105, 216410 (2010).

Figure: Momentum distribution function n(k) of metallic (left) and Mott insulating (right)

states.

0

0.5

1

yz yz

zxzx

xy

0

0.5

1

yz yz

zxzx

xy

Oxygen hole states in layered perovskite nickelate R2-xSrxNiO4

M. Uchida1, Y. Yamasaki

2, J. Okamoto

2, Y. Kaneko

3,

H. Nakao2, Y. Murakami

2, and Y. Tokura

1,3,4

1Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan

2Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization,

Tsukuba, Ibaraki 305-0801, Japan 3Multiferroics Project, ERATO, Japan Science and Technology Agency (JST), Tokyo 113-8656, Japan

4Cross-Correlated Materials Research Group (CMRG) and Correlated Electron Research Group (CERG), ASI,

RIKEN, Wako 351-0198, Japan

High-Tc superconductivity appears close to the Mott transition induced by doping holes

into antiferromagnetic parent insulators. Such filling-control insulator-metal transitions are

widely observed for transition-metal oxides with strongly correlated electrons, yet the

emergence of high-Tc superconductivity remains unique for the layered cuprates. Layered

nickelate R2-xSrxNiO4 (R being rare earth element) with K2NiF4 type structure is a rare

example of a two-dimensional antiferromagnetic insulator-metal transition system, providing

a contrastive counterpart to superconducting R2-xSrxCuO4 with the same crystal structure [1].

RSNO shows diagonal-stripe and checkerboard charge ordering at x~1/3 and 1/2, respectively,

and then undergoes an insulator-metal transition at x~1.

We have succeeded in growing single crystals of Nd2-xSrxNiO4 up to the metallic region by

using a high-pressure floating zone method and investigated the orbital characters of doped

holes by systematically measuring polarization-dependent O K- and Ni L-edge absorption

spectra. Figure shows the doping dependence of the O K-edge absorption spectra for E || c.

Two peaks appear above x=0.6 and 1.0, respectively, suggesting that the checkerboard type

charge ordering persists above x=1/2 with introducing the excess holes to 3z2-r

2 orbital states

and that the insulator-metal transition occurs with its melting at x~1.

[1] M. Uchida et al., Phys. Rev. Lett. 106, 027001 (2011).

FIG. Doping variation of O K-

edge absorption spectra for E || c

in Nd2-xSrxNiO4.

527 528 529 530 531 532

x=0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Photon Energy (eV)

Nd2-xSrxNiO4

O K-edge E || c

Electronic structure of the electron-doped iron-based superconductors Ba(Fe1-xTMx)2As2

(TM = Ni, Cu) observed by angle-resolved photoemission spectroscopy

S. Ideta1, T. Yoshida

1,6, I. Nishi

1, A. Fujimori

1,6, M. Nakajima

1,4,6, H. Kotani

2,

M. Kubota2, K. Ono

2, Y. Nakashima

3, M. Matsuo

3, T. Sasagawa

3, K. Kihou

4,6,

Y. Tomioka4,6

, C. H. Lee4,6

, A. Iyo4,6

, H. Eisaki4,6

, T. Ito4,6

, S. Uchida4,6

, R. Arita5


2 KEK, Photon Factory, Tsukuba, Ibaraki 305-0801, Japan

3 Materials and Structures Laboratory, Tokyo institute of Technology, Kanagawa 226-8503, Japan

4 National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8568, Japan 5 Department of Applied Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

6 JST, Transformative Research-Project on Iron Pnictides (TRIP), Chiyoda, Tokyo 102-0075, Japan

The electron-doped iron-based superconductors Ba(Fe1-xTMx)2As2 (TM-Ba122, TM = Co, Ni, Cu) show superconductivity and the maximum transition temperature reaches ~ 25 K at x ~ 0.07 (TM = Co), ~ 18 K at x ~ 0.05 (TM = Ni) and ~ 2 K at x ~ 0.044 (TM = Cu) [1, 2]. According to the rigid-band model, in Ni-Ba122 (Cu-Ba122), it is expected that the doped electron concentration is almost twice (three times) as large as that of Co-Ba122. In previous angle-resolved photoemission spectroscopy (ARPES) studies of Co-Ba122, the three-dimensional hole and electron Fermi surfaces (FSs) [3, 4] and the superconducting gaps [5] have been observed. Also, the shift of the chemical potential and the number of hole and electron carriers have been estimated from the ARPES data, and interpreted in terms of the rigid-band model [3]. On the other hand, according to a density functional calculation, it has been pointed out that electrons doped by the impurity atoms such as Co, Ni and Cu are mostly located at the substituted impurity site [6]. In this study, we have studied Ba(Fe1-xTMx)2As2 (TM = Ni, Cu) with Ni concentration x = 0.0375, 0.05, and 0.08, and with Cu concentration x = 0.06 and 0.08. Particularly, we focus on the shape of the three-dimensional FSs in these compounds. Figure 1 shows the result of FS mapping in k//-kz space for the electron FS for the x = 0.06 and 0.08 samples. We find that the volume of the electron FSs are smaller than the value which is expected from the rigid band picture. In order to estimate the hole and electron carrier concentrations, we have deduced the FS volume for hole and electron FSs. Thus obtained number of carriers for Ni- and Cu-Ba122 is compared with that for Co-Ba122. We have also performed FS mapping in kx - ky momentum space to discuss the nesting properties of these electron-doped systems. [1] P. C. Canfield et al., PRB 80, 060501(R) (2009). [2] N. Ni et al., PRB 82, 024519 (2010). [3] V. Brouet et al., PRB 80, 165115 (2009). [4] W. Malaeb et al., JPSJ 78, 123706 (2009). [5] K. Terashima et al., PNAS 106, 7330–7333 (2009). [6] H. Wadati, et al., PRL 105, 157004 (2010).

Fig. 1: Three dimensionality of Fermi surface of Ba(Fe1-xCux)2As2 in k||-kz space on x = 0.06 and x = 0.08.

Soft x-ray magnetic circular dichroism study of the diluted magnetic semiconductor

Zn1-xCrxTe

Y. Yamazaki1, T. Kataoka

1, V.R. Singh

1, A. Fujimori

1,

F.-H. Chang2, H.-J. Lin

2, D.J. Huang

2, C.T. Chen

2,

K. Ishikawa3, K. Zhang

3, S. Kuroda

3


2 National Synchrotron Radiation Research Center, Hsinchu, Taiwan

3 Institute of Materials Science, University of Tsukuba, Ibaraki 305-8577, Japan

Spintronics is defined as the technology which has both of spin and charge degrees of

freedom. One of the fields of spintronics is search for and characterization of diluted

magnetic semiconductors (DMSs). DMSs are considered for creating new devices, such as

high-capacity random access memory or spin FET. The diluted magnetic semiconductor

(DMS) Zn1-xCrxTe attracts much attention because ferromagnetism at room temperature has

been confirmed by both magnetization and magnetic circular dichroism (MCD)

measurements [1]. Recently, it was found that the doping of iodine (I), which is expected to

act as an electron donor, leads to clear enhancement of the ferromagnetism in Zn1-xCrxTe [2],

whereas the doping of nitrogen (N), which is expected to act as an electron acceptor,

suppresses the ferromagnetic behavior [3]. To realize a larger magnetization and a higher

Curie-temperature, it is important to study the effects of I- and N-doping on the electronic

structure associated with the ferromagnetism. Because X-ray absorption spectroscopy (XAS)

and X-ray magnetic circular dichroism (XMCD) are suitable methods to reveal such

microscopic nature of compounds, we have performed XAS and XMCD measurements on

undoped, I-doped and N-doped samples.

The XAS spectra show clear peak shifts toward higher energies in the heavily N-doped

sample. This indicates that N-doping works as hole donor, and Cr3+

states are added to the

Cr2+

state in the heavily N-doped sample. The XMCD spectra of lightly N-doped, undoped

and I-doped samples measured under various magnetic fields show ferromagnetic behaviors,

while for heavily N-doped sample, XMCD signal was not observed. Higher XMCD

intensities were observed in I-doped sample than in undoped one, consistent with the results

of magnetization measurements [4].

[1] H. Saito et al., Phys. Rev. Lett., 90, 202172 (2003).

[2] S. Kuroda et al., Nat. Mater., 6, 440 (2007). [3] N. Ozaki et al., Appl. Phys. Lett., 87, 192116 (2005).

[4] N. Ozaki et al., Phys.Rev. Lett. 97, 037201 (2006).

Versatile helimagnetic phases under high magnetic fields in the cubic perovskite SrFeO3

S. Ishiwata1,2

, M. Tokunaga3, Y. Kaneko

4, D. Okuyama

5, Y. Tokunaga

4,

T. Arima6, S. Wakimoto

7, K. Kakurai

7, Y. Taguchi

2,5, and Y. Tokura

1,2,4,5

1Department of Applied Physics and Quantum-Phase Electronics Center (QPEC), University of Tokyo, Tokyo

113-8656, Japan 2Correlated Electron Research Group (CERG), RIKEN, Wako 351-0198, Japan

3Institute for Solid State Physics, University of Tokyo, Tokyo 113-8656, Japan

4Multiferroics Project, ERATO, Japan Science and Technology Agency, c/o RIKEN, Wako 351-0198, Japan

5Cross-Correlated Materials Research Group (CMRG) RIKEN, Wako 351-0198, Japan

6Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan

7Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan

Recently, the anomalous Hall effect (AHE) observed in non-coplanar spin structures with

spin chirality has been of great interest due to possible spintronic applications. One such

example is a topologically stable, a vortex like spin texture, skyrmion stabilized under

magnetic fields in B20-type chiral magntes [1-3]. The title compound SrFeO3 prepared as an

epitaxial thin film has been reported to show AHE below 110 K [4], potentially originating

from a chiral spin texture stabilized under magnetic fields. This compound has been known as

a simple cubic perovskite with a proper-screw-type spin ordering below 130 K and reported to

show several transitions. However, the origins of the AHE and the phase transitions have not

been clarified due to difficulty in preparation of bulk single crystalline samples.

In this work, with the use of single crystalline SrFeO3 prepared by a high-pressure

technique, we measured magnetization and resistivity under pulsed high fields up to 45 T, and

identified 5 kinds of ordered, presumably helimagnetic phases. Under static magnetic fields,

we found that at least two of them below 110 K shows unconventional AHE, whose origin

will be discussed in terms of a spin chirality mechanism.

[1] A. Neubauer et. al., Phys. Rev. Lett. 102, 186602 (2009).

[2] X. Z. Yu et al., Nature 465, 901 (2010).

[3] X. Z. Yu et al., Nature Materials in press.

[4] N. Hayashi et. al., J. Mater. Chem., 11, 2235 (2001).

The origin of spin-split band at Sb(111) surface

T. Kadono1, K. Miyamoto

2, R. Nishimura

3, K. Kanomaru

3, M. Nagano

4, T. Shishidou

4, T.

Oguchi4, S. Qiao

5, K. Shimada

2, H. Namatame

2, A. Kimura

3 and M. Taniguchi

2,3

1 Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

2 Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-0046, Japan

3 Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan

4 Department of Quantum Matter, ADSM, Hiroshima University, Higashi-Hiroshima 739-8530, Japan

5 Advanced Materials Laboratory, Fudan University, Shanghai 200433, China

Crystal surfaces with a strong spin-orbit coupling have attracted a great attention because

it would cause a spin splitting despite non-magnetic property. At surfaces, spin degeneracy

could be lifted due to a space inversion asymmetry and a spin-orbit coupling. Recently, a spin

split energy band has been observed in the surface states of Au(111) and Sb(111) [1,2].

Surprisingly, the size of splitting for Sb(111) is comparable to that for Au(111) although the

spin-orbit coupling strength of outer p shell is much weaker for Sb [1]. In order to clarify the

origin of such the marked spin-split feature, spin- and angle- resolved photoelectron

spectroscopy (spin-ARPES) has been applied to Sb(111) surface.

Figure 1 shows the E-k// plot of spin-up component in the binding energy (EB) range of 0 –

0.45 eV along Γ-M of surface Brillouin zone. The intensity maxima of photoemission spectra

in the lower EB region are traced with open triangles. The band shows an upward energy

dispersion with a bottom (EB = 0.2 eV) at Γ and

crosses Fermi level (EF) just at +0.15Å-1

(-0.05 Å-

1) in the positive (negative) k// region. It can be

noticed that this energy dispersion is asymmetric

with respect to Γ, which clearly indicates the

Rashba-type spin splitting due to broken space

inversion symmetry. Combined with the ab-

initio slab calculation, the large spin-splitting is

caused by the strong asymmetry of constituent

wave function.

[1] S. LaShell et al., Phys. Rev. Lett. 77, 3419

(1996).

[2] K. Sugawara et al., Phys. Rev. Lett. 96,

064611 (2006).

Fig. 1: Experimental E-k// relation of

Sb(111) surface in spin-up channel. The

peak maxima are traced as denoted by

open triangles.

Angle-resolved photoemission spectroscopy study of hole-doped and electron-doped

cuprate Y1-zLaz(Ba1-xLax)2Cu3Oy

W. Uemura1*

, S. Ideta1, I. Nishi

1, T. Yoshida

1, A. Fujimori

1,

M. Kubota2, K. Ono

2, K. Segawa

3, Y. Ando

3

1 Department of Physics, University of Tokyo, Tokyo, Japan

2 Photon Factory, Institute of Materials Structure Science, KEK, Japan

3 Institute of Scientific and Industrial Research, Osaka University, Japan

Since the discovery of the high-Tc cuprate superconductors, much effort has been

made in the field of condensed matter physics. However, the mechanism of high-Tc

superconductivity is still unclear so far. As for the sign of doped carriers, the high- Tc cuprate

superconductors can be classified into two types, that is hole-doped and electron-doped

cuprate. Symmetry or asymmetry of the electronic states in the two types of cuprates is an

essential aspect for understanding the physical properties of the high-Tc cuprates. This should

be considered when we try to construct a theory of the cuprate high-Tc superconductors. In

this study, we report angle-resolved photoemission spectroscopy (ARPES) results on the

ambipolar cuprate Y1-zLaz(Ba1-xLax)2Cu3Oy (YLBLCO) [1], which can become either hole-

doped or electron-doped cuprate with the same crystal structure. In our ARPES results of the

hole- and electron-doped YLBLCO, dispersive feature has been observed in the nodal

direction, however, quasi-particle peaks have not been observed, indicating strong localization

behavior compared to the lightly doped YBa2Cu3Oy [2]. The observed shifts of the dispersive

feature between the hole- and electron-doped samples is consistent with a previous core-level

photoemission study [3] which showed a chemical potential jump between the hole-doped and

electron-doped YLBLCO.

[1] K. Segawa et al., Nature Physics 6, 579 (2010).

[2] H. Yagi et al., arXiv:1002.0655.

[3] M. Ikeda et al., Phys. Rev. B 82, 020503 (2010).

Microwave power dependence of microwave absorption in Bi2212 crystals

G. Ahmad1, M. Shahabuddin

1

1 Department of Physics, Jamia Millia Islamia University, Jamia Nagar, New Delhi, 110025, India

To save spaces occupied by transmission base stations, tunable high temperature

superconducting microwave filters is needed. The stacked superconducting thin films are

engaged as the tunable microwaves filters. They are operated under magnetic fields [1-2]. In

the present work, we investigated the microwave power dependence of microwave absorption

(MA) in Bi2212 crystals at 77.8 K. The microwave absorption spectra of sample measured at

various microwave power at low temperature. At low microwave power of 0.1 mW,

microwave absorption spectrum shows only a sharp first peak near zero magnetic field, which

corresponds to Meissner phase for magnetic field parallel to c-axis.

[1] Z. Y. Shen et. al., IEEE Trans. Appl. Supercond. 7, 2446 (1997)

[2] A. Lauder et al., Adv. Mater. 10, 1249 (1998)

FeOx

InOx

Sub.

3.389Å

26.34Å FeOx

InOx

FeOx

InOx

InOx

c = 26.08Å

a = 3.389ÅIn

Fe

O5nm

Disturbed layer

Film

Magnetic and electronic properties of two-dimensional triangular antiferromagnet

RFe2O4 thin films grown by pulsed laser deposition

M. Seki1, F. Iwamoto

1, H. Yamahara

2,

H. Yokota2 and H. Tabata

1,2

1 Department of Electrical Engineering and Information Systems, University of Tokyo, Bunkyo-ku, Tokyo 113-

8656, Japan 2 Department of Bioengineering, University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan

Complex oxides with multi-layered structure have attracted great interest in recent

years. They may exhibit exotic properties such as high-temperature superconductivity and

large thermoelectric effects, originating from the confinement of electrons in the two-

dimensional layers. The triangular antiferromagnet RFe2O4 (R = Y, Yb, Tm, Lu, In, etc) is one

of the layered oxides. RFe2O4 is composed of the alternate stacking of the hexagonal Fe-O

layer and R-O layer along the c-axis as shown in Fig. 1. In the few years, studies on RFe2O4

have been stimulated by the discovery of a colossal dielectric constant in LuFe2O4 [1].

However, there has been no report concerning the fabrication of these compounds. We report

herein the epitaxial growth of RFe2O4 thin films using a pulsed laser deposition and their

unique physical properties originating from the charge ordering of the Fe ions in the triangular

lattices. The key points for the growth of high quality films are high deposition temperature

(650-800°C) and control of volatilization of elements during film deposition. We found that

the thickness of Fe-O unit layers can be

controlled by changing oxygen partial pressure

and target composition. The films exhibit

distinct anisotropy in their magnetic and

electronic properties, suggesting the strong

magnetic coupling of Fe ions in the two-

dimensional triangular lattice plane.

[1] N. Ikeda et al., Nature 436, 1136 (2005).

[2] M. Seki et al., J. Cryst. Growth 312, 2273

(2010).

[3] M. Seki et al., Appl. Phys. Express 3, 105801 (2010).

Fig. 1 TEM images and schematic crystal

structure of InFe2O4.

Soft x-ray photoemission study of La1-xSrxTiO3 thin films

K. Ishigami1*

, Kohei Yoshimatsu2, Masaru Takizawa

3, Hiroshi Kumigashira

2

Masaharu Oshima2, Teppei Yoshida

4, Atsushi Fujimori

4

1 Department of Complexity Science and Engineering, The University of Tokyo, Chiba 277-8561, Japan

2 Department of Applied Chemistry, The University of Tokyo, Tokyo 113-8656, Japan

3 Research Organization of Science and Engineering, Ritsumeikan University, Shiga 525-8577, Japan

4 Department of Physics, The University of Tokyo, Tokyo 113-0033, Japan

La1-xSrxTiO3 (LSTO), which has the perovskite-type structure, is a basic material for

studying the mechanism of filling-controlled Mott transition. With decreasing x (with

increasing d-band filling), LSTO changes from the band insulator (SrTiO3, d0) to

paramagnetic metal, antiferromagnetic metal, to antiferromagnetic insulator (LaTiO3, d1) [1,2].

In the previous studies, critical behaviors of the electric structure have been observed in the

vicinity of the metal-insulator transition [2,3]. However, because of large contribution of

surface states to the incoherent part of the photoemission spectra, it has been difficult to

quantitatively evaluate the spectral weight transfer from the coherent part to the incoherent

part with d-band filling [3].

By preparing samples with atomically flat surfaces, the contribution of surface states in the

photoemission spectra would be decreased. In addition, by using in-situ bulk sensitive soft x-

ray photoemission spectroscopy, one can obtain spectrum which reflects bulk electronic state.

Therefore, we have grown LSTO thin films by the laser molecular beam epitaxy method, and

performed in-situ soft x-ray photoemission experiment.

As shown in Fig. 1, photoemission spectrum of LSTO clearly shows a sharp coherent part

of the Ti 3d band near the Fermi level and the incoherent part is suppressed compared with

that of the bulk spectrum. The La 4d and Sr 3d core

levels are shifted to the lower energies with d-band

filling, which is consistent with the chemical potential

shift with electron doping.

[1] Y. Tokura et al., Phys. Rev. Lett. 70, 2126 (1993).

[2] K. Kumagai et al., Phys. Rev. B 48, 7636 (1993).

[3] T. Yoshida et al., Europhys. Lett. 59, 258 (2002).

Fig. 2 Photoemission spectra of

LSTO thin film near the Fermi level.

Incoherent part has been subtracted

from original spectra.

Magnetic-field induced competition of two multiferroic orders in a triangular-lattice

helimagnet MnI2

T. Kurumaji1, S. Seki

1, S. Ishiwata

1,2, H. Murakawa

3,

Y. Tokunaga3, Y. Kaneko

3 and Y. Tokura

1,2,3

1Department of Applied Physics and Quantum Phase Electronics Center (QPEC), University of Tokyo, Tokyo

113-8656, Japan 2Cross-Correlated Materials Research Group (CMRG) and Correlated Electron Research Group (CERG),

RIKEN Advanced Science Institute, Wako 351-0198, Japan 3Multiferroics Project, ERATO, Japan Science and Technology Agency (JST), Tokyo 113-8656

Magnetic and dielectric properties with varying magnitude and direction of magnetic field

H have been investigated for a triangular lattice helimagnet MnI2. The in-plane electric

polarization P emerges in the proper screw magnetic ground state below 3.5 K, showing the

rearrangement of six possible multiferroic domains as controlled by the in-plane H. With

every 60o-rotation of H around the [001]-axis, discontinuous 120

o-flop of P-vector is

observed as a result of the flop of magnetic modulation vector q. With increasing the in-plane

H above 3 T, however, the stable q-direction changes from q||⟨1-10⟩ to q||⟨110⟩, leading to a

change of P-flop patterns under rotating H. At the critical field region (~3 T), due to the phase

competition and resultant enhanced q-flexibility, P-vector smoothly rotates clockwisely twice

while H-vector rotates counter-clockwisely once.

Ferroelectricity induced by spin-dependent metal-ligand hybridization in Ba2CoGe2O7

H. Murakawa1, Y. Onose

1,2, S. Miyahara

1, N. Furukawa

1,3 and Y. Tokura

1,2,4

1Multiferroics Project, ERATO Japan Science and Technology Agency (JST),

c/o Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan

2Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan

3Department of Physics and Mathematics, Aoyama Gakuin University, Sagamihara, Kanagawa 229-8558, Japan

4Cross Correlated Materials Research Group (CMRG) and Correlated Electron Research Group (CERG),

RIKEN-ASI, Wako, Saitama 351-0198, Japan

We have investigated the variation of induced ferroelectric polarization under magnetic

field with various directions and magnitudes in a staggered antiferromagnet Ba2CoGe2O7.

While the ferroelectric polarization cannot be explained by the well-accepted spin current

model nor exchange striction mechanism, we have shown that it is induced by the spin-

dependent p-d hybridization between the transition-metal (Co) and ligand (O) via the spin-

orbit interaction. In this mechanism, a single spin orientation is responsible for the electric

polarization. On the basis of the correspondence between the direction of electric polarization

and the magnetic state, we have also demonstrated the electrical control of the magnetization

direction.

[1] H. Murakawa et al., Phys. Rev. Lett. 103 137202 (2010).

Figure (a) Coordination of the spin

moment (S) in a CoO4 tetrahedron in

Ba2CoGe2O7. In a spin-dependent

metal-ligand hybridization mechanism,

the induced polarization (P) is

expressed as the attached equation,

where ei is the vector connecting the

Co and the i-th ligand O ions. (b) The

temperature dependence of [001]

component of P (Pc) in the magnetic

field 5 T along various directions.

Pressure-induced spin state transition in BiCoO3.

Kengo Oka1, W. Chen

2, H. Yusa

3, A.A. Belik

3, E. Takayama-Muromachi

3,

M. Mizumaki4, N. Ishimatsu

5, N. Hiraoka

6, M. Tsujimoto

7, M. Tucker

8, J. P. Attfield

9,

Y. Shimakawa2, M. Azuma

1

1Materials and Structure Laboratory, Tokyo Institute of Technology, Nagatsuta, Yokohama 226-8503, Japan

2Institute for Chemical Research, Kyoto University, Uji, Kyoto, 611-0011, Japan

3National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

4Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan

5Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan

6National Synchrotron Radiation Research Center (NSRRC), 101 Hshin-Ann Road, Taiwan, 30076, Republic of

China 7Institute for Integrated Cell-Material Sciences, Yoshida-Ushinomiyacho, Kyoto 606-8501, Japan 8ISIS facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, United Kingdom

9Centre for Science at Extreme Conditions and School of Chemistry, University of Edinburgh, Mayfield Road,

Edinburgh, EH9 3JZ, United Kingdom

The perovskite BiCoO3 was recently synthesized at high pressure and is notable as a

parent compound for lead-free piezoceramics. It is isostructural with PbTiO3 which is the

mother compound of the commercially used piezoelectric material Pb(Zr,Ti)O3 (PZT).

However, the transition of BiCoO3 to a paraelectric phase has not been observed under

atmospheric pressure (AP) because of sample decomposition. Investigation of the structural

transition under high pressure conditions is therefore of particular interest.

Another interest in BiCoO3 is the spin state of Co3+

ion. BiCoO3 B of

the ordered magnetic moment at 5 K, indicating that Co3+

is in the high-spin (HS) t2g4eg

2 state.

Recently, we proposed a scenario where the HS d6 electronic configuration of Co

3+ amplifies

the structural distortion of BiCoO3. In this context, a change to either the IS or LS states is

expected in the paraelectric phase.

In this study, the structural and electronic properties of BiCoO3 under high pressure

have been investigated. Synchrotron X-ray and neutron powder diffraction studies show that

the structure changes from polar PbTiO3-type to centrosymmetric GdFeO3-type at 2-3 GPa

with a large volume decrease of 13% at room temperature revealing a spin state change. The

pressure-temperature phase diagram of BiCoO3 has been constructed enabling the transition

temperature at ambient pressure to be estimated as 800 – 900 K.

[1] K. Oka et al., J. Am. Chem. Soc. 132, 9438 (2010).

Theory of magnetoelectric resonance

in two-dimensional S=3/2 antiferromagnet Ba2CoGe2O7

S. Miyahara1 and N. Furukawa

1,2

1 Multiferroics Project (MF), ERATO, Japan Science and Technology Agency (JST),

c/o Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan 2 Department of Physics and Mathematics, Aoyama Gakuin University, Kanagawa 252-5258, Japan

We investigate magnetic excitations in an S=3/2 Heisenberg model representing two-

dimensional antiferromagnet Ba2CoGe2O7 [1]. Below TN = 6.7 K, the Co magnetic moments

show an antiferromagnetic ordering [2]. Recently, the material is paid attention to due to the

magnetic field induced ferroelectric polarization [3,4]. Such multiferroics behaviors can be

explained well by considering a spin-dependent metal-ligand hybridization mechanism on a

Heisenberg model [4,5]. In terahertz absorption experiment of the compound, Goldstone

mode as well as novel magnetic excitations, conventional magnetic resonance at 2 meV and

both electric- and magnetic-active excitation at 4 meV, have been observed [6]. By

introducing a hard uniaxial anisotropy term Λ(Sz)2, three modes can be explained naturally.

We also indicate that, via the spin-dependent metal-ligand hybridization mechanism, the 4

meV excitation is an electric-active mode through the coupling between spin and electric-

dipole. Moreover, at 4 meV excitation, interference between magnetic and electric responses

emerges as a cross correlated effect. Such cross correlation effects explain the non-reciprocal

linear directional dichroism observed in Ba2CoGe2O7.

[1] S. Miyahara et al.,, arXiv:1101.3679 (2011).

[2] A. Zheludev et al., Phys. Rev. B 68 024428 (2003).

[3] H.T. Yi et al., Appl. Phys. Lett. 92 212904 (2008).

[4] H. Murakawa et al., Phys. Rev. Lett. 105 137202 (2010).

[5] T. Arima, J. Phys. Soc. Jpn. 76 073702 (2007).

[6] I. Kézsmárki et al., Phys. Rev. Lett. accepted (2011).

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