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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
P a g e | 3
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
4 | P a g e
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
P a g e | 5
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
6 | P a g e
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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