IV International Symposiumon Strong Nonlinear Vibronic and Electronic Interactions in Solids

BOOK OF ABSTRACTS

TARTU 2013

IV International Symposium

on Strong Nonlinear Vibronic

and Electronic Interactions in Solids

Topics

Nonlinear dynamics of crystal lattices

Strongly correlated electronic and magnetic systems

Modern problems of solid state spectroscopy

Organizing Committee V. Hizhnyakov (Institute of Physics, University of Tartu, Estonia)

R. K. Kremer (Max Planck Institute, Stuttgart, Germany)

G. Seibold (Brandenburg Technical University, Cottbus, Germany)

Supported by Institute of Physics, University of Tartu, Estonia; Max Planck Institute for Solid State Research, Stuttgart, Germany; Brandenburg Technical University, Cottbus, Germany; Meyer-Struckmann Foundation, Germany; European Union through the Regional Development Fund

1-3 MAY TARTU

2013

2 | P a g e

Programme

The registration and sessions take place

in the Peterson Hall, Dorpat Conference Centre

Tuesday, 30 April 2013

19:00 Welcome party at the University Café

Wednesday, 1 May 2013

9:00 – 10:00 Registration

10:00 Opening

Lattice dynamics Chair: E. Sigmund

10:10 A. J. Sievers Experiments on the localization and switching of

vibrational energy in nonlinear lattices

10:35 M. Sorokin Effect of intrinsic localized modes on nucleation of a new

phase in solids

11:00 V. Hizhnyakov Modeling of self-localized vibrations and defect

formation in solids

11:25 G. Tsironis Nonlinear modes in PT-symmetric metamaterials

11:50-

12:10

Coffee break

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Strong correlations and magnetism Chair: J. P. Toennies

12:10 G. Benedek Giant excitation numbers of Fuchs-Kliewer phonons by

Ne+ to neutral Ne scattering from a LiF(001) surface at

grazing incidence

12:35 G. Seibold Ferromagnetic order in underdoped cuprates

13:00 J. Wosnitza The FFLO state in quasi-two-dimensional organic

superconductors

13:25 A. Sherman Magnetic response of hole- and electron-doped cuprates

perovskites

13:50-

15:20

Lunch

Spectroscopy and ab initio calculations Chair: G. Benedek

15:20 J.P. Toennies Electron-phonon coupling in thin lead films from helium

atom scattering

15:45 R.K. Kremer Temperature dependence of semiconducting gaps

16:10 M. Dressel Quantum behavior of water molecules confined in nano-

cages of gemstone

16:35 T. Rõõm Spin wave excitations in the multiferroic Ba2CoGe2O7

17:00-

17:20

Coffee break

Spectroscopy and physical chemistry Chair: A. Bianconi

17:20 L. Kador Fluorescence Lifetime Imaging Microscopy in the

Frequency Domain (FD-FLIM) – Materials Analysis

with the Polar-Plot Approach

17:45 D. Terentyev Dislocations in metals under irradiation

18:10 M. Schreiber Chemistry Nobel prize 2011 for a quasi-scientist: The

forbidden beauty of quasicrystals

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Thursday, 2 May 2013

Lattice dynamics Chair: A. J. Sievers

9:00 S. Flach Nonlinear Waves in Disordered Systems

9:25 J. Archilla Effect of ZBL potential on kinks in repulsive lattides

9:50 Y. Kosevich Wandering and self-trapped exciton-polaron solitons in

coupled macromolecules and coupled semiconductor

nanowires and nanoribbons

10:15 V. Dubinko Reaction rates in crystalline solids with account of

radiation-induced quodons

10:40 M. Pärs Optical switching of an organic photochromic triad

11:05-

11.25

Coffee break

Magnetism and superconductivity Chair: M. Dressel

11:25 A. Bianconi Complex granular high temperature superconductors

11:50 D. Suter Strongly coupled individual spins for sensing and

quantum computing

12:15 J. Richter Strongly correlated systems on highly frustrated lattices:

From magnons to electrons

12:40 A.V. Mikheyenkov Singlet description of the 2D frustrates Heisenberg

model with arbitrary signs of nearest and next-nearest

exchanges

13:05 U. Nagel Terahertz spectroscopy of spin waves in multiferroic

BiFe3 in high magnetic fields

13:30-

15:00

Lunch

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Magnetism Chair: A. Bussmann-Holder

15:00 R. Stern Frustration and BEC of magnons in Han Purple

15:25 E. Joon Soliton lattice phase of spin-Peierls state

15:50 A.A. Tsirlin Frustrated pentagonal Cairo lattice in Bi4Fe5O13F

16:15 O. Janson Microscopic modeling of the S=1/2 Heisenberg

ferrimagnet Cu2OSeO3

16:40-

18:00 Poster session & Coffee+snacks

(Chair: A. Sherman)

19:00-

22:00

Conference dinner in the Atlantis Restaurant

Friday, 3 May 2013

Magnetism and superconductivity Chair: J. Wosnitza

9:00 A. Bussmann-Holder The phase diagram of EuxSr1-xTiO3: from almost

multiferroic to quantum paraelectric

9:25 E. von Oelsen Spin waves in degenerate two-orbital Hubbard models

9:50 A. Vargunin Two-gap superconductivity: interband interaction in the

role of external field

10:15-

10:35

Coffee break

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Ab initio calculations and superconductivity Chair: S. Flach

10:35 M. Brik Semi-empirical and first principles analysis of the

microscopic crystal field effects and electron-vibrational

interaction for 3d ions in solids

11:00 N. Kristoffel Multiband superconductivity and the pseudogap

11:25 A. Pishtshev A comparative DFT study of dynamical covalency

in delafossite-type crystalline materials

11:50 M. Schreiber The modified sharpened index hms and other variants in

the Hirsch index zoo

12:15-

12:30

Closing

P a g e | 7

List of posters

1. H. Kaasik, V. Hizhnyakov, Crossover of the efficiency of the spontaneous down

conversion at strong laser field

2. A. Kalda, MAS NMR study of Cu2CdB2O6

3. T. Katuwal, THz Spectroscopy study of azurite at low temperature

4. P. Konsin, B. Sorkin, Electrostatic doping of high-Tc cuprate ultrathin films and

transverse electric field effects

5. V. Krasnenko, V. Boltrushko, M. Klopov,V. Hizhnyakov, Structures of conjoined

graphene nanoribbons and carbon nanotubes

6. A. Leitmäe, 139

La NMR study of the triple-layer T’-La4Ni3O8

7. K. Pae, V. Hizhnyakov, Time-dependent Jahn-Teller effect in case of E e –problem

8. V. Palm, M. Rähn, J. Jäme, V. Hizhnyakov, Excitation of surface plasmon polaritons

in Al-coated SNOM tips

9. L. Peedu, Infrared spectroscopy and quantum 5D calculations: H2@C70

10. I. Rebane, Photon-gated persistent spectral hole-burning in a four-level system by

pulses

11. P. Rubin, A. Sherman, M. Schreiber, Magnetic phase diagram of the two-dimensional

J1-J3 Heisenberg model with S=1 on a triangular lattice

12. E. Sigmund, V. Hizhnyakov, G. Seibold, Interplay between electronic

inhomogeneities and pairing interactions in high­Tc superconductors

13. I. Tehver, V. Hizhnyakov, G. Benedek, Quasi-elastic light scattering of doped 3He

liquid on Fermi excitations

14. T. Vaikjärv, Optical manifestations of energy relaxation in quasidegenerate electronic

states

8 | P a g e

P a g e | 9

ORAL

CONTRIBUTIONS

10 | P a g e

Experiments on the localization and switching of vibrational energy in nonlinear lattices

A. J. Sievers

Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, NY 14853-2501

An intrinsic localized mode (ILM) represents a localized vibrational excitation in a

nonlinear lattice. Such a mode will stay in resonance as the driver frequency is changed

adiabatically until a bifurcation point is reached, at which point the ILM switches and

disappears. Experiments probing these excitations continue to produce surprises. Our recent

studies demonstrate both the production and manipulation of localized energy along

micromechanical arrays, and the E&M generation of countable intrinsic localized modes in a

1-D atomic spin lattice. The dynamics behind switching in a micromechanical array is

examined through experimental measurements and numerical simulations. Linear response

spectra of a driven micromechanical array containing an ILM were measured in the frequency

region between two fundamentally different kinds of bifurcation points that separate the large

amplitude ILM state from the two low amplitude vibrational states. Particularly interesting is

the lower transition, which occurs when the four-wave mixing partner of the natural

frequency of the ILM intersects the top most extended band mode of the same symmetry. The

properties of linear local modes associated with the driven ILM are also identified

experimentally for the first time but play no role in these transitions.

P a g e | 11

Effect of intrinsic localized modes on nucleation of a new phase in solids.

V.I.Dubinko1, M.V.Sorokin

2

1National Science Center ‘Kharkov Institute of Physics and Technology’,

Akademicheskaya St. 1, 61108 Kharkov, Ukraine 2National Research Centre ‘Kurchatov Institute’, Kurchatov Sq. 1, 123182 Moscow, Russia

Nucleation as the first stage of new phase formation, which determines the subsequent

phase transition kinetics, is broadly studied in basic research and applications. Kinetic

approach to nucleation describes stochastic overcoming of the Gibbs barrier for the critical

nuclei formation. This process provides ‘diffusion’ of the nuclei in the size space, whereas the

‘drift’ forces tend to dissolve subcritical nuclei. It appears that the elementary steps in the size

space are determined by the reaction kinetics (impingement/evaporation of atoms) and by

diffusion of atoms in the matrix, both of which can be affected by the presence of intrinsic

localized modes (ILM’s), a.k.a. discrete breathers. Large amplitude oscillations of atoms

about their equilibrium positions in the vicinity of ILM’s cause local potentials of alternating

sign, which can be described in terms of time-periodic modulations of the potential barriers

for chemical reactions and diffusion [1]. The reaction rates averaged over large macroscopic

volumes and times including many ILM’s can be increased by many orders of magnitudes,

depending on the ILMs statistics.

In this report we take this effect into account in evaluation of the nucleation rates of

precipitates of a new phase both in thermal equilibrium and in thermal spikes in solids under

irradiation with swift particles.

[1] V. I. Dubinko, P. A. Selyshchev, and J. F. R. Archilla, Reaction-rate theory with account of the

crystal anharmonicity, Phys. Rev. E 83 (2011) No 4, doi: 10.1103/PhysRevE.83.041124.

12 | P a g e

Modeling of self-localized vibrations and defect formation in solids

M. Haas1, V. Hizhnyakov

1, M. Klopov

2, A. Pishtshev

1, A. Shelkan

1

1Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia

2Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia

Modeling of nonlinear dynamics of solids presents a serious problem in material science.

Here we present a MD method which includes: 1) long-range linear interatomic forces

described by means of phonon Greens functions and 2) short-range non-linear (anharmonic)

forces considered explicitly. The method allows one to take into account the effects of the

macroscopic field on the intrinsic localized modes (ILMs) in insulators. We also performed

MD simulations of recoil processes in alkali halide crystals following the scattering of X-rays

or neutrons a) by applying the new algorithm allowing one to take long-range (Coulomb)

forces into account and b) by using LAMMPS software package. At small energies (<10 eV)

the recoil can induce intrinsic localized modes and linear local modes associated with them.

In metals, as a result of the screening of the atomic interactions by free electrons, the odd

anharmonicities may be essentially reduced. Due to this reduction the frequencies of intrinsic

localized and linear local modes can be positioned above the phonon spectrum. The MD

simulations of the atomic motion in metallic Ni and Nb confirm this prediction. If the recoil

energy exceeds several tens of eV the vacancy and interstitials can be formed, in a strong

dependence on the direction of the recoil momentum. In fcc lattices the recoil momentum in

(110) direction can produce the single vacancy and the crowdion while in case of recoil

momentum in (100) and in (111) directions the bi-vacancy and the crowdion can be formed.

P a g e | 13

Nonlinear modes in PT-symmetric metamaterials

G. P. Tsironis

Department of Physics, University of Crete and Institute of Electronic Structure and Laser,

FORTH, P. O. Box 2208, 71003 Heraklion, Greece

Synthetic systems with matched gain and loss may form parity-time PT-symmetric

metamaterials described through non-hermitian Hamiltonians and showing a phase transition

in between an exact and a broken phase as a function of the gain/loss power [1]. The PT-

symmetry breaking has been experimentally observed in optical lattices [2]. We introduce a

PT-symmetric metamaterial consisted of split-ring resonator (SRR) dimers, one with loss and

the other with equal amount of gain, coupled magnetically while nonlinearity and gain are

introduced through tunnel Esaki diodes. Within the framework of the equivalent circuit model

[3], extended for the PT-dimer chain, the dynamics of the charge qn accumulated in the

capacitor of the n-th SRR is governed by 2 3

2 2 1 2 2 2 1 0 2 1 2 1 2 1sin( )M n n M n n n n nq q q q q q q (1) 2 3

2 1 2 2 1 0 2 2 2sin( )M n n M n n n nq q q q q q (2)

where ,M M are the magnetic interaction coefficients, and are dimensionless nonlinear

coefficients, is the gain/loss coefficient ( 0 ), 0 is the amplitude of the external driving

voltage, while and are the driving frequency and temporal variable, respectively,

normalized to the inductive-capacitive (LC) resonance frequency 0 and inverse LC

resonance frequency 1

0 , respectively, 0 01/ LC with 0C being the linear capacitance.

In the absence of nonlinearity, for fixed ,M M , the bandwidths as a function of the

gain/loss parameter show the onset of the PT-phase transition and a resulting band

modification. The presence of nonlinearity may induce nonlinearly localized modes in the

form of discrete breathers with the largest part of the total energy concentrated into two

neighboring sites belonging to the same gain/loss dimer [3].

The PT-symmetric nonlinear metamaterial may be used for dynamic tuning in the range of

the modified band and switching to the broken phase.

[1] R. El-Ganainy,et al., Opt. Lett. 32, 2632 (2007); K. G. Makris, et al, Phys. Rev. Lett. 100, 103904

(2008).

[2] A. Guo et al., Phys. Rev. Lett. 103, 093902 (2009); C. E. Rüter et al., Nature Physics 6, 192

(2010); A. Szameit, et al., Phys. Rev. A 84, 021806(R) (2011).

[3] N. Lazarides, M. Eleftheriou, and G. P. Tsironis, Phys. Rev. Lett. 97, 157406 (2006); N. Lazarides

and G. P. Tsironis, cond. mat. 1210.2410v1.

14 | P a g e

Giant excitation numbers of Fuchs-Kliewer phonons in grazing-incidence Ne+ scattering

from the LiF (100) surface

A.A. Lucas1,2

, M. Sunjic1,3

and G. Benedek1,4

1 Donostia International Physics Centre (DIPC), University of the Basque Country (EHU-UPV),

Paseo M. Lardizabal 4, 20018 Donostia-San Sebastian, Spain. 2 Department of Physics, University of Namur, 61 rue de Bruxelles, B5000 Namur, Belgium.

3 Department of Physics, University of Zagreb, Bijenicka 32, HR 10000 Zagreb, Croatia.

4 Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca,Via R. Cozzi 53, 20125 Milano,

Italy.

An analytic model is developed to describe the inelastic processes occurring when keV

Ne+ ions are scattered at grazing incidence by the (100) surface of LiF. The large energy

losses (up to 30 eV) of the reflected Ne+ particles reported by Borisov et al are shown to arise

specifically from the long-range coupling between the projectiles and Fuchs-Kliewer (FK)

optical phonons of LiF whose fields extend far outside the surface. The strength of the

coupling is determined, which allows to compute the average number (several hundreds) of

excited FK phonon quanta ( 0.071S eV ) and hence the mean energy losses. For

emerging, neutralized Ne0, a distinct energy loss mechanism is shown to occur, namely the

excitation of FK phonons and other types of surface collective modes associated with the

screening of the F0 hole left behind by the neutralization process. This novel mechanism

contributes a large fraction of the loss, additional to that suffered by the incident Ne+ ion. The

model explains the experimental observations quantitatively. The paper ends with a

discussion of the large energy broadening of the observed loss peaks.

P a g e | 15

Ferronematic order in underdoped cuprates

G. Seibold1

, M. Capati2, C. Di Castro

2, M. Grilli

2, and J. Lorenzana

2

1Institut für Physik, BTU Cottbus, PBox 101344, 03013 Cottbus, Germany

2ISCCNR, CNISM and Dipartimento di Fisica, Universitа di Roma ``La Sapienza'',

P. Aldo Moro 2, 00185 Roma, Italy

A low energy incommensurate spin response has been detected in many high-temperature

superconductors. In the LSCO system the associated spin modulation is rotated below the

doping concentration x=0.055 and the presence of an orthorhombic lattice distortion suggests

a one-dimensional magnetic scattering along the diagonal direction which is even static.

Contrary to the LaBaCuO and LaNdCuO compounds where the static incommensurate spin

modulation is due to the formation of charge stripes along the CuO direction, no associated

charge order has been found in the spin glass phase. In order to explain this puzzling situation

we propose a model where doping of holes favors the formation of stripe segments which

have a spin vortex and an antivortex fixed to the extremes. Within the extended Hubbard

model we find that the length and orientation of segments is governed by the ratio between

next-nearest and nearest neighbor hopping and limited by the long-range Coulomb

interaction. The influence of the structural distortion and the short-range anisotropic

interaction between segments leads to a charge nematic with macroscopic polarization due to

the ferro orientation of the segments. We show that this state, which we call a ferronematic,

implies an incommensurate response in the spin channel which is in very good agreement

with elastic neutron scattering data from lanthanum cuprates in the spin-glass phase.

16 | P a g e

The FFLO state in quasi-two-dimensional organic superconductors

J. Wosnitza

Hochfeld-Magnetlabor Dresden (HLD), Helmholtz-Zentrum Dresden-Rossendorf, D-01314 Dresden,

Germany

In 1964, Fulde and Ferrell (FF) as well as Larkin and Ovchinnikov (LO) predicted

independently the existence of novel inhomogeneous superconducting states at high magnetic

fields and low temperatures. In the so-called FFLO state superconductivity can survive even

beyond the Pauli paramagnetic limit. Prime candidates for exhibiting the FFLO state are the

quasi-two-dimensional (2D) organic superconductors. These are mostly clean-limit

superconductors and when the magnetic field is aligned parallel to the conducting planes the

orbital critical field is greatly enhanced, much beyond the Pauli limit. We performed high-

resolution specific-heat and torque-magnetization experiments in magnetic fields up to 32 T

for such 2D organic superconductors [1]. Besides an upturn of the upper critical field towards

lowest temperatures, we observe a second thermodynamic transition within the

superconducting phase signaling the existence of an additional superconducting phase. These

features appear only in a very narrow angular region close to parallel-field orientation as

evidenced by comprehensive angular- and field-dependent specific-heat measurements for

one organic superconductor [2]. Our results give strong evidence for the realization of the

FFLO state in organic superconductors.

Part of this work was supported by EuroMagNET II (EU contract No. 228043). Work

done in cooperation with R. Beyer, B. Bergk, P.H.M. Böttger, S. Yasin, R. Lortz, A. Demuer,

I. Sheikin, Y. Wang, Y. Nakazawa, G. Zwicknagl, and J.A. Schlueter.

R. Lortz, Y. Wang, A. Demuer, P.H.M. Böttger, B. Bergk, G. Zwicknagl, Y. Nakazawa, and J.

Wosnitza, Phys. Rev. Lett. 99, 187002 (2007); B. Bergk, A. Demuer, I. Sheikin, Y. Wang, J.

Wosnitza, Y. Nakazawa, and R. Lortz, Phys. Rev. B 83, 064506 (2011).

R. Beyer, B. Bergk, S. Yasin, J.A. Schlueter, and J. Wosnitza, Phys. Rev. Lett. 109, 027003 (2012).

P a g e | 17

Magnetic response of hole- and electron-doped cuprate perovskites

A. Sherman1 and M. Schreiber

2

1Institute of Physics, University of Tartu, Estonia

2Institut für Physik, Technische Universität Chemnitz, Germany

The t-J model and Mori projection operator formalism are used for calculating the

magnetic susceptibility of p- and n-type cuprates in the superconducting and pseudogap

phases. In the p-type case, the peculiar hole dispersion together with the energy and

momentum conservation laws provide an incommensurate low-frequency response. The

theory reproduces the hourglass dispersion of the susceptibility maxima with the upper branch

reflecting the dispersion of localized spin excitations and the lower branch being due to

incommensurate maxima of their damping. The intensive resonance peak appears when the

hourglass waist falls below the bottom of the electron-hole continuum. In the pseudogap

phase, the Fermi arcs lead to a quasi-elastic incommensurate response at low temperatures.

This result explains the lack of the superconducting gap in the susceptibility of phase-

separated underdoped lanthanum cuprates. The theory accounts for the magnetic stripe

reorientation from the axial to diagonal direction at low hole concentrations. In the n-type

case, the electron band folding across the antiferromagnetic Brillouin zone border, which is

inherent in these crystals, leads to a commensurate low-frequency response. As a result, the

dispersion of the susceptibility maxima is cone-shaped with the apex point at the

antiferromagnetic momentum. The same band folding causes the appearance of a

supplementary spin-excitation branch. The coexistence of the usual and supplementary

branches explains two maxima observed in the frequency dependence of the susceptibility.

This work was supported by the European Union through the European Regional

Development Fund (Project TK114) and by the Estonian Science Foundation (Grant

ETF9371).

18 | P a g e

Electron-phonon coupling in thin lead films from helium atom scattering

J. Peter Toennies

Max Planck Institute for Dynamics and Self-Organization,

Am Fassberg 17, 37077 Göttingen, Germany

The coupling between the electrons and phonons of a metal is of fundamental importance

for understanding many basic transport processes, including heat conductivity and electrical

conductivity. The electron-phonon coupling constant is a basic ingredient in the Bardeen,

Cooper, Schrieffer theory of superconductivity. The same coupling also comes into play

whenever a solid is exposed to excitations from the outside, such as in photoemission,

inelastic electron scattering, tunnelling spectroscopies or at the surface when exposed to gas

phase collisions.

Rather unexpected new information on the electron-phonon coupling comes from the

phonon dispersion curves of thin lead films with between N= 3 – 8 monolayers measured by

Inelastic Helium Atom Scattering (IHAS). [1] The time-of-flight spectra reveal an unusually

large number of modes corresponding to more than ½ of the total number of the possible 2 N

modes in the planar scattering geometry. Sophisticated density function (DFT) calculations

have been employed to explore the electron charge density oscillations in response to the

phonon induced atomic displacements. Previously similar calculations for the Cu(111) surface

revealed that the electron density distributions throughout the surface region are strongly

coupled to the surface phonons. [2] The observed excitation of subsurface phonons in this

case could be explained by assuming that they are excited via the electronic charge distortion

resulting from the He atom impact and not by the conventional mechanism of impulsive

collisions with the surface atoms. In the lead films a similar effect combined with the

unusually large electron-phonon coupling constant in lead films explains the excitation of the

large number of phonons even those which are farthest from atom intensities are closely

related to the mode specific electron-phonon coupling constants and even provide their wave

vector dependence for the first time. Thus IHAS has been demonstrated to be powerful

method for obtaining this important information.

[1] J. Braun, P. Ruggerone, Ge Zhang, J. P. Toennies, and G. B. Benedek, Phys. Rev. B 79, 205423

(2009)

[2] V. Chis, B. Hellsing, G. Benedek, M. Bernasconi, E. V. Chulkov, and J. P. Toennies, Phys. Rev.

Lett. 101, 206102 (2008)

[3] I. Yu. Sklyadneva, E. V. Chulkov, P. M. Echenique, R. Heid, K.-P. Bohnen, G. Benedek and J. P.

Toennies, Phys. Rev. Let. 107, 095502 (2011)

P a g e | 19

Temperature dependence of semiconductor gaps

Reinhard K. Kremer and Manuel Cardona

Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany

The various semiempirical approaches used to fit the temperature dependence of

semiconductor gaps as obtained by ellipsometry (direct gaps) [1] and photoluminescence

(indirect gaps) [2] measurements are critically reviewed. In order to describe the temperature

dependence of the semiconductor gaps, both electron-phonon and thermal expansion

contributions have to be considered. We demonstrate that including phonon statistical factors

(Bose-Einstein) and the phonon densities of states are best suited to describe the measured

data. Approaches using several frequencies, corresponding to characteristic peaks in the

phonon densities of states, fit the data equally well or better than single (averaged) frequency

fits previously considered. The often used Varshni equation (T 2) has been shown to fail for

T0 K, whereas a T 4 [2] temperature dependence is expected and confirmed by theory and

experiment. Electron-phonon coupling constants of opposite sign enable us to reproduce the

unusual non-monotonic behavior found in several silver and copper semiconductor

compounds.[3,4]

[1] P. Lautenschlager, M. Garriga, L. Viña, and M. Cardona, Phys. Rev. B 36, 4821 (1987).

[2] M. Cardona, T. A. Meyer and M. L.W. Thewalt, Phys. Rev. Lett. 91, 196403-1 (2004).

[3] A. Göbel, T. Ruf, M. Cardona, C. T. Lin, J. Wrzesinski, M. Steube, K. Reimann, J.-C. Merle, and

M. Joucla, Phys. Rev. B 57, 15183 (1998).

[4] J. Bhosale, A. K. Ramdas, A. Burger, A. Muñoz, A. H. Romero, M. Cardona, R. Lauck, and R. K.

Kremer, Phys. Rev. B 86, 195208 (2012).

20 | P a g e

Quantum behaviour of water molecules confined in nano-cages of gemstone

M. Dressel1, B.P. Gorshunov

2, E.S. Zhukova

2, V.V. Lebedev

3, G. S. Shakurov

4,

R. K. Kremer5, E. V. Pestrjakov

6, V. G. Thomas

7, D. A. Fursenko

7, V. I. Torgashev

8

11. Physikalisches Institut, Universität Stuttgart, Germany

2A.M. Prokhorov General Physics Institute, Russian Academy of Sciences,

and Moscow Institute of Physics and Technology, Dolgoprudny Moscow Region Russia 3Landau Institute for Theoretical Physics, Russian Academy of Siences, Chernogolovka

and Moscow Institute of Physics and Technology, Dolgoprudny Moscow Region Russia 4Kazan Physical-Technical Institute, 10/7 Sibirsky trakt, 420029 Kazan, Russia

5Max-Planck-Institut für Festkörperforschung, Heisenbergstraβe 1, 70569, Stuttgart, Germany

6Institute of Laser Physics, 13/3 Ac. Lavrentyev's Prosp., 630090 Novosibirsk, Russia

7Institute of Geology and Mineralogy, 3 Koptyug st., 630090, Novosibirsk, Russia

8Faculty of Physics, Southern Federal University, 344090 Rostov-on-Don, Russia

Water is essential for life; it is the chemical compound most pervasive and widest spread

on earth, the liquid most studied by physics, chemistry and biology. Nevertheless, despite the

simple structure of the H2O molecule, water probably is least understood of all liquids. The

situation becomes more complex in the vicinity with solid surfaces or single macromolecules,

for instance proteins, where a hydration layer forms with a gradual transition from bound and

biological water towards bulk water in which the hydrogen-bond network becomes

increasingly dynamical. Even in ice, weak hydrogen bonds allow libration of the water

molecule. Recently it was shown, that water confined at the nanoscale of about 1 nm does not

crystallize upon cooling as low as 173 K. Here we report optical investigations of H2O

molecules in the pores of a crystal lattice where single water molecules finally reveal their

quantum behavior. The energy levels of the molecule positioned within the sixfold rotational-

symmetric potential of the beryl nanopores hybridize, causing a series of discrete energy

levels between which librational and translational transitions are observed in the THz range of

frequency.

P a g e | 21

Spin wave excitations in the multiferroic Ba2CoGe2O7

T. Rõõm1, U. Nagel

1,K. Penc

2, J. Romhanyi

2

D. Szaller3, S. Bordacs

3, I. Kezsmarki

3, A. Antal

3, T. Feher

3, A. Janossy

3

H. Engelkamp4,H. Murakawa

5, Y. Tokura

5

1NICPB, Tallinn

2Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, Budapest

3Department of Physics, Budapest University of Technology

4High Field Magnet Laboratory, Institute for Molecules and Materials, Radboud University, Nijmegen

5Quantum-Phase Electronics Center, Department of Applied Physics, The University of Tokyo

Ba2CoGe2O7 is a multiferroic material where spin waves exhibit giant directional

dichroism and natural optical activity at THz frequencies due to the large ac magnetoelectric

effect [S. Bordacs et al., Nature Physics 8, 734 (2012)]. We studied spin excitations in the

magnetically ordered phase of the non-centrosymmetric Ba2CoGe2O7 in magnetic fields up to

33 T [Penc et al., Phys. Rev. Lett. 108, 257203 (2012)]. In the ESR and THz absorption

spectra we found several spin excitations beyond the two conventional magnon modes

expected for such a two-sublattice antiferromagnet. A multiboson spin-wave theory describes

these unconventional modes, including spin-stretching modes, characterized by an oscillating

magnetic dipole and quadrupole moment. The lack of inversion symmetry allows each mode

to become electric dipole active.

22 | P a g e

Fluorescence Lifetime Imaging Microscopy in the Frequency Domain (FD-FLIM) -

materials analysis with the polar-plot approach

Lothar Kador

University of Bayreuth, Germany

Fluorescence lifetimes can be measured with a cw laser which is modulated in the MHz

range. Then the fluorescence emission is also modulated but, due to the non-zero lifetime, it

features a phase shift and a reduced modulation depth as compared to the excitation. The

lifetimes calculated from these two effects are identical in the case of single-exponential

fluorescence decay and differ otherwise. For the interpretation of the data the so-called polar-

plot (or phasor) approach is particularly useful which is equivalent to the Cole-Cole plot in

dielectric spectroscopy. The distribution of data points corresponding to different spots on a

sample allows one to assess and interpret material inhomogeneities. Experimental data and

their polar plots will be presented for three different examples: The laser dye Rhodamine 6G

doped into different polymer matrices, an auto-fluorescent conjugated polythiophene

polymer, and the inorganic semiconductor gallium selenide (GaSe).

P a g e | 23

Dislocations in metals under irradiation

D. Terentyev

Nuclear Materials Science Institute, SCK-CEN, Boeretang 200, B-2400, Mol, Belgium

Microstructural evolution of metals and metallic alloys under irradiation conditions and

'post-irradiation' response is largely determined by pre-existing microstructure. One of the

most important microstructural features of metallic alloys is dislocation. Primary, dislocations

offer means for plastic deformation, thus ensuring ductility of materials. Thermally activated

movement of dislocations and their interaction with pre-existing 'natural' lattice defects such

as grain boundaries provides natural strength of materials being dependent on temperature and

applied deformation speed. Up on irradiation, however, dislocations will interact with

radiation-induced mobile defects (i.e. point defects and their clusters) thus acting as sinks for

them, on the one hand. On the other hand, radiation-induced defects of relatively large size

(i.e. few nanometers and larger) will act as obstacles for dislocation movement causing so-

called hardening making material less ductile. This type of degradation mechanism is called

embrittlement. For structural materials, embrittlement under irradiation is one of the key

issues.

In this presentation, we give a review of recent atomistic simulations done in BCC and

FCC metals devoted to: (i) atomistic description of dislocations, their mechanisms of

movement, interaction with grain boundaries; (ii) absorption of radiation-induced defects by

dislocations and interaction of He (typical product of transmutation) with dislocations; (iii)

interaction of moving dislocations with typical radiation defects in BCC and FCC metals such

as: voids, precipitates, dislocation loops.

24 | P a g e

Chemistry Nobel prize 2011 for a quasi-scientist: the forbidden beauty of quasicrystals

Michael Schreiber

Institut für Physik, Technische Universität Chemnitz, 09107 Chemnitz, Germany

Quasicrystals and their symmetry are topical research objects in mathematics, physics,

chemistry, material sciences, and also interesting in arts and architecture.

Quasicrystals were discovered in the early 1980s in metallic alloys. Their diffraction

pattern consists of Bragg peaks, like in a periodic crystal, but with a crystallographically

forbidden symmetry like icosahedral symmetry. Their structure is usually described in terms

of quasiperiodic tilings of space, which play the role of the lattice structure in conventional

crystals. A simple one-dimensional example is the Fibonacci chain. Different methods are

presented to construct such quasi-periodic systems like the Penrose tiling in two dimensions.

Although quasicrystals are usually composed of metallic elements, they show very low

electric conductivity which decreases with temperature and with the structural perfection.

This can be related to the electronic states. In quasicrystals, there are no extended Bloch

waves. There exists a competition between the aperiodicity, which tends to localize the

electron, and the repetitivity of the structure (the same structural motives appearing over and

over again), which tends to have a delocalizing effect because electrons tunnel between

identical local environments. In simple model Hamiltonians one observes electronic states

that are neither extended over the entire system nor exponentially localized, but so-called

“critical” states that show a multifractal probability distribution.

Very huge quasiperiodic tilings can be constructed as direct products of quasiperiodic

chains, so that extremely large systems sizes become accessible. Wave packets on these

tilings show anomalous diffusion in a stepwise process of fast expansion followed by time

intervals of confined wave packet width. These and other particular physical properties of

quasi-crystalline materials are discussed.

Light-propagation through a stack of two media with different diffractive indices, aligned

according to different quasiperiodic sequences, is also studied. Finally topical technological

applications are explained. Examples in art and architecture are also shown.

P a g e | 25

Nonlinear waves in disordered systems

Sergej Flach

New Zealand Institute for Advanced Study

Centre for Theoretical Physics and Chemistry

Massey University

Private Bag 102 904 NSMC, 0746 Auckland, New Zealand

Various extended media are capable of localizing waves, in particular in low dimensions.

Examples are random potentials (Anderson localization), quasiperiodic potentials (Aubry-

Andre localization), Wannier-Stark ladders (Bloch oscillations). Going from real to

momentum space adds also the kicked quantum rotor (dynamical localization). Wave

localization relies on the phase coherence of the waves. Experiments with ultracold atomic

clouds in optical potentials, and light propagating through structured media, have confirmed

that. When atom-atom interaction is added and is treated on a mean field level, or when light

intensity is increased, the wave equations turn nonlinear. This has far reaching consequencies,

since nonlinearity may annihilate integrability, lead to deterministic chaos, and ultimately

destroy phase coherence. Insulators turn into conductors, wave packet localization is

destroyed, and a number of very intricate mathematical problems on how to connect to the

linear wave theory pop up. I will give an introduction into this fascinating field, discuss and

explain the main results, extend to quantum aspects, and discuss open problems.

26 | P a g e

Effect of ZBL potential on kinks in repulsive lattices.

Juan Archilla

University of Sevilla, Spain

Supersonic kinks have been shown to exist in lattices where the interaction between

particles or atoms is only repulsive and the atoms are kept inside the system because of

external forces. Theory and numerical experiments show that the velocities and energies of

kinks diverge when the amplitude of the kinks approaches the interatomic distance.

It is however unrealistic the fact that for large energies the atoms become too close. At

those distances the overlapping of the electron wavefuncion produces strong repulsion that is

modeled with empirical short range potentials. Considering the applicability of the repulsive

lattice to the interlayer potassium sheet in silicates, we calculate and use the Ziegler-Biersack-

Littmark (ZBL) potential between the potassium ions and study the consequences on kink

properties, both numerically and analytically. The main conclusion is that kinks propagate

with large energy having however realistic distances between them.

P a g e | 27

Wandering and self-trapped exciton-polaron solitons in coupled macromolecules

and coupled semiconductor nanowires and nanoribbons

Yu. A. Kosevich

Semenov Institute of Chemical Physics, Russian Academy of Sciences, 119991 Moscow, Russia

The coupled electronic and lattice dynamics inherent to the exciton-polaron formation in

quasi-1D system was recently probed in linear-chain complex by time resolved femtosecond

impulsive excitation techniques. In this presentation, we describe a model for the transport of

coherent excitons in coupled quasi-one-dimensional (quasi-1D) systems like coupled

macromolecules or coupled semiconductor nanowires and nanoribbons through the

"wandering exciton-polaron". This model describes the formation of the exciton-polaron

through the interaction with acoustic phonons via the deformation potential. The exciton

transport is described as the motion of the exciton-polaron soliton, which is temporarily self-

trapped in one of the coupled quasi-1D subsystems, both along and across the subsystems.

Correspondingly an external electron or hole can travel in coupled macromolecules and

coupled semiconductor nanowires and nanoribbons in the form of "wandering electronic

polaron". We also describe the Rabi-like oscillations (exciton-polaron wandering) of the

exciton relative populations in coupled resonant quasi-1D subsystems, in which the relative

phase of the coupled coherent excitons or exciton condensates oscillates around / 2 or

/ 2 mod π.

28 | P a g e

Reaction rates in crystalline solids with account of radiation-induced quodons

V.I. Dubinko

NSC Kharkov Institute of Physics and Technology, Kharkov 61108, Ukraine

Recent experimental and molecular dynamics results give evidence for the existence of

the discrete solitons (DS’s), a.k.a. intrinsic localized modes (ILM’s), discrete breathers

(DB’s) and quodons, which are stable, highly localized lattice excitations that can transfer

energy along close-packed crystal directions. The DS interaction with crystal defects results

in the amplification of the reaction rates, such as atom ejection and diffusion. In this

presentation, recent results on the DS detecting in three-dimensional crystal structures by

means of molecular dynamics are discussed, and a new concept of the rate theory

modification in solids under irradiation is presented, which is based on the interaction of

crystal defects with quodon “gas”.

P a g e | 29

Optical switching of an organic photochromic triad

M. Pärs1, M. Gradmann

1, R. Schmidt1

, C.C. Hofmann1, K. Gräf

2, P. Bauer

2,

M. Thelakkat2, and J. Köhler

1

1 Experimental Physics IV, University of Bayreuth, 95440 Bayreuth, Germany

2 Applied Functional Polymers, University of Bayreuth, 95440 Bayreuth, Germany

We investigate the photophysical properties of a molecular triad (see left panel on figure)

what consists of a central photochromic dithienylcyclopentene (DCP) unit and two highly

efficient perylenebisimide (PBI) fluorophores. The DCP unit can be reversibly interconverted

between two bistable forms by light, whereas the fluorescence intensity of the PBI shows the

modulation with high contrast ratio as a function of the state of the DCP (see right panel on

figure)[1]. We will present a model that describes the experimental observations, such as the

fluorescence modulation contrast ratio, the reaction rates and the reaction quantum yields as a

function of optical radiation intensities. Based on our experimental findings, the functionality

of an optical gate is demonstrated.

[1] M. Pärs, C.C. Hofmann, K. Willinger, P. Bauer, M. Thelakkat, J. Köhler, Angew. Chem. Int. Ed.

2011, 50, 11405-11408

30 | P a g e

Complex granular high temperature superconductors

Antonio Bianconi

Sapienza University of Rome, Italy

We report the quantitative determination of nanoscale phase separation in YBaCuO and

LaCuO systems by Scanning nano-X-ray Diffraction (SXD). Multiple types of phase

separation are shown to be driven by competing phases of self-organized pseudo-Jahn-Teller

polarons. We clearly show two main types of granular superconductors: striped nano-grains

of polarons in an anti-ferromagnetic medium and striped polarons nano-grains in a metal

medium. Photo-induced manipulation of nanoscale textures for new materials functionality is

described.

P a g e | 31

Strongly coupled individual spins for sensing and quantum computing

Dieter Suter

Technische Universität Dortmund, D-44221 Dortmund, Germany

The electronic and nuclear spins associated with the nitrogen-vacancy center in diamond

have become some of the most promising solid-state qubits. The properties that make them so

attractive include the limited Hilbert space, excellent controllability, and long coherence

times, all leading to the possibility of implementing quantum algorithms with high precision.

In addition, the relatively easy initialization and readout of individual qubits and the

possibility of room-temperature operation lead to lower hurdles for experimental

implementations than in other solid-state systems. The realization of this potential requires the

implementation of quantum registers on the basis of NV centers and high-fidelity robust

quantum gate operations. This includes the protection of the quantum registers against

environmental noise as well as the design and implementation of gates that are insensitive to

experimental imperfections, such as deviations in the amplitudes of control fields. Protection

can be achieved by suitable pulse sequences that modulate the interaction with the

environment in such a way that the resulting average effect of the environment is minimized

while the effect of the control fields is retained. Apart from applications in quantum

information processing, these systems are also attractive tools for quantum-limited sensors.

32 | P a g e

Strongly correlated systems on highly frustrated lattices: from magnons to electrons

J. Richter

Institut fuer Theoretische Physik, Universitaet Magdeburg,

P.O. Box 4120, D-39016 Magdeburg, Germany

For antiferromagnetic Heisenberg spin systems as well as for Hubbard electrons on

various frustrated lattices a class of exact eigenstates can be constructed [1,2]. Such

eigenstates can be found, e.g., for the 1D sawtooth and kagome chains, the 2D kagome and

checkerboard lattices, and the 3D pyrochlore lattices. The exact many-particle eigenstates

consist of independent magnons (electrons) localized on finite areas of the lattice and become

ground states for certain values of total magnetization (electron concentrations). Important

structural elements of the relevant systems are triangles being attached to polygons or lines.

Then the magnons (electrons) can be trapped on these polygons or lines. For electrons the

scenario of localized eigenstates is related to the so-called flat-band ferromagnetism [2,3].

The correlated systems having localized eigenstates exhibit a highly degenerate ground-

state manifold at the saturation field hsat (at a characteristic value of the chemical potential 0)

for magnons (electrons) [2,4,5]. The degeneracy grows exponentially with the system size

and leads to a finite residual entropy.

By mapping the localized magnon (electron) degrees of freedom onto a hard-core lattice

gas one may find explicit analytical expressions for the low-temperature thermodynamics in

the vicinity of hsat(0). Though the scenario of localized eigenstates is similar for spin and

electron systems, the different statistics of spins and electrons leads to different construction

rules for the localized eigenstates and, as a result, to a different hard-core lattice gas

description.

[1] J. Schulenburg, A. Honecker, J. Schnack, J. Richter, and H.-J.Schmidt, Phys. Rev. Lett. 88,

167207 (2002); J. Richter, J. Schulenburg, A. Honecker, J. Schnack, and H.-J. Schmidt, J.Phys.:

Condens. Matter 16, S779 (2004).

[2] O. Derzhko, J. Richter, A. Honecker, M. Maksymenko, and R. Moessner, Phys. Rev. B 81, 014421

(2010); M. Maksymenko, A. Honecker, R. Moessner, J. Richter, and O. Derzhko, Phys. Rev. Lett.

109, 096404 (2012).

[3] A. Mielke, J. Phys. A 24, L73 (1991); H. Tasaki, Prog. Theor. Phys. 99, 489 (1998).

[4] O. Derzhko and J. Richter, Phys. Rev. B 70, 104415 (2004); O. Derzhko and J. Richter, Eur. Phys.

J. B 52, 23 (2006); J. Richter, O. Derzhko and T. Krokhmalskii, Phys. Rev. B 82, 214412 (2010);

M. Maksymenko, O. Derzhko, and J. Richter, Eur. Phys. J. B 84, 397 (2011).

[5] M.E.Zhitomirsky and H.Tsunetsugu, Phys. Rev. B 70, 100403(R) (2004); M.E. Zhitomirsky and

H. Tsunetsugu, Phys. Rev. B 75, 224416 (2007).

P a g e | 33

Singlet description of the 2D frustrates Heisenberg model with arbitrary signs of nearest

and next-nearest exchanges

A.V.Mikheyenkov1,2

, A.F.Barabanov1, A.V.Shvartsberg

2

1 Institute for High Pressure Physics, RAS, 142190 Troitsk, Moscow Region, Russia

2 Moscow Institute of Physics and Technology, 9, Institutskii per.,

141700 Dolgoprudny, Moscow Region, Russia

The two-dimensional S=1/2 J1–J2 quantum Heisenberg model is considered in the frames

of spherically symmetric self-consistent approach (alternative notation – spin-rotation-

invariant Green’s function method). The spin-spin correlation functions and energy are

obtained for arbitrary signs and values of exchange constants (for the entire “J1–J2 circle”),

both for zero temperature and for T>0.

The calculations, performed without any adjustable parameters, commonly used in the

method, lead to the qualitative agreement with the available numerical and analytical data. It

is argued, that proper tuning of the method parameters (vertex corrections and damping value)

gives also quantitative agreement for any particular region of “J1–J2 circle”.

For T=0 three long-range order phases (FM, AFM and stripe) and two regions of spin

liquid are indicated. It is shown, in particular, that the transition from ferromagnetic state to

spin liquid is continuous and goes via the narrow region of “soft” ferromagnet (Sz2<1/4).

There is no spin liquid state between AFM and FM (near the point J1=0, J2=-1) but the long-

range order in this region is considerably softened.

We also argue (for the example of purely AFM model J1=1, J2=0) that the results depend

qualitatively on the spin length S. In contrast to the S=1/2 case, for S≥1 the spin gap is

exponentially small at all temperatures T≤ J1, the correlation length being exponentially large.

34 | P a g e

Terahertz spectroscopy of spin waves in multiferroic BiFeO3 in high magnetic fields

U. Nagel 1, T. Katuwal

1, H. Engelkamp

2, D. Talbayev

3,

Hee Taek Yi 4, S.-W. Cheong

4, T. Rõõm

1

1 National Institute of Chemical Physics and Biophysics, Akadeemia tee 23, 12618 Tallinn, Estonia

2 High Field Magnet Laboratory, Institute for Molecules and Materials, Radboud University

Nijmegen, Toernooiveld 7, NL-6525 ED Nijmegen, The Netherlands 3 Department of Physics, Tulane University, 5032 Percival Stern Hall, New Orleans, LA 70118, USA

4 Rutgers Center for Emergent Materials and Department of Physics and Astronomy, Rutgers

University, 136 Frelinghuysen Rd., Piscataway, New Jersey 08854, USA

BiFeO3 is a G-type antiferromagnet (AFM) below 640 K in which the Fe3+

magnetic

moments are ordered antiferromagnetically between adjacent pseudocubic (111) planes with

ferroelectric polarization P || [111]. Ferromagnetic ordering within the planes is broken by a

long-wavelength cycloidal spin order with three possible directions of the ordering vector qi ||

{[110],[011],[101]} P. The spins of a cycloid qi are in the plane determined by P and qi.

Dzyaloshinskii-Moriya interaction in [111] direction cants spins out of the cycloid plane.

Most of INS and THz spectroscopy studies on BiFeO3 have been done in zero applied

magnetic field. Raman work demonstrated that the Raman-active magnon frequencies depend

strongly on applied electric field. High field ESR was done in magnetic fields up to 25 T, but

was limited to frequencies lower than the main cycloid resonances and one of the AFM mode

resonances.

In this work, we present terahertz spectra of BiFeO3 single crystals at low temperature and

high magnetic field up to 31 T which allow us to identify the excitations of the cycloid and

follow their magnetic field dependence until the cycloidal order is destroyed in high magnetic

field Hc 19 T along the [001] (pseudocubic) axis. We have studied far-infrared active

magnetic modes in a single ferroelectric domain BiFeO3 crystal at low temperature.

From the magnetic field dependence of spectra we get information about the anisotropic

magnetic interactions in BiFeO3 in conditions where the high magnetic fields compete with

internal fields. We found that the zero field spectra at low temperature are different before and

after application of a high magnetic field and this change persists at low temperature. We

interpret this as a change from a state with three magnetic domains into a state with one

domain with the lowest energy in high magnetic field.

P a g e | 35

Frustration and BEC of magnons in Han Purple

R. Stern1, I. Heinmaa

1, V. V. Mazurenko

2, A. A. Tsirlin

1,

D. V. Sheptyakov3, S. Krämer

4, and T. Kimura

5

1National Institute of Chemical Physics and Biophysics, Akadeemia tee 23, 12618 Tallinn, Estonia

2Ural State Technical University, Mira Street 19, 620002, Ekaterinburg, Russia

3Laboratory for Neutron Scattering, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland

4Laboratoire National des Champs Magn´etiques Intenses, LNCMI - CNRS (UPR3228)

5Division of Materials Physics, Graduate School of Engineering Science,

Osaka University, Toyonaka, Osaka 560-8531, Japan

We present a revised microscopic model of Han Purple, BaCuSi2O6, a model compound

that features magnetic bilayers with weakly coupled spin dimers.

The low-temperature crystal structure of BaCuSi2O6 has been recently investigated with

high-resolution synchrotron x-ray and neutron powder diffraction techniques and has been

found to be on average (ignoring the incommensurate modulation) orthorhombic, with the

most probable space group Ibam. The Cu-Cu dimers in this material are forming two types of

layers with distinctly different interatomic distances. Subtle changes also modify the

interlayer Cu-Cu exchange paths.

In order to understand the nature of the two-dimensional Bose-Einstein condensed (BEC)

phase in BaCuSi2O6, we performed detailed 63

Cu and 29

Si NMR above the critical magnetic

field, Hc1=23.4 T. The two different alternating layers present in the system have very

different local magnetizations close to Hc1; one is very weak, and its size and field

dependence are highly sensitive to the nature of inter-layer coupling. Its precise value could

only be determined by "on-site" 63

Cu NMR.

Using the low-temperature structural data, we obtain also via DFT calculations two types

of nonequivalent spin dimers, in excellent agreement with our earlier 63,65

Cu NMR data. We

further argue that leading inter-dimer couplings connect the upper site of one dimer to the

bottom site of the contiguous dimer, and not the upper–to-upper and bottom–to–bottom sites,

as assumed previously. This finding is supported by existing inelastic neutron scattering

results. We discuss the consequences of our results to frustration in BaCuSi2O6, to existing

theoretical interpretation of our high field NMR results as well as to available theories of the

Bose-Einstein condensation (BEC) of magnons observed in this compound.

36 | P a g e

Soliton lattice phase of spin-Peierls state

I. Heinmaa, E. Joon, R. Stern

National Institute of Chemical Physics and Biophysics, Akadeemia tee 23, Tallinn 12618, Estonia

One-dimensional magnetic spin ½ chains undergo spin-Peierls (SP) transition caused by

crystal lattice dimerization at low temperatures. The dimerization or doubling of a unit cell is

a result of spin-phonon magnetoelastic coupling and takes place in CuGeO3 at TSP= 14K.

However, this state is unstable against the high magnetic field H>Hc=12.5T, which creates

domain walls inside the dimerized phase. This soliton lattice phase exists between lower and

higher TSP in other spin-Peierls compounds too: TiClO (65K/95K) and TiBrO (27K/48K), but

it isn’t caused by magnetic field. For establishing the cause of soliton lattice phase we have

investigated the spin-Peierls compound TiPO4 with highest TSP=112K (73K/112K) by 31

P and 47,49

Ti NMR in high magnetic field 8.5 and 14.1 Tesla. In paramagnetic phase T> TSP the

Knight shift follows susceptibility and at T<73K the NMR line splits indicating dimerization.

In temperature interval 73K<T<112K inhomogeneous distribution of magnetization is clearly

seen demonstrating spin paired and normal phase coexisting. Additional measurements and

comparing with the data of the compounds mentioned above allowed us conclude that the

soliton lattice phase in these crystals is caused by inter-chain spin-spin interaction.

P a g e | 37

Frustrated pentagonal Cairo lattice in Bi4Fe5O13F

Alexander A. Tsirlin1, Dmitry Batuk

2, Artem M. Abakumov

2

1National Institute of Chemical Physics and Biophysics, Akadeemia tee 23, 12618 Tallinn, Estonia

2EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium

Cairo lattice reveals an unusual topology of frustrated magnetic interactions that form

pentagonal units. We present crystal structure and magnetism of Bi4Fe5O13F, a recently

discovered spin-5/2 compound, which is one of the few material prototypes of the Cairo spin

lattice. Thermodynamic measurements reveal a sequence of phase transitions, with the onset

of magnetic order at TN =178K and subsequent transformations at T1=62K and T2=71K. The

low-temperature magnetic structure is non-collinear and commensurate, in agreement with

theoretical expectations for the Cairo-lattice spin model. However, additional magnetic

transitions below TN have not been anticipated by theory. A comparative microscopic analysis

of Bi4Fe5O13F and the sister compound Bi2Fe4O9 will be presented.

38 | P a g e

Microscopic modeling of the S=1/2 Heisenberg ferrimagnet Cu2OSeO3

O. Janson1, I. Rousochatzakis

2, A. A. Tsirlin

1, U. K. Roessler

2, J. van den Brink

2,

and H. Rosner3

1 National Institute of Chemical Physics and Biophysics, Akadeemia tee 23, 12618 Tallinn, Estonia

2 Leibniz Institute for Solid State and Materials Research, Dresden, Germany 3 Max Planck Institute for Chemical Physics of Solids, Dresden, Germany

Cu2OSeO3 is a S=1/2 Heisenberg ferrimagnet with a sizable magnetoelectric coupling.

Recent experimental studies reported magnetic-field-induced emergence of skyrmions in this

insulating material. Based on extensive DFT band structure calculations, we evaluate

isotropic (Heisenberg) as well as anisotropic (Dzyaloshinskii-Moriya) magnetic exchange

couplings. Five relevant magnetic couplings form a complex, non-frustrated spin model,

which can be described as a pyrochlore lattice of magnetic tetrahedra. A peculiar feature of

this lattice is the alternation of ``strong'' tetrahedra (the constituent spins are strongly coupled)

and ``weak'' tetrahedra. Profiting from the separation of the energy scales, we develop an

effective model, treating strong tetrahedra either as a classical S=1 object or as a coherent

quantum superposition of classical states. For the latter case, we find an excellent agreement

with the quantum Monte Carlo simulations of the full model and the experimental

magnetization and neutron diffraction data. We demonstrate that the developed effective

model can be further used to model the field-induced behavior, including the formation of

skyrmions.

P a g e | 39

The phase diagram of EuxSr1-xTiO3: from almost multiferroic to quantum paraelectric

Annette Bussmann-Holder, Reinhard Kremer, und Jürgen Köhler

Max-Planck-Institute for Solid State Research, Heisenbergstr. 1, D-70569 Stuttgart, Germany

EuTiO3 (ETO) has been shown to have a pronounced magneto-dielectric coupling since

the low temperature dielectric constant is strongly magnetic field dependent. Its close analogy

with SrTiO3 (STO) has motivated us to model this compound with the same lattice dynamical

parameters and explore its high temperature dynamics. The important outcome was the

prediction that a structural phase transition, driven by a zone boundary mode instability as

observed in STO, should be present in ETO at much higher temperatures as compared to

STO. Indeed specific heat measurements have identified this transition at TS=282K, more

than twice as large as TS of STO. Since STO and ETO have the same lattice parameters, a

straightforward investigation of the phase diagram of their mixed crystals has been carried

through by different methods. Interestingly, both phase transitions, the high temperature one

at TS and the low temperature transition to an antiferromagnetic phase at TN=5.5K in ETO,

depend nonlinearly on x in EuxSr1-xTiO3. The consequences of these findings are discussed

and evidences for high temperature unconventional spin dynamics presented.

40 | P a g e

Spin waves in degenerate two-orbital Hubbard models

E. von Oelsen

Brandenburg Technical University, Cottbus, Germany

We examine a two-dimensional Hubbard model with two degenerate orbitals per lattice

site. Medium to strong Coulomb (U,U’) and Hund’s exchange (J) interactions are included.

Such models play an important role in the attempt to understand the basic physics of pnictides

and other transition-metal compounds. We focus our investigations both on the instabilities of

the homogeneous paramagnet towards magnetically ordered phases and the stability of the

possibly existing stable (anti-)ferromagnetic phases. Critically interactions as a function of

doping are calculated and the resulting phase diagrams will be presented. Our calculculations

are based on the time-dependent Gutzwiller theory which has recently been developed for

multi-band Hubbard hamiltonians. Recently published results [1] illustrate the power of the

TDGA, resulting in phase diagrams that exhibit a strong U- and J-dependance (in contrast to

convenient Hartree–Fock calculations).

[1] E. von Oelsen, G. Seibold, and J. Bünemann, Phys. Rev. Lett. 107, 076402, 2011

P a g e | 41

Two-gap superconductivity: interband interaction in the role of external field

Artjom Vargunin, Küllike Rägo, Teet Örd

Institute of Physics, University of Tartu, Tähe 4, 51010 Tartu, Estonia

By studying thermodynamics of a two-gap superconducting system, various

manifestations of the former intrinsic phase transition in the band with weaker

superconductivity are analyzed. It is shown that weak interband pair-transfer interaction acts

as external field on that phase transition, similarly to magnetic field for ferromagnetics or

electric field for ferroelectrics. As a result, the singularities of thermodynamic functions

related initially to the intrinsic phase transition point become smeared and all located at

different temperatures.

42 | P a g e

Semi-empirical and first principles analysis of the microscopic crystal field effects and

electron-vibrational interaction for 3d ions in solids

M.G. Brik

Institute of Physics, University of Tartu, Riia 142, Tartu 51014, Estonia

In the present work some selected examples of analysis of the microscopic crystal field

effects and electron-vibrational interactions for the transition metal ions in solids will be

given. The exchange charge model of crystal field [1] and the CASTEP module [2] of

Materials Studio package [3] were used as the main tools in the performed calculations. The

two mentioned approached were applied to different systems; some details are given below.

Semi-empirical analysis of the energy level schemes for the isoelectronic impurity ions in

a crystal field and an analysis of the microscopic crystal field effects were performed for the

Cr3+

, Mn4+

, and Fe5+

ions in SrTiO3 [4]. As a result, the impurities energy levels, the Jahn-

Teller stabilization energies, and deformations of the impurity centers due to the combined

effects of the a1g and eg normal modes of the octahedral complex were all evaluated.

Ab-initio calculations were performed for the KZnF3:Ni2+

system to model the

microscopic crystal field effects on the nickel 3d electron states by calculating the electronic

band structure at the ambient and elevated hydrostatic pressure and extracting from these

calculations the distance dependence of the crystal field strength 10Dq and main parameters

of the electron-vibrational coupling, such as Huang-Rhys factor and Stokes shift [5].

For all considered systems detailed comparison with available experimental and/or other

theoretical data is given.

[1] B.Z. Malkin, in: A.A. Kaplyanskii, B.M. Macfarlane (Eds.), Spectroscopy of Solids Containing

Rare-Earth Ions, North-Holland, Amsterdam, 1987, pp. 33-50.

[2] M.D. Segall, P.J.D. Lindan, M.J. Probert, C.J. Pickard, P.J. Hasnip, S.J. Clark, M.C. Payne,

J. Phys.: Condens. Matter 14 (2002) 2717.

[3] http://accelrys.com/products/materials-studio/

[4] M.G. Brik, N.M. Avram, J. Phys.: Condens. Matter 21 (2009) 155502.

[5] M.G. Brik, G.A. Kumar, D.K. Sardar, Mat. Chem. Phys. 136 (2012) 90.

P a g e | 43

Multiband superconductivity and the pseudogap

N. Kristoffel and P.Rubin

Institute of Physics, University of Tartu, Riia 142, 51014, Tartu,Estonia

The pseudogap (PG) can appear as a natural event in multiband superconductivity. In

such systems external bare normal state band gaps can exist and the chemical potential can be

positioned selectively in the bands.Vanishing of the bare gaps (e.g.by doping) determines the

critical points on the phase diagram. Here the Lifshitz type transitions with the reconstruction

of the active part of the Fermi surface appear. Build-up and destruction of the chemical

potential with the overlapping bands drive the transition temperature domain on the phase

diagram. The PG corresponds to the minimal quasiparticle excitations of a band not bearing

the chemical potential.

Various two- and three-band systems with interband pairing channels have been analysed

and illustrated. In the two-band cases the critical points appear on the Tc dome edges and

does not lead to “usual” PG type behavior. In multiband superconducting systems the critical

points positions are not limited in respect of the Tc dome. The PG and the superconducting

gap (SCG) of another band can coexist at the same phase diagram parameter. At a well

developed PG the corresponding energy scales can be rather different. The PG associated

states enter the superconducting game effectively near the critical points. This can appear also

in indirect manner through the participation of the PG band states in positioning the chemical

potential (closed PG band pairing channel). In the critical region the normal state and

superconducting contributions into the PG quasiparticle energy become comparable. Passing

it the PG continues as the SCG of the same band. Generally a critical point near the Tc

maximum refers to the participation of the PG band states in the pairing. The appearance of a

PG on the phase diagram can be considered as an indication to the multiband nature of the

superconductivity mechanism (not vice versa).

The behaviour of the cuprate PG can be explained by a model including an itinerant band

besides nodal and antinodal (gapped) novel defect-polaronic state distributions created by

doping.

This study was supported by the European Union through the European Regional

Development Fund (Centre of Excellence “Mesosystems: theory and applications“, TK114)

and the Estonian Science Foundation Grant No 8991.

[1] N.Kristoffel, P. Rubin, Phys. Lett. A 374,70 (2009)

[2] N.Kristoffel, P.Rubin, Intern. J .Modern Phys. 26,1250144 (2012)

44 | P a g e

A comparative DFT study of dynamical covalency in delafossite-type crystalline

materials

A. Pishtshev1, G. Seibold

2

1Institute of Physics, University of Tartu, Tartu, Estonia

2Institut für Physik, BTU Cottbus, Cottbus, Germany

In this work, we use density functional theory (DFT) simulations to investigate structural

energetics and chemical bonding in several members of delafossite-type crystalline systems:

CuMO2 (M=B, Al, Ga, In, La), and NaHF2 . Electronic structure calculations were performed

within PBE-based GGA and hybrid functional schemes. For the given materials, we analyze

the corresponding trends in local charge geometries, the bonding properties, the cation-anion

electron transfer, and the interplay between the polar lattice and electronic degrees of freedom

in terms of effective charges and the electron localization function. In the context of the

underlying dynamical processes in forming of electronic configurations, which potentially are

important for creation of the dynamic covalency channels, the obtained results are used to

perform a comparative analysis of relative strengths and weaknesses of ionic and covalent

contributions into structural bonding, and to estimate an impact of specific covalent linkage of

many-body nature connected with the relevant coupling of valence electrons and ionic cores.

P a g e | 45

The modified sharpened index hms and other variants in the Hirsch index zoo

Michael Schreiber

Institut für Physik, Technische Universität Chemnitz, 09107 Chemnitz, Germany

The Hirsch index or h-index is widely used to quantify the impact of an individual’s

scientific research output. I present an analysis of two case studies, one for 8 famous

physicists and another for 26 not-so-prominent colleagues. Difficulties with the determination

of the index and its interpretation are discussed.

Fractionalized counting of the publications (rather than the citations) is an appropriate

way to distribute the impact of a paper among all the coauthors of a multi-authored

manuscript, leading to a simple modification hm of the h-index. On the other hand the

exclusion of self-citations allows one to sharpen the index (yielding hs), what is appropriate,

because self-citations are usually not reflecting the significance of a publication. Combining

the two procedures gives the modified sharpened index hms. The additional effort in

determining the modified sharpened index hms is worth performing in order to obtain a fairer

evaluation of the citation records.

In order to take into account the highly skewed frequency distribution of citations, Egghe

proposed the g-index as an improvement of the h-index. The g-index discriminates better

between different citation patterns. Fractionalized counting of the multi-authored manuscripts

leads again to a simple modification gm. The exclusion of self-citations allows one to sharpen

the index yielding gs. In my opinion the modified sharpened index gms is the most appropriate

way to estimate the visibility of a scientist’s research. Whether this is a measure of

importance and significance is a debatable and debated question.

Therefore, as an alternative, percentile-based indicators have attracted more attention

recently. Topical developments like the fractional scoring are presented and applied for two

different case studies with empirical data.

46 | P a g e

Friedrich Wilhelm Ostwald, Nobel Laureate for Chemistry 1909

Wolfgang Hönle

formerly Max-Planck-Institute for Chemical Physics, Dresden(Germany)

Wilhelm Ostwald (Latvian: Vilhelms Ostvalds) was born in

Riga (Sept. 2nd, 1853), as the second son of the baltic-german

cooper Gottfried Wilhelm Ostwald (1824–1903) and his wife

Elisabeth Leuckel (1824–1903). He graduated 1875 from the

University of Tartu, Estonia and under the guidance of Carl

Schmidt he received his Ph.D. in 1878. He taught in Tartu from

1875 to 1881 and at Riga

Polytechnicum from 1881 to 1887 and in 1887 he became

Professor for Physical Chemistry in Leipzig and Citizen of Saxony.

In 1906 he retired and lived in his country estate in Großbothen

near Leipzig/Saxony, where he kept on working in several research fields.

In 1909 Ostwald received the Nobel Prize in Chemistry for his work on catalysis,

chemical equilibria and reaction velocities. Ostwald was a member of the International

Committee on Atomic Weights until 1917. He developed a colour theory as well as supported

by his Nobel Price money several inter-national language movements such as Esperanto and

later Ido. Besides he was the founder and financier of ‘Die Brücke’, an International institute

to organise intellectual work. Wilhelm Ostwald was strongly engaged in numerous

philosophical writings and adopted the philosophy of Monism and became President of the

Monistic Alliance in 1911.

Ostwald together with Svante Arrhenius, Jacobus Henricus van’t Hoff and Walther Nernst

are the founders of Physical Chemistry. Ostwald taught several hundred of physical chemistry

students more than 70 of them held academic chairs all over the world and he founded several

important scientific journals. The Ostwald law of dilution as well as the Ostwald ripening

commemorate him until today. On April 4th, 1932 Wilhelm Ostwald died in a hospital in

Leipzig and was buried in the given up quarry on his estate in Großbothen near Leipzig.

My talk will cover his life, his family and his scientific work.

P a g e | 47

POSTERS

48 | P a g e

Crossover of the efficiency of the spontaneous down conversion at strong laser field

V. Hizhnyakov, H. Kaasik

Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia

Spontaneous down conversion of photons - the creation of photon pairs on the expense of

the single photon, is the nonlinear quantum phenomenon which takes place due to existence

of the zero-point fluctuations. The process is of remarkable interest also from practical point

of view while generated photon pairs are in the entangled state. At the usual conditions the

process has extremely low probability and is observed only in case of positive feedback e.g.

in parametric oscillators. However, at present there exist possibilities to strongly enhance (at

least locally) the intensity of laser light and in this way to increase the intensity of the down

conversion. In this communication we consider how the efficiency of this process changes

with increasing of the intensity of excitation. We restrict ourselves with the case of laser light

focused at a small spot of a dielectric with non-zero second-order susceptibility. We will

show that increasing of the intensity I of the laser light will result in the increasing of the two-

photon emission only if I remains smaller than the critical intensity I0. The further increasing

of I will cause decreasing of the two-photon emission. The crossover of the efficiency of the

process takes place if the field of the laser light becomes comparable to the atomic field. This

condition may be fulfilled e.g. by using femto-pulse excitation with 10-6

J of pulse energy.

P a g e | 49

MAS NMR study of Cu2CdB2O6, a decorated Shastry-Sutherland spin-1/2 lattice

Alan Kalda, Ivo Heinmaa, Enno Joon, Oleg Janson, Alexander Tsirlin, Raivo Stern

National Institute of Chemical Physics and Biophysics, Akadeemia tee 23, 12618 Tallinn, Estonia

We report high resolution solid state 11

B nuclear magnetic resonance (NMR) results on a

decorated Shastry-Sutherland lattice spin-1/2 quantum magnet Cu2CdB2O6.

In this study, magic angle spinning (MAS) technique is used to obtain 11

B NMR spectra of

Cu2CdB2O6 in 14.4T external magnetic field. We rotate the sample powder in a 1mm (outer

diameter) ceramic MAS rotor at the speed of 40-50kHz at temperatures between

30K<T<300K. Such a CryoMAS probe, developed at NICPB, was used for the experiment.

NMR spectra reveal two distinctive boron sites (B1 and B2). Temperature dependent Knight’s

shift is measured and evaluated for both B1 and B2. Furthermore, we demonstrate linear

relation appearing between magnetic susceptibility [1] and Knight’s shift for B1, whereas for

B2 this relation is not entirely linear.

[1] O. Janson, I. Rousochatzakis, A. A. Tsirlin, J. Richter, Yu. Skourski, H. Rosner, (2012),

Decorated Shastry-Sutherland lattice in the spin-1/2 magnet CdCu2(BO3)2, Physical Review B

85,064404

50 | P a g e

THz spectroscopic study of azurite at low temperature

T. Katuwal1, U. Nagel

1, T. Rõõm

1, R. K. Kremer

2

1National Institute of Chemical Physics and Biophysics, Tallinn

2Max-Planck-Institute für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Stuttgart

In order to investigate the low temperature magnetic modes of Cu3(CO3)2(OH)2 (azurite)

we employed terahertz (THz) spectroscopic measurements on the natural single crystals. The

transmission spectra were recorded at the temperatures down to 4 K and magnetic field up to

12 T using polarized THz radiation. Frequency-field diagrams are plotted for the low energy

resonances. When electric field component e of light is polarized along the a/c-axes, two

modes (monomer and dimer) are found to be active. However, when e is along the diamond-

chain (b axis) the dimer mode is inactive. These facts suggest that the monomer mode is

independent of polarization of light, but the dimer mode depends on light polarization. The

exchange interaction J2 is estimated to be 900 GHz =43 K.

P a g e | 51

Electrostatic doping of high-Tc cuprate ultrathin films and transverse electric field

effects

Peet Konsin and Boris Sorkin

Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia

In [1] the electric field effects on the bulk superconducting phase transition temperature

(the bulk and surface conductivities are equal) of thin cuprate films in the metal-dielectric-

superconductor structures are studied. In this report an ultrathin high-Tc superconducting film

being in contact with a normal metal from one side and with a dielectric layer from another

side and other interfaces between the film and dielectric substrates are studied. We have

considered in the Thomas-Fermi approximation two-dimensional electron gas in the electric

field and found the relative carrier concentration Dp2 . It is shown that this concentration Dp2

can be the electrostatic doping parameter in superconducting films. The quantity Dp2 depends

on the electric induction ( ED ld , , d is the dielectric constant of the insulator, l is the

phenomenological parameter describing an ionic liquid), the Thomas-Fermi screening length

TFl , the film thickness L and the carrier concentration n in the sample in the absence of the

induction D . It is shown that in the negative electric field ( 0D ) the concentration 02 Dp

and xp D 2 (e.g. x in La2-xSrxCuO4). The dependences x on D are calculated in the

accumulation regime for the sub-monolayer (in La2-xSrxCuO4 the molecular layer), the

monolayer (a single unit cell thick film) and other cases. At this, the inductions D ~108 V/cm

are necessary for the inducing electrostatic doping effects in ultrathin films. We estimated the

superconducting transition temperature in the whole range of the phase diagram of LSCO

(between x =0.05÷0.27). We discuss recent experiments on the electric field effects

( x =0.06÷0.14) obtained in [2,3].

The research was supported by the Estonian Research Council (project IUT2-27) and by

the European Union through the European Regional Development Fund (projects TK114 and

SF0180013s07AP).

[1] P. Konsin and B. Sorkin, Phys. Rev. B 58, 5795 (1998).

[2] A.T. Bollinger, G. Dubuis, J. Yoon, D. Pavuna, J. Misevich and I. Božović, Nature 472, 458-460

(2011).

[3] G. Dubuis, A.T. Bollinger, D. Pavuna and I. Božović, J. Appl. Phys. 111, 1112 (2012).

52 | P a g e

MD and electron density calculations of conjoined graphene and carbon nanotubes

V. Krasnenko1, V. Boltrushko

1, M. Klopov

2,V. Hizhnyakov

1

1Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia

2Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia

Recent studies of conjoined graphene nanoribbons (GNR) and carbon nanotubes (CNT)

have demonstrated that these materials may have new important mechanic, electric and

catalytic properties and can act as energy and hydrogen storage. These expectations rely on

the extraordinary electric and mechanical properties of these materials. In this communication

we have performed a numerical study of some conjoined CNTs and GNR structures. A series

of MD simulations of GNRs and CNTs have been performed by using LAMMPS, Material

Studio and VASP software packages.

The typical total number of carbon atoms included in our MD simulations when using a

LAMMPS package was several thousands. The diameter of nanotubes was varying from 1.5

nm up to to 0.4 nm. It was found that all nanotubes, except the smallest ones (with diameter

less than 0.5 nm) may be entirely wrapped over by GNR. The distance between the wrapped

ribbon and the nanotube is 0.34 nm - the typical distance between grapheme sheets. If the

length of the nanotube is smaller than the width of the ribbon then, depending on the

difference of the width of CNT and GNR, the latter may be wrapped around the CNT (narrow

ribbon) or bent to an angle. In case the CNT was initially situated at the end of the long GNR

with a fixed position of its end, the nanotube was revolving upon the GNR.

To check the results, we have also performed the calculations of GNRs and CNTs, using a

CASTEP module of Material Studio and a VASP package, allowing one to include into

calculations the exchange correlation functional. In these calculations, to optimize the

geometry, one calculates electronic densities, electronic energies and the corresponding forces

to the atoms by using a super-cell technique. As compared to LAMPS, this algorithm is rather

time-consuming. Therefore, the calculations were made for super-cells which include only

few hundreds of atoms. We have found that the CNT with GNR wrapped around it at a

distance 0.34 nm graphene ribbon indeed form a static structure.

We have also found a metastable structure of CNT and GNR with the minimum distance

0.164 nm. In this structure the tube and the graphene ribbon are strongly deformed: the

nearest (adversary) rings of CNT and GNR are tilted towards each other so that one atom of

the 6C ring of CNT is drawn to one (nearest) atom of the ring of GNR to much shorter

distance (0.164 nm) than other atoms (situated at the mean distance 0.3 nm). This structure

has an analogy with the metastable structures of two graphene sheets with the distance 0.156

nm and a benzene molecule stacked on a graphene sheet with the distance 0.1563 nm.

However, in the latter structures the adversarial 6C rings remain parallel. A theoretical

inspection shows that the underlying mechanism of the discovered strong deformation (tilting

of the adversarial 6C rings) of conjoined CNT and GNR is the pseudo-Jahn-Teller effect.

P a g e | 53

139La NMR study of the triple-layer T’-La4Ni3O8

Alexander Leitmäe1, Ivo Heinmaa

1, Enno Joon

1, Nicholas J. Curro

2, Raivo Stern

1

1National Institute of Chemical Physics and Biophysics, Akadeemia tee 23, 12618 Tallinn, Estonia

2Department of Physics, University of California, Davis, CA 95616, USA

The triple-layer T’-La4Ni3O8 [1] is of scientific interest due to the possibility of achieving

Ni-based high-temperature superconductivity, because of Ni atoms having +1,+2 valencies in

the “infinite” NiO2 planes analoguous to cuprates. Also, a phase transition to

antiferromagnetic order has been established at TN = 105K [2] which could be an important

prerequisite for superconductivity. With the cuprate analogy, La atoms will enable hole

conductivity to Ni. The La nuclei are situated in 2 positions in the lattice – the fluoride block

and between the NiO2 layers.

Detailed nuclear magnetic resonance (NMR) experiments were made in order to

investigate the La nuclei. The results indicate distinctly different chemical surroundings for

the two La lattice sites, which is in agreement with the lattice structure. Using the obtained

data, it was possible to evalulate the values of the electric field gradient experienced by the La

nuclei. The gradient turned out to be greater for the nuclei in the fluoride block.

[1] V. V. Poltavets, K. A. Lokshin, A. H. Nevidomskyy, M. Croft, T. A. Tyson, J. Hadermann, G.

Van Tendeloo, T. Egami, G. Kotliar, N. ApRoberts-Warren, A. P. Dioguardi, N.J. Curro, and M.

Greenblatt, Phys. Rev. Lett. 104, 206403 (2010).

[2] N. ApRoberts-Warren, A. P. Dioguardi, V. V. Poltavets, M. Greenblatt, P. Klavins, and N. J.

Curro, Phys. Rev. B 83, 014402 (2011).

54 | P a g e

Time-dependent Jahn-Teller effect in case of E e –problem

Kaja Pae, V. Hizhnyakov

Institute of Physics University of Tartu, Riia 142, 51014 Tartu, Estonia

We are considering the effect of non-totally symmetric vibrations on the electronic

transition from a non-degenerated to the two-fold degenerated electronic state of an impurity

centre in solids. In the case of a two-fold degenerate state of E-representation it leads to JTE

with doubly degenerate vibrations of e-symmetry – the E e -problem, which is studied in

present survey. In this case the potential energy in the space of configurational coordinates

has the famous Mexican hat shape and possesses the conical intersection.

We have developed a rigorous quantum mechanical method which allows one to calculate

the optical spectra of impurity centres in crystals with the JTE in the excited state, taking into

account the non-totally symmetric phonons of the bulk [1,2]. The method works for arbitrary

vibronic interaction with a few (pseudo)local modes and weak interaction with phonons.

Rigorous quantum mechanical method enables us to find density matrix which describes

the population of the vibronic levels and time evolution of the coherence. Knowing this

matrix one can calculate the time dependence of any quantity of the vibronic subsystem. Here

we consider the time-dependence of the distribution function of the configurational coordinate

in the case of the optical excitation of the centre by a spectrally non-selective short light

pulse. Our consideration allows us for the first time give a rigorous description of the long-

time-scale relaxation of the system through the conical intersection, caused by the emission of

phonons to the bulk. The research was supported by the ETF grant TLOFY0145 and by the projects GLOFY7741 and

SLOFY114T.

[1] Hizhnyakov, V.; Pae, K.; Vaikjärv, T. (2012). Optical Jahn-Teller effect in the case of local modes

and phonons. Chemical Physics Letters, 525–526, 64 - 68.

[2] Pae, K; Hizhnyakov, V. (2013). Nonadiabaticity in a Jahn-Teller system probed by absorption and

resonance Raman scattering. Journal of Chemical Physics, in press.

P a g e | 55

Excitation of surface plasmon polaritons in Al-coated SNOM tips

V. Palm, M. Rähn, J. Jäme, V. Hizhnyakov

Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia

We use the mesoscopic effect of spectral modulation [1] caused by the interference of two

photonic modes transmitted by a SNOM tip to detect surface plasmon polariton (SPP)

excitation in 200 nm and 150 nm Al-coated bent-type SNOM tips. In spectra of the broadband

light transmitted by SNOM tips regions of highly regular spectral modulation can be

observed, indicating spectral intervals in which only two photonic modes (apparently HE11

and TM01) are transmitted with significant and comparable amplitudes. Demonstrated is a

remarkable sensitivity of observed spectral features to mesoscopically small displacements of

the fiber input in respect to the broadband light source; “proper” adjustment is always

necessary to satisfy the two-mode condition. [2,3] An obtained modulation period yields the

value of optical path difference (OPD) for the pair of modes. Due to the inherent modal

dispersion of the fiber, this OPD value depends linearly on the fiber tail length l. An

additional contribution to OPD can be generated in a metal-coated SNOM tip due to a mode-

sensitive photon-plasmon coupling resulting in excitation of SPPs with different phase

velocities.

For the both SNOM tips studied regular patterns with nearly frequency-independent

modulation periods could be observed; analyses of obtained linear OPD (l) dependences

reveal that in both cases the modal dispersion in a SNOM tip is much higher and has an

opposite sign compared to the fiber tail. According to our theoretical considerations, this

effect can be attributed to a mode-selective excitation of SPPs in a SNOM tip. For our 150 nm

SNOM tip it was also possible to observe a different type of regular spectral pattern, which

exhibits a frequency-dependent modulation period and apparently corresponds to a different

pair of modes.

Acknowledged is the support from Estonian Science Foundation Grant Nos. 7741, 8167

and project TLOFY0145, EU project TK114, and ESF project 1.2.0401.09-0079 in Estonia.

[1] M. Rähn, M. Pärs, V. Palm, R. Jaaniso, V. Hizhnyakov, Optics Communications 283, 2457

(2010).

[2] V. Palm, M. Rähn, V. Hizhnyakov, Optics Communications 285, 4579 (2012).

[3] V. Palm, M. Rähn, J. Jäme, V. Hizhnyakov, Proc. SPIE 8457, 84572S (2012).

56 | P a g e

Infrared spectroscopy and quantum 5D calculations: H2@C70.

L. Peedu1, Min Ge

1, U. Nagel

1, T. Rõõm

1,

Y. Murata2, K. Komatsu

2, Minzhong Xu

3, Z. Bacic

3

1 National Institute of Chemical Physics and Biophysics, Akadeemia tee 23, 12618 Tallinn, Estonia

2Institute for Chemical Research, Kyoto University, Kyoto

3Department of Chemistry, New York University, New York

We studied endohedral H2 translation and rotation eigen levels and eigen values in C70

using infrared spectroscopy and quantum calculation on a five dimensional potential energy

surface. The translation and rotation transitions appear as sidebands to the fundamental

vibration transition v = 0 to 1 of H2 in the mid-infrared spectra. The translation and rotation

eigen levels and states were calculated using two potential enery surfaces, one for the v = 0

and the other for the v = 1 vibration states. The potential energy surface was defined as a sum

of three-site H2 - C pair Lennard-Jones type potentials. The Lennard-Jones interaction

parameters otpimized for the infrared spectra of H2@C60 were used. The translational modes

of H2 are split into two well separated groups in C70. The lower energy translation z mode is

along the long axis of C70 and another higher xy mode is in the equatorial plane perpendicular

to the z axis. The agreement between theory and experiment is good for the z mode while the

energy of the xy mode is slightly underestimated.

P a g e | 57

Photon-gated persistent spectral hole-burning in a four-level system by pulses

Inna Rebane

Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia.

Persistent spectral hole-burning [1] is widely used as a method of eliminating the

inhomogeneous broadening of the spectra of electronic transitions in impurity systems. On the

photon-gated spectral hole-burning the selective excitation of the first step is followed by the

non-selective excitation of the second step, which leads to a persistent photochemical

transformation of the impurity. It is shown theoretically that on spectral hole-burning in a

four-level {0, 1, 2, 3} system, if the first pulse (excitation from level 0 to level 1) is a coherent

and a single-sides exponential, then after relaxation from level 1 to level 2, in the case of

excitation 2→3 by an extremely short second pulse on the increase of the time delay between

pulses, monotonous narrowing of the spectral hole in the inhomogeneous distribution function

of the frequency of the first-step electronic transition takes place. The width of this hole can

be narrower than the hole width obtainable in spectral hole-burning by monochromatic light.

In the limit case of the increasing of the time delay between pulses, it is possible to eliminate

the energy relaxation-induced hole width and there remains pure phase-relaxation-induced

hole width.

[1] B.M. Kharlamov, R.I. Personov and L.A. Bykovskaya, Optics Commun. 12 (1974) 191;

A.A. Gorokhovskiy, R.K. Kaarli and L.A. Rebane, Optics Commun. 16 (1976) 282.

58 | P a g e

Magnetic phase diagram of the two-dimensional J1-J3 Heisenberg model with S=1 on a

triangular lattice

P. Rubin1, A. Sherman

1, M. Schreiber

2

1Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia

2Institut für Physik, Technische Universität, D-09107 Chemnitz, Germany

Using Mori’s projection operator technique the spin Green’s functions of the Heisenberg

model with S=1 on the two - dimensional triangular lattice are calculated. The ferromagnetic

nearest- and antiferromagnetic 3rd-nearest-neighbor couplings, J1 = -(1-p) J and J3 = pJ, J >

0, are taken into account in the entire range of the parameter p. Mori's projection operator

method retains the rotation symmetry and does not anticipate any magnetic ordering.

For zero temperature several phase transitions are observed. At p 0.2 the ground state is

transformed from the ferromagnetic spin structure into a disordered state, which in its turn is

changed to an antiferromagnetic long-range ordered state with the incommensurate ordering

vector 1.16,0Q at 0.31p . With the growing p the ordering vector moves along the line

CQ Q to the commensurate point 2 / 3,0Q , which is reached at 1p . The final state

with an antiferromagnetic long-range order can be conceived as four interpenetrating

sublattices with the 120º spin structure on each of them. We apply the results to NiGa2S4.

[1] P. Rubin, A. Sherman, M. Schreiber, Physics Letters A 376, 1062 (2012)

P a g e | 59

Interplay between electronic inhomogeneities and pairing interactions

in high-Tc superconductors

E. Sigmund1, V. Hizhnyakov

2 and G. Seibold

1

1Institut für Physik, BTU Cottbus, PBox 101344, 03013 Cottbus, Germany

2Institute of Physics, Tartu University, Riia 142, 51014 Tartu, Estonia

Since the discovery of high-temperature superconductivity by Bednorz and Müller

numerous experiments have evidenced the existence of electronic inhomogeneities in these

compounds. While early on these inhomogeneities where believed to be predominantly

chemically driven, e.g. due to material imperfections like disorder introduced by the dopant

ions, it was subsequently realized that the strongly correlated character of the cuprate

superconductors and thus the electronic subsystem itself can favor the formation of

inhomogeneities on the nano-scale. The presence of electronic inhomogeneities strongly

reduces the screening of the electron-ion interaction high-temperature superconductors. We

show that this implies the existence of an non-totally screened long-range contribution the

electron-lattice coupling and opens an additional channel for the formation of copper pairs.

We calculate the superconducting order parameter taking into account a) the long-range and

the short-range parts of the electron-lattice interaction and b) the Coulomb repulsion between

charge-carriers. It turns out that whereas the long-range electron-lattice coupling determines

the anisotropy of the order parameter, the Coulomb repulsion and the short-range interactions

determine its symmetry. Thus, different high-Tc superconductor may have s- or d-wave

symmetry, depending on the relative strength of the interactions involved in the pairing.

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Quasi-elastic light scattering of doped 3He liquid on Fermi excitations

I. Tehver1, V. Hizhnyakov

1, G. Benedek

2,3

1Institute of Physics, University of Tartu, Riia 142, 202400 Tartu, Estonia

2Donostia International Physics Center (DIPC), Paseo de Lardizàbal 4, 20018 Donostia/San

Sebastian, Spain 3Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca,Via Cozzi 53, 20125 Milano,

Italy

Inelastic (Raman) scattering of pure He liquids is very weak due to the small polarizability

of He atoms. However the addition of impurity atoms or molecules may allow one to observe

such a scattering, especially if excitation takes place close to a resonance with an electronic

transition in the impurities. Unlikely to all other materials, the light scattering in quantum

liquid 3He may involve a unique mechanism, the creation and annihilation of particle-hole and

collective atom excitations, which are specific to a liquid Fermi like 3He. In the low energy

limit there is finite density of states of the quasi-particles and holes close to the Fermi level.

Therefore the density of states (DOS) of particle-hole excitations in the low-energy limit is

strongly enhanced as compared to the DOS of collective atom excitations (phonons) existing

in 3He. This makes possible to observe Fermi excitations in the scattering spectrum as a quasi-

elastic tail of the Rayleigh line. In this communication we present a theory of the mentioned

quasi-elastic scattering of light. We will show that this scattering causes a background to the

Rayleigh line, whose intensity linearly increases on the red side. The study of the shape of the

background at the distance of several Fermi energies from the Rayleigh line gives a unique

possibility to investigate the transformation of the collective excitation of atoms (phonons)

into single-particle excitations of 3He Fermi liquid.

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Optical manifestations of energy relaxation in quasi-degenerate electronic states

Taavi Vaikjärv

Institute of Physics University of Tartu, Riia 142, 51014 Tartu, Estonia

Excited states in molecules and impurity centers in crystals with high symmetry are

usually degenerate. The dynamics which occurs in the vicinity of the symmetric configuration

is essentially determined by the non-adiabaticity of the electronic and nuclear motion. We are

concerned with optical probing of quasi-degenerate electronic states which allows one to get

information about complicated vibronic dynamics governed by the pseudo-Jahn-Teller effect.

We present a method which allows one to study theoretically the dynamics of the time

evolution of the excited states in these systems and manifestations of this evolution in the

optical spectra. We note that the phonon continuum in the excited state is taken into account.

Our study is based on the recently proposed method [1] of calculation of optical

transitions in centers with the Jahn-Teller and pseudo-Jahn-Teller effect in the final state. The

time evolution of the states is calculated in the density matrix form using the master equation

[2]. The result of this work is calculated evolution of the relaxation process. The method is

applicable for calculations for long as well as intermediate and short times. The most

interesting thing to notice is that the relaxation rates of vibronic states depends greatly on the

distance of nearest neighbor states.

[1] K. Pae, T. Vaikjärv, V. Hizhnyakov Program & Abstracts: 20th International Symposium on the

Jahn-Teller Effect, 77 – 79 (2010).

[2] G. Lindblad, Commun. Math. Phys. 48 119 (1976).

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List of participants

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