DyProSo 2015

13 Sept 2015, 17:00 → 17 Sept 2015, 19:45 Europe/Berlin

Freising

Michael Leitner, Pascal Neibecker (Heinz Maier-Leibnitz Zentrum (MLZ)), Silvia Valentin-Hantschel, Winfried Petry

 

35th Symposium on Dynamical Properties of Solids

The 35th International Symposium on Dynamical Properties of Solids - DyProSo XXXV - was held from 13 to 17 September 2015 at the Bildungszentrum Freising near  Munich. The conference was hosted by the  Heinz Maier-Leibnitz Zentrum (MLZ) for neutron research (www.mlz-garching.de).

Spread over four days, the scientific programme featured 15 invited talks and 29 contributed talks on theoretical, experimental and methodological aspects of dynamical properties in solids and soft matter, categorized into the topical sessions
- Amorphous and soft matter
- Multiferroics and ferroelectrics
- Electrons and spins
- Diffusive dynamics
- Phonons and magnons
- Theoretical and experimental methods
- Excitations of strongly correlated systems
- Two-dimensional systems
- Materials under high pressure,
as well as a poster session.

The symposium format implied an informal atmosphere with a limited number of 80 participants, and the scientific exchange was promoted by the renouncement of parallel sessions and the generous schedule. The conference programme was complemented by an optional visit of the research neutron source Heinz Maier-Leibnitz (FRM II) or a guided tour around  the Freisinger Domberg.

The next installment of the DyProSo series will take place in 2017 and will be organized by Prof. Piotr Zielinski, Institute of Nuclear Physics, Polish Academy of Sciences, Cracow.
We look forward to seeing you there!

On behalf of the International Advisory Board

Winfried Petry (Conference chairman)

Sponsors

 

Poster 20150212_DyProSo_Poster.pdf

Sunday 13 September

17:00 → 19:00 Registration

19:00 → 20:00 Welcome Rezeption

Monday 14 September

08:30 → 10:40 Amorphous and Soft Matter

08:30 Role of Disorder in the Thermodynamics and Atomic Dynamics of Glasses

The heat capacity of glasses at temperatures of about ~10 K for a long time was considered to be anomalously higher than that of the corresponding crystals. The related excess of the low-energy vibrational states, the so-called 'boson' peak, was similarly considered to be an anomaly distinguishing glasses from crystals and related to their disordered state. Recent results [1] reveal that (i) the difference in the discussed properties occurs not because the glass is structurally disordered, but because it usually has lower density than that of the corresponding crystal, (ii) the heat capacity of glasses and crystals with same densities is quite similar, and (iii) the boson peak is the glassy counterpart of the van Hove singularity of the corresponding crystal. We analyze the generality of the new results and discuss the compatibility of the suggested interpretation of the boson peak with available experimental data. Analyzing the relation of the new results to various theoretical models, we discuss a possible experimental approach to explore further the nature of the low-frequency vibrational excitations in glasses [2]. [1] A.I.Chumakov, G.Monaco, A.Fontana, et al, Phys. Rev. Lett. 112 (2014) 025502. [2] A.I.Chumakov and G.Monaco, J. Non-Cryst. Solids 407 (2015) 126.

Speaker: Dr Aleksandr Chumakov (European Synchrotron Radiation Facility)

09:10 Boson peak in two liquid crystal glass-formers – results of the neutron scattering

The inelastic neutron scattering spectra were measured for two liquid crystal chiral glass-formers: (S)-4-(2-methylbutyl)-4'-cyanobiphenyl (5*CB) and (S)-4-(1-methylheptyloxy)-4’-cyano-biphenyl (8*OCB) in the temperature range from 4 to 30 K. The experiments were performed for phase I, glass of phase II and glass of cholesteric phase for 5*CB and phase II, glass of phase I and glass of isotropic liquid for 8*OCB. Boson peak was observed for phase I and glass of phase II for 5*CB and for glass of isotropic liquid for 8*OCB (Fig. 1). For other experimental runs the boson peak was not observed or was of much less intensity. For all runs the broadening of the elastic peak, characteristic for conformational jumps or reorientation of molecular groups, was observed. The estimated correlation time of these motions was of the order of picoseconds. The tunnel splitting suggested for temperatures lower than 1 K by relaxation calorimetry experiment was not observed.

Speaker: Dr Jan Krawczyk (Institute of Nuclear Physics PAN)

09:35 Light-Controlled Topological Charge in a Nematic Liquid Crystal

Creating, imaging, and transforming the topological charge in a superconductor, a superfluid, a system of cold atoms, or a soft ferromagnet is a difficult—if not impossible—task, because of the hortness of the length-scales and lack of control. The length scale and softness of defects in liquid crystals allow for the easy observation of charges, but it is difficult to control charge creation. Recently, we demonstrated [1,2] full control over the creation, manipulation and analysis of topological charges that are pinned to a microfibre in a nematic liquid crystal. Oppositely charged pairs are created via the Kibble-Zurek mechanism by applying a laser-induced local temperature quench in the presence of symmetry-breaking boundaries. The pairs are long-lived, oppositely charged rings or points that either attract and annihilate, or form a long-lived, charge-neutral loop made of two segments with a fractional topological charge. This indicates the sensitivity of the Kibble-Zurek mechanism and the coarsening dynamics of entangled defects at late times to the connectedness of space and symmetry-breaking boundary conditions which might have implications also on the cosmological level. Furthermore, we show that any even number of topological charges could be deliberately created on topologically simple objects, which opens new routes to the design and assembly of topologically complex colloidal structures. [1] M. Nikkhou, M. Škarabot, S. Čopar, M. Ravnik, S. Žumer and I. Muševič, Nature Physics 11, 183 (2015), doi:10.1038/nphys3194. [2] M. Nikkhou, M. Škarabot, and I. Muševič, Eur. Phys. J. E 38: 23 (2015), doi:10.1140/epje/i2015-15023-6.

Speaker: Prof. Igor Musevic (J. Stefan Institute)

10:15 Immense elastic softening and the responsible molecular mechanisms near the demixing phase transition of thermo-responsive polymer solutions

Functional materials based on stimuli-responsive polymers are of great interest in view of their current use, and foreseen future implementation, in many everyday life applications. Frequent applications are sensors, drug delivery systems, soft robotics and responsive surfaces. The environmental responsiveness of such hydrogels, thin films or micellar solutions bases on a structural instability, which occurs at a demixing transition with a lower critical solution temperature (LCST). Even though numerous theoretical and experimental studies have been carried out, central problems regarding the phase separation mechanisms remain unsolved. Using aqueous solutions of the model thermo-responsive polymer poly(N-isopropyl acrylamide) (PNIPAM), we identify the volume expansion coefficient and the isothermal compressibility as order parameter susceptibilities of the demixing transition [1-3]. An intriguing finding based on Brillouin spectroscopy is that immense strain-softening, and hence a third order elastic constant, governs the phase separation, instead of the linear elastic properties [2]. In order to better assess the relationship with the underlying molecular mechanisms, we also focus on the molecular origins of the demixing transition [1, 4]. Important variations in hydrophobic and H-bond interactions occur within the phase-separating polymer solutions. Our studies based on quasi-elastic neutron scattering show how the local diffusion behavior of the hydration water varies during the partial dehydration of the PNIPAM chains in the course of the demixing transition [4]. Novel insights into the impact of the molecular processes on the macroscopic order parameter susceptibilities of the phase separation of stimuli-responsive polymer systems are finally given. References: [1] M. Philipp, et al, Soft Matter, 2012, 8, 11387. [2] M. Philipp, et al, Soft Matter 2013, 9, 5034. [3] M. Philipp, et al, Soft Matter 2013, 9, 9887. [4] M. Philipp, et al, The Journal of Physical Chemistry B 2014, 118, 4253.

Speaker: Dr Martine Philipp (TU München)

10:40 → 11:10 Coffee break

11:10 → 12:15 Multiferroics and ferroelectrics

11:10 Néel-type Skyrmion Lattice in a Polar Magnetic Semiconductor

Following the early prediction of the skyrmion lattice (SkL)―a periodic array of spin vortices―by Bogdanov and coworkers [1], it has been observed recently in various magnetic crystals mostly with chiral structure [2]. Although non-chiral but polar crystals with Cnv symmetry were identified as ideal SkL hosts in pioneering theoretical studies, this archetype of SkL has remained experimentally unexplored. Here, we report the discovery of a SkL in the polar magnetic semiconductor GaV4S8 with rhombohedral (C3v) symmetry and easy axis anisotropy [3]. The SkL exists over an unusually broad temperature range compared with other bulk crystals and the orientation of the vortices is not controlled by the external magnetic field but instead confined to the magnetic easy axis. Supporting theory attributes these unique features to a new Néel-type of SkL describable as a superposition of spin cycloids in contrast to the Bloch-type SkL in chiral magnets described in terms of spin helices. We found that the strong orientational confinement of the vortices ensures the robustness of two distinct skyrmionic states with a core magnetization pointing either up or down the easy axis. This may facilitate a unique magnetic control of the SkL by magnetic fields applied perpendicular to the vortex cores in contrast to cubic helimagnets where vortex cores instantaneously co-align with the magnetic field. In addition, the polar crystal structure of GaV4S8 may be exploited for a non-dissipative electric field control of the SkL. [1] A. N. Bogdanov and A. Hubert, J. Magn. Magn. Mater. 138, 255 (1994). [2] S. Mühlbaueret al., Science 323, 915 (2009). [3] I. Kézsmárki et al., http://arxiv.org/abs/1502.08049

Speaker: Prof. István Kézsmárki (Department of Physics, Budapest University of Technology and Economics)

11:50 Magnetic and orbital excitations in the multiferroic skyrmion host GaV4S8

The lacunar spinel GaV4S8 undergoes orbital ordering at 44 K and reveals a complex magnetic phase diagram below 13 K, including a ferromagnetic, cycloidal and Néel-type skyrmion lattice phase.[1] Skyrmions are topologically protected nano-scale spin vortices with fascinating physical properties and high potential for future data storage. Based on magnetic susceptibility, heat capacity and pyrocurrent measurements, all as function of temperature and magnetic field, we construct a detailed phase diagram and in addition, we provide a thorough study of the polar properties of GaV4S8, revealing that its orbitally ordered phase is ferroelectric with sizable polarization of ~ 1 μC/cm2. Moreover, spin-driven excess polarizations emerge in all magnetic phases; hence, GaV4S8 hosts three different multiferroic phases including the skyrmion lattice formed by ferroelectric spin vortices.[2] By taking into account the crystal symmetry and spin patterns of the magnetically ordered phases, exchange striction is identified as the main microscopic mechanism behind the spin-driven ferroelectric polarization in each multiferroic phase. The polar crystal structure of GaV4S8 is unique among the known skyrmion-lattice host materials and the ferroelectric SkL phase may be exploited for non-dissipative electric-field control of skyrmions. In the second part of this talk we present detailed results using THz and coplanar waveguide (CPW) absorption spectroscopy. We find an intriguing relaxation dynamics in the THz range indicating the divergence of relaxation times coupled to the orbital dynamics and establishing an orbitally driven ferroelectric phase below the Jahn-Teller transition. In addition, using CPW absorption spectroscopy we study magnetic excitations of the skyrmion, the helical and the collinear spin phases.[3] [1] I. Kézsmárki et al., arXiv: 1502.08049, unpublished, (2015) [2] E. Ruff et al., arXiv: 1504.00309, unpublished, (2015) [3] D. Ehlers et al., unpublished, (2015)

Speaker: Prof. Alois Loidl (University of Augsburg)

12:15 → 13:30 Lunch

13:30 → 15:00 Multiferroics and ferroelectrics

13:30 Flexoelectricity from density-functional perturbation theory

Flexoelectricity describes the electric polarization that is linearly induced by a strain gradient, and is being intensely investigated as a tantalizing new route to converting mechanical stimulation into electrical signals and vice versa. While several breakthrough experiments have been reported in the past few years, progress on the theoretical front has been comparatively slow. The main difficulty with calculating the flexoelectric response of a material is the inherent breakdown of translational periodicity that a strain gradient entails, which at first sight questions the very applicability of traditional plane-wave pseudopotential methods. In this talk I will show how these obstacles can be overcome by combining density-functional perturbation theory with generalized coordinate transformations [1,2], gaining access to the full microscopic response (in terms of electronic charge density, polarization and atomic displacements) of a crystal or nanostructure to an arbitrary deformation field. As a practical demonstration, I will present results on the full flexoelectric response of SrTiO3, including atomic relaxations and surface effects. [3] I will show that, upon bending a SrTiO3 slab, one obtains a positive voltage if the crystal lattice is terminated by a TiO2 layer, a negative voltage if the termination is of the SrO type. This points to a dramatic dependence of the flexoelectric effect on the details of the surface: an atomically thin termination layer can affect the magnitude, and even the sign, of the response of a macroscopically thick object. [1] M. Stengel, Phys. Rev. B 88, 174106 (2013). [2] M. Stengel, Nature Communications 4, 2693 (2013). [3] M. Stengel, Phys. Rev. B 90, 201112(R) (2014).

Speaker: Dr Massimiliano Stengel (ICMAB-CSIC)

14:10 High resolution polarized Raman scattering study of multiferroic MnWO4

Multiferroic materials with magnetic and electric ordering within a single phase gained attention during the last years due to their potential for industrial applications. Materials with a cycloidal spin arrangement are of particular interest because the electrical polarisation is directly caused by the complex magnetic order. Hence, both ordering phenomena are tightly coupled in this category of compounds. One example is MnWO4, which passes successively three antiferromagnetic phase transitions on cooling. The phase AF2 between 12.5 K and 7 K exhibits a cycloidal spin arrangement with an incommensurate magnetic propagation vector. This ordering induces a spontaneous polarisation by the inverse Dzyaloshinski-Moriya-interaction. As a consequence, the interaction between spin and polarisation could possibly lead to a change in the spectrum of lattice vibrations. Due to the tiny polarization, it was believed up to now that this effect was too small to be observed experimentally. Using high resolution polarized Raman scattering, however, we were able to detect clear signatures of this magnetoelectric interaction. The Raman spectra were taken from a MnWO4 single crystal in the temperature range from 100 K to 6 K covering the three magnetic phase transitions. A careful analysis of the experimental data allowed the precise determination of both, the wave numbers and intrinsic linewidths. Most of the 18 Raman-active phonons show a significant shift in wave number below the magnetic ordering temperature that cannot be explained merely on the basis of lattice dynamics, but should be attributed to spin-phonon-interaction. The most striking effect is observed for the Ag mode with the highest frequency (about 884 cm-1), which corresponds to an oxygen stretching vibration and exhibits a pronounced softening on cooling.

Speakers: Mr Fabian Ziegler (Georg-August-Universität Göttingen), Dr Holger Gibhardt (Georg-August-Universität Göttingen)

14:35 Magnetic and magnetoelectric excitations in hexagonal multiferroics RMnO3 probed by neutron scattering and THz spectroscopy

Broadband THz spectroscopy is a very useful tool to study complex magnetic/electric order in condensed matter i.e. in multiferroic compounds since both magnetic (magnons) and electric (optical phonons) excitations lie in this energy range. When cross coupling between magnetic and electric order occurs, new kind of excitations may emerge: these are called electro-magnons. Coupling two complementary experimental techniques, THz/FIR spectroscopy on synchrotron source and inelastic neutron scattering, we have focused on two members of the multiferroics hexagonal manganites RMnO3, with R=Er and Ho. These compounds order electrically below 800 K and magnetically around 80 K with a 120° frustrated arrangement of the Mn3+ ions. We have fully characterized the low energy spectra (magnon, phonon, crystal field transitions) of these compounds and their excitation rules as regards the electric and magnetic fields of the THz wave. In ErMnO3, we have observed the complete loss of the magnetic character of a magnon transmuted into an electroactive excitation [Chaix, et al. Phys. Rev. Lett. 112, 137201 (2014)]. We attribute this magnetoelectric dynamical process to the hybridization between a crystal field level transition of the Er magnetic rare earth and a Mn magnon. In HoMnO3, spectacular modifications of the Mn spin waves and Ho crystal field level transitions are observed at a temperature of 40 K when a spin reorientation of the Mn3+ magnetic moments occurs, together with the ordering of some Ho ions. At lower temperature, the spin waves dispersion perpendicular to the Mn triangular planes vanish, the Mn ordered structure being maintained in the molecular field of the rare earth ions. Both studies highlight the crucial role of the strong coupling between Mn and rare earth ions in the dynamical properties of these hexagonal manganites.

Speaker: Dr Sophie DE BRION (Institut Néel)

15:00 → 15:30 Coffee break

15:30 → 15:55 Multiferroics and ferroelectrics

15:30 Probing ferroelectricity and magnetoelectric effect in RMnO3 (R = Tb, Dy) by Fe3+ B-site substitution

Multiferroics, where spontaneous long-range magnetic and polar orderings coexist, represent an attractive class of compounds combining rich physics with potential for multifunctional applications. TbMnO3 and DyMnO3 are multiferroics compounds exhibiting magnetic ordering of Mn3+ ions, with a sinusoidal modulated collinear structure below TN = 41K and 39K, and a cycloidal one below Tlock = 27K and 18K, respectively, which is accompanied by the emergence of spontaneous electric polarization [1,2], accordingly to the Dzyaloshinskii-Moriya model [3]. Usually, the magnetoelectric coupling has rather small magnitude. However, larger magnetoelectric effect can be found in frustrated magnetic materials. In rare-earth perovskite manganites, the magnetic frustration can be induced by modifications of exchange interactions among nearest and next-nearest neighbors of Mn3+. To tune the balance between these ferro and antiferromagnetic interactions, we have studied the effect of Mn3+ substitution by Fe3+ on selected physical properties of TbMn1-xFexO3, with x = 0 to 0.05, and DyMn1-yFeyO3, with y = 0 to 0.03, since Fe3+ has the same ionic radius as Mn3+ but it is not Jahn-Teller active. This substitution induces small structural distortions and changes the magnetic interactions, which play an important role on the magnetoelectric properties. We have found that already at x = 0.05 and y = 0.03 the ferroelectricity is lost. Interestingly though, within this range, as x/y increases there is a strong increase of the magnetoelectric effect. The polarization becomes so sensible to the magnetic field, that for the highest x/y, it can be almost suppressed. The magnetoelectric (x/y, T) phase diagrams are proposed. 1 N Aliouane et al 2008 J. Phys.: Condens. Matter 20 434215 2 T Kimura and Y Tokura 2008 J. Phys.: Condens. Matter 20 434204 3 Cheong SW and Mostovoy M 2007 Nature Materials Jan 6(1):13-20

Speaker: Mr Rui Vilarinho Silva (IFIMUP-IN, Faculty of Sciences, University of Porto, Portugal)

15:55 → 17:40 Electrons and spins

15:55 Slow dynamics in magnetic pyrochlore oxide systems

Geometrically frustrated magnetic systems provide a large variety of unusual magnetic ground states. Among these, the pyrochlore oxide compounds (formula A2B2O7, where A is a magnetic rare-earth, and B a transition metal) have focused much attention, because their lattice, made of corner-sharing tetrahedra, is a source of strong magnetic frustration. It results in the stabilization of exotic magnetic ground states, such as classical or quantum spin-ices, spin-liquids or unconventional magnetic orders combined with spin fluctuations. I will present a review of magnetization and ac susceptibility studies on these compounds which evidence the existence of slow dynamics at very low temperature. These dynamics coexist with fast fluctuations and / or magnetic ordering. They can be the signature of emergent excitations, such as magnetic monopoles in classical spin-ices (A=Ho, Dy) but in other systems (A=Tb, Er, Nd…), their origin remains an open issue. They could be due to loop dynamics, domain-wall dynamics, as well as induced by the presence of structural defects.

Speaker: Dr Elsa LHOTEL (Institut Néel CNRS)

16:35 Probing the Dynamical Properties of Nanoscale Solid-State Systems in Real Space and Real Time

One of the biggest dreams in solid-state physics is getting quantitative access to the dynamical properties of tiny-tiny systems in real time and real space. Any technique that averages over an ensemble behavior thus inherently cannot be used and is out of scope here. Equally, indirect k-space methods will fail at these small length scales as well, due to the same limitations. In order to fulfill the above requests, we recently developed scanning-probe-methods (SPM) towards both a superb spatial and temporal resolution. The high spatial resolution is inherently known in SPM from tons of splendid works performed on metallic, semiconducting, insulating, organic and biological nanomaterials, reporting a topographic resolution down to the single atomic and molecular level. What is novel, though, is that these SPM methods can be easily tuned towards an unprecedented time resolution down to pico-seconds. We will introduce into these novel techniques with two examples of solid-state measure­ments. The first system addresses ultrafast electronic transport properties as exemplified on a thin film organic field-effect transistor (OFET). To monitor the charge recombination in-situ we modified a standard non-contact scanning force microscope (nc-SFM) operated in the Kelvin force probe mode (KPFM) to measure surface potentials, into a time-resolved KPFM (tr-KPFM) technique [1]. tr-KPFM is able not only to delineate and track the charge wave fronts of injected electrons in real time, but equally proves, why today’s OFET devices fail to achieve faster switching speeds, simply due to Schottky barrier issues. The second example will be discussed by quantifying the excited state lifetime of electrons in an optically pumped SiGe semiconductor device [2]. Here, a similar side-band demodulation scheme as for tr-KPFM is implemented into scattering scanning near-field optical microscopy (tr-s-SNOM). We find the charge carrier lifetimes to drastically depend on their ground state population density, as monitored by varying the Si-to-Ge concentration. References: [1] J. Murawski et al., (2015) submitted. [2] F. Kuschewski et al., Scientific Reports (2015) in print.

Speaker: Prof. Lukas M. Eng (TU Dresden)

17:15 Domain wall dynamics, local diffusion, and phonons in a lead-free relaxor ferroelectric

Ferroelectric materials are used in many applications, e.g. as ultrasonic transducers. The most commonly used ferroelectrics like Pb_xZr_(1-x)TiO_3 contain lead, which makes them potential hazards for human health and our environment. Consequently, lead-free ferroelectrics are currently being developed with the aim of replacing the lead-containing materials in the medium term. However, the microscopic mechanisms which determine the ferroelectric properties have to be identified before the properties can be optimized for specific applications. The ferroelectric parameters of solid solutions near the morphotropic phase boundary of (1-x)Bi_(1/2)Na_(1/2)TiO_3-xBaTiO_3 around x = 0.06 were cited as d_33 = 125 pC/N and ε_33^T / ε_0 = 580. These values are comparable to those of commonly used lead-containing ferroelectrics. The diffuse x-ray scattering experiments performed by our group revealed features related to the local octahedral tilting order and stacking faults between different tilt domains. These features react strongly to the application of an external electric field and their temperature dependence is clearly correlated with the dielectric permittivity. Potential phonon anomalies may also help to identify the mechanisms leading to the good ferroelectric properties. We investigated the temperature dependence of the structural dynamics of a single crystal with x = 0.04 using inelastic and quasielastic neutron scattering. Most phonon branches are not well defined, but hints of possible anomalies are visible. The quasielastic scattering (QENS) was measured at different positions in reciprocal space, focusing on the field-dependent diffuse scattering. We observed a very strong temperature and Q dependence. This indicates that different mechanisms like enhanced domain wall mobility and local cation hopping strongly influence the dielectric properties in different temperature ranges.

Speaker: Mr Florian Pforr (Technische Universität Darmstadt)

18:00 → 19:00 Dinner

19:00 → 21:00 Poster session w/ wine/beer

19:00 7: Ab initio study of the strain-mode coupling in SrBi2Nb2O9

The Aurivillius compound SrBi2Nb2O9 (SBN) has commonly been considered as a potential candidate for nonvolatile memories, and as a consequence, it has been thoroughly investigated both experimentally [1-3] and computationally [4,5]. The phase diagram of SBN is governed by three relevant distortions, and it has been shown that the trilinear coupling among them stabilizes the ferroelectric ground state [4-6]. An analogous critical influence of the trilinear coupling is also observed in the isomorphous SrBi2Ta2O9 (SBT), where slight differences in the mode interplay induce the presence of an intermediate non-polar phase [3,5,6]. In this work we evaluate by ab initio calculations the couplings of the two components of the strain tensor that do not break the parent tetragonal symmetry with the three relevant symmetry adapted modes. We find that the three modes show significant couplings with strain. Elastic enthalpy is also calculated and we find that a particular combination of applied stress and misfit strain can be used to produce an enhancement of the spontaneous polarization as obtained in other materials [7]. 1. A. D. Rae, J. G. Thompson and R. L. Withers. Acta Cryst. B, 48, 418 (1992). 2. Ismunandar, B.J. Kennedy, Gunawan and Marsongkohadi, Jour. Solid State Chem. 126(1), 135 (1996). 3. P. Boullay, J. Tellier, D. Mercurio, M. Manier, F.J. Zúniga and J.M. Perez-Mato, Solid State Sci. 14(9) 1367 (2012). 4. J. M. Perez-Mato, M. Aroyo, Alberto García, P. Blaha, K. Schwarz, J. Schweifer and K. Parlinski, Phys. Rev. B 70, 214111 (2004). 5. Urko Petralanda and I. Etxebarria. In press. 6. I. Etxebarria, J. M. Perez-Mato, and P. Boullay, Ferroelectrics 401, 17 (2010). 7. O. Dieguez, K. M. Rabe and D. Vanderbilt, Phys. Rev. B 72, 144101 (2005).

Speaker: Dr Iñigo Etxebarria (Fisika Aplikatua II Saila. University of the Basque Country)

19:00 5: Ab initio vibrational properties of the MXene materials functionalized by fluorine, oxygen and hydroxyl group

A new family of 2D-like nano-sized binary and ternary transition metal carbides (known also as MXenes) has received significant scientific interest due to their intriguing functionalities and technological applications. Physical and chemical properties of these material can be triggered by terminating their surfaces by various adatoms or molecules. This contribution presents first-principle studies of the influence of fluorine and oxygen adatoms as well as the surface terminating OH group on the lattice dynamics of pristine Ti2C monolayers.

Speaker: Mrs Magdalena Wąsik (Institute of Technology, Pedagogical University of Cracow, 30-084 Cracow, Podchorazych 2)

19:00 11: Analytic approach to anharmonic modes of lattice vibrations.

The classical theory of lattice dynamics deals with harmonic crystals. While this theory is very effective with describing large number of physical phenomena connected with lattice vibrations, it leaves out an important aspect of the lattice dynamics: anharmonic effects. Without this component many important physical phenomena cannot be properly described: thermal expansion, phase transitions, thermal equilibrium, thermal conductivity, multi-phonon processes - to name just a few. Over the years a number of effective methods have been developed to deal with cases where the anharmonicity could be treated as a small perturbation in the harmonic model. In many situations this approach is useful and effective (e.g. Quasi Harmonic Approximation for dealing with thermal expansion). Unfortunately, in cases where forces in the crystal are strongly anharmonic (e.g. phase transitions, phonon modes with strongly anharmonic potentials) this approach is much less effective. In crystals the anharmonic component may take various forms. In many materials (e.g. PbTe, TiO2) one or more of vibrational modes is characterized by a strongly anharmonic potential of the general form of the fourth order polynomial. The equation of motion in such a potential can be solved analytically and the result can be further analysed to obtain experimentally verifiable properties: mode frequency as a function of temperature, thermal displacements, line profile etc. Even in cases where the potential does not allow for analytical solution, the procedure can still be carried out, however with higher computational cost, using numerical integration of the equation of motion. This work presents a computational scheme for this type of calculation and demonstrates the derivation of temperature dependence of phonon frequency in the case of anharmonic mode in rutile titanium dioxide. The calculation is based on the DFT-derived potential for the mode.

Speaker: Dr Pawel T. Jochym (Dept. of Computational Material Science, Institute of Nuclear Physics PAN, Cracow, Poland)

19:00 2: Broadband dielectric response of polyaniline pellets as nanocomposites of metallic emeraldine salt and dielectric base

Recently, we have studied dielectric spectroscopy of variously conducting polyaniline (PANI) pellets in a very broad frequency (10ˇ−2–10ˇ13 Hz) and temperature (10–300 K) range [1]. The DC conductivity varied between ≈10 S/cm for the emeraldine salt and ≈10ˇ–12 S/cm for the deprotonated emeraldine, the PANI base. Mechanism of the conduction consists of polaron transfer along the PANI chains and the reason for such dramatic differences is a result of various degrees of disorder within the chains and their arrangement [2]. Since the fully ordered metallic PANI films with DC conductivity of ≈10ˇ3 S/cm were also reported [3], we attempted here to model the whole dielectric and conductivity spectrum of our emeraldine salt pellets as a nanocomposite of the metallic PANI and our amorphous PANI base. For modelling of the conductivity including the THz and infrared part with vibrational modes we have used the effective medium approach based on Bruggeman and generalised Lichtenecker model [1]. Both models are discussed from the view point of topology and percolation of the conductive fraction and preferences of the latter model are demonstrated. [1] J. Petzelt, D. Nuzhnyy, V. Bovtun, M. Savinov, M. Kempa, I. Rychetsky: Broadband dielectric and conductivity of inhomogeneous and composite conductors, Phys. Stat. Sol. A 210, 2259 (2013). [2] A. B. Kaiser: Electronic transport properties of conducting polymers and carbon nanotubes, Rep. Prog. Phys. 64, 1 (2001). [3] K. Lee, S. Cho, S. H. Park, A. J. Heeger, C.-W. Lee: Metallic transport in polyaniline, Nature 44, 65 (2006).

Speaker: Dr Jan Petzelt (Institute of Physics AS CR, Na Slovance 2, 182 21 Prague 8, Czech Republic)

19:00 4: Directional dichroism at the spin-wave excitations of multiferroic Ni3TeO6

In physical systems simultaneously breaking time-reversal and spatial inversion symmetries the strength of absorption for two counter-propagating light beams can be different irrespective of the polarization state of light [1], which phenomenon is termed as non-reciprocal directional dichroism. Until recent experiments on multiferroic materials [2, 3] this effect was generally found to be weak. Directional dichroism of multiferroics in the far-infrared spectral range is the consequence of the optical magnetoelectric effect, i.e. the coupled dynamics of spins and local electric dipoles[4]. Spin-wave modes in multiferroics can simultaneously be excited by the electric and magnetic components of light, hence, they can be viewed as the “elementary excitations” of such hybrid magnetoelectric response. Indeed multiferroic Ni3TeO6 [5] shows strong directional dichroism in its spin excitations, even for unpolarized light. The temperature- and magnetic field dependence of these resonances was followed up to the Néel temperature and up to 30 T, respectively.

Speaker: David Szaller (Budapest University of Technology and Economics)

19:00 1: Dynamics of water confined in chrysotile asbestos studied by inelastic neutron scattering

We report the new data on the molecular dynamics of water confined in the “channels” of the crystal structure of chrysotile asbestos obtained by neutron spectroscopy. The neutron scattering measurements have been carried out on the time-of-flight spectrometer Sequoia at SNS (Oak Ridge, USA). The energy range up to 600 meV was covered with the instrument conditions selected to emphasize different parts of the full range of water vibration dynamics. The neutron spectra of the dry and wet samples were recorded at the identical conditions and the difference spectra were obtained by subtracting the ‘dry’ spectra from the ‘wet’ ones. Special care was taken to keep the preferred orientation of the fibers (c-axis) in the samples so that the resulting misorientation of the channels does not exceed ±12.5 degrees from a selected direction. The neutron scattering measurements were performed with the two sample geometries: with the channel axes along the neutron wave vector transfer Q in the scattering plane and perpendicular to the scattering plane. This permitted us to track the preferred direction of the hydrogen vibrations corresponding to different spectral ranges: acoustic, librational, molecular frequencies. We have found evidences for particularly strong anisotropy of confined water vibrations in the libration band (50-130 meV) which appears to be split onto 3 peaks. Such sharp peaks different bulk-ice phases were observed so far only in the inelastic neutron scattering spectra of the proton ordered phases (ice-II, ice-VIII and partially ordered ice-VI) while the proton disordered phases exhibit in this range practically featureless spectra, similar to that of ice-Ih. These observations have been confirmed in the measurements of deuterated water in the protonated asbestos what permitted us to move out from the range of strong vibration bands observed in the asbestos itself.

Speaker: Dr Alexandre IVANOV (Institut Laue-Langevin, Grenoble)

19:00 14: Effect of CFO and PZT fillers on Dielectric and Ultrasonic Properties of P(VDF-TrFE) Copolymer Based Composites

Composites are considered to be any multiphase materials that show a combination of properties of their components. PZT, CoFe2O4 and BaTiO3 are one of ingredients that can improve the dielectric properties and the ultrasonic properties of polymer based composites. In this work we report on both the dielectric and the ultrasonic properties of the conventional polymer polyvinylidene fluoride / trifluoroethylene (P(VDF-TrFE)) of the composition 70/30 mol% with various concentrations of (Pb0,75Ba0,24Sr0,01)(Zr0,53Ti0,47)O3 (BPZT) and CoFe2O4 (CFO) fillers. By the means of dielectric spectroscopy it is shown that the dielectric properties may be tuned by varying the volume fraction of the ferroelectric fillers. The dependencies of the dielectric properties of the composites on filler volume fraction are reported and analyzed in terms of an analytical model (Lichtenecker’s effective medium approximation) applying electrodynamic boundary conditions. Experimental study of ultrasonic wave attenuation, velocity and piezoresponse in these composites has been performed over wide temperature range (100 K – 410 K) using ultrasonic automatic pulse-echo technique. The temperature dependences of ultrasonic velocity and attenuation showed anomalies attributed to the glass transition and paraelectric-ferroelectric phase transition. Above Curie temperature Tc the piezoresponse vanishes in beforehand polarized samples.

Speaker: Dr Vytautas Samulionis (Vilnius University)

19:00 16: Effects of 18 O isotope substitution in multiferroic RMnO3 (R=Tb, Dy)

Multiferroic materials demonstrate desirable attributes for next-generation multifunctional devices as they exhibit coexisting ferroelectric and magnetic orders. In type-II multiferroics, coupling exists that allows ferroelectricity to be manipulated via magnetic order and vice versa, offering potential in high-density information storage and sensor applications. Despite extensive investigations into the subject, questions of the physics of magnetoelectric coupling in multiferroics remain, and competing theories propose different mechanisms. The aim of this investigation was to study changes in the statics and dynamics of structural, ferroelectric and magnetic orders with oxygen-18 isotope substitution to shine light into the coupling mechanism in multiferroic RMnO3 (R=Tb, Dy) systems. We have performed Raman spectroscopy on 16O and 18O-substituted TbMnO3 single crystals. Oxygen-18 isotope substitution reduces all phonon frequencies significantly. However, specific heat measurements determine no changes in Mn3+ (28 and 41 K) magnetic phase transition temperatures. Pronounced anomalies in peak position and linewidth at the magnetic and ferroelectric phase transitions. While the anomalies at the sinusoidal magnetic phase transition (41 K) are in accordance to the theory of spin-phonon coupling, further deviations develop upon entering the ferroelectric phase (28 K). Furthermore, neutron diffraction measurements on 16O and 18O-substituted DyMnO3 powders show structural deviations at the ferroelectric phase transition (17 K) in the order of 100 fm. These results indicate that structure is actively involved in the emergence of ferroelectricity in these materials.

Speaker: P.J. Graham (School of Physics, University of New South Wales, Australia)

19:00 3: Elastic constants in multiferroic Bi2Mn4O10

Mullite-type Bi2Mn4O10 [1] is an example for a multiferroic compound with a Néel temperature of 39 K, although its crystal structure [2] is not typical for allowing ferroelectricity. The dipoles of the constituent Mn4+O6 (along c-axis, bond valence sum BVS = 3.90(2) v.u.) and Mn3+O5 (along b-axis, BVS = 3.04(2) v.u.) polyhedra are oriented in different directions [3]. Both its nuclear and magnetic structural features differ from those of other rare-earth members of the R2Mn4O10 family. While each member of R2Mn4O10 family possesses incommensurate magnetic character [4], Bi2Mn4O10 exhibits a commensurate [5] magnetic structure. In order to characterize its lattice dynamical properties we have studied the acoustic phonon branches of a single crystal of Bi2Mn4O10 at room temperature using inelastic neutron scattering (INS). Experiments have been performed at the three-axes spectrometer PUMA@FRM II. The dispersion curves of the acoustic phonon branches in the Brillouin zone have been measured for different propagation directions close to the zone center (Gamma-point) and for different polarization vectors. The linear slopes of the corresponding dispersion curves are related to a specific combination of the elastic constants cij. We were able to determine the complete set of nine elastic constants cij provided by the orthorhombic crystal (Pbam). The results clearly demonstrate the pronounced mechanical anisotropy of Bi2Mn4O10. References [1] H. Schneider, et al., Int. J. Mat. Res. 103 (2012) 422 - 429. [2] M. Burianek, et al., Int. J. Mat. Res. 103 (2012) 449 - 455. [3] L. M. Volkova, D.V. Marinin, J. Phys.: Condens. Matter 21 (2009) 015903. [4] S. Petit, et al., Phys. Rev. B 87 (2013) 140301. [5] C. Vecchini, et al., Phys. Rev. B 77 (2008) 134434.

Speakers: Mr Fabian Ziegler (Georg-August-Universität Göttingen), Prof. Götz Eckold (Georg-August-Universität Göttingen), Dr Holger Gibhardt (Georg-August-Universität Göttingen), Dr Oleg Sobolev (Georg-August-Universität Göttingen)

19:00 15: Electric control of the unidirectional transmission in the ferrotoroidic LiCoPO4

Multiferroics allow the magnetic control of electric polarization and the electric control of magnetization via the static magnetoelectric effect (ME). Dynamic or optical ME effects gained high interest because they give rise to unidirectional transmission - as recently observed in low-temperature multiferroics. This phenomenon allows the development of optical diodes, which transmit unpolarized light in one, but not in the opposite direction. Ferroelectric polarization and spontaneous magnetization in multiferroics can be exploited to switch the transmission direction with either magnetic or electric field as shown in Fig. 1. In prior optical directional effects in solids were only investigated with magnetic field, but were never tested in the presence of external electric field before. LiCoPO4 is a well known multiferroic material with an exotic, recently discovered degree of freedom known as torroidal moment [4]. Neutron diffraction and magnetization studies have revealed complex magnetic phases in both compounds pointing to the existence of several competing energy scales. The exotic magnetic interactions and the possible finite macroscopic torroidal moment of this material promote it to an ideal candidate for novel optical magnetoelectric experiments. By means of far-infrared absorption measurements we have revealed the existence of unidirectional transmission which could be controlled by either magnetic or electric fields (Fig.1). The samples were cooled down to the antiferromagnetically ordered phase in crossed electric and magnetic fields and the measurements were performed in the presence of these static fields.

Speaker: Mr Vilmos Kocsis (Department of Physics,Budapest University of Technology and Economics)

19:00 13: EPR Spectroscopy of Manganese-Doped Perovskite-Type Metal-Organic Framework

Recently, a new type of porous materials called coordination polymers or metal-organic frameworks (MOFs) emerged and attracted attention of the scientific community. These crystalline compounds are unique due to the highly porous structures which can be utilized for gas adsorption related applications. Some of MOF materials contain paramagnetic transition-metal ions, resulting in peculiar magnetic properties of these compounds. In addition, the organic part in some coordination polymers consists of polar molecules, which below a certain phase transition temperature order into a ferroelectric phase. Lately, a promising MOF [(CH3)2NH2][Zn(HCOO)3] with perovskite-type architecture and inherent ferroelectricity was synthesized. It is believed that the ferroelectric phase in this material is due to the ordering of (CH3)2NH2+ ions, but, however, the precise phase transition mechanism is still obscure. In this work we investigate the [(CH3)2NH2][Zn(HCOO)3] MOF doped with 0.05 mol% paramagnetic Mn2+ ions using the continuous-wave (CW) and pulsed EPR methods. The temperature dependent X-band CW and field-sweep as well as Q-band CW EPR spectra reveal that the local Mn2+ ion-probes are indeed sensitive to the local structural changes occurring at the phase transition point. Spectral simulations were used to obtain the g, hyperfine A and fine-structure D tensors and temperature dependence of their components allowing to further characterize the observed phase transition and the MOF structure. Following the temperature dependence of the axial zero-field splitting parameter D, it was concluded that the phase transition into the ferroelectric phase is of the first order.

Speaker: Mr Mantas Simenas (Faculty of Physics, Vilnius University, Sauletekio av. 9, Vilnius, Lithuania LT-10222)

19:00 6: Evolution of the helimagnon dispersion

We present measurements of the magnetic field-dependency of the helimagnetic band structure in MnSi. For low fields the helimagnons were previously discovered and mapped out by Janoschek et al. (Phys. Rev. B, 81(21), 214436, 2010) and Kugler et al. (arXiv:1502.06977, 2015), respectively. The high-field ferromagnetic dispersion, on the other hand, has been well known since the 1970s when it was measured by Ishikawa et al. (Phys. Rev. B, 16(11), 4956, 1977) Preliminary measurements we performed on the triple-axis spectrometer MACS (at NIST) suggest that the helimagnetic bands can still be seen even in the ferromagnetic regime (we used fields up to 1 T). This would still be in agreement with Ishikawa as they used thermal neutrons and thus had such a large resolution volume as to not being able to discern individual bands. Our measurements are currently being continued at the instruments MIRA (at MLZ) and later at TASP (at PSI). So far at MIRA we could see the band structure collapse into a single-magnon dispersion for very high fields of a few tesla. For fields below one tesla we saw a shifting in energy of mainly the first band. Here, the spectrum appears as a single non-symmetric peak as the first band moves towards the position of the other bands.

Speaker: Tobias Weber

19:00 10: Guided surface waves on interfaces of media with positive and negative Poisson’s ratio

Surfaces and interfaces are waveguides for some kinds of waves. The Stoneley wave occurs on the interface of perfectly bonded isotropic elastic materials [1]. The kind of waves are of importance e.g in geoscience [2] and in the design of delay lines in acoustical waveguides [3]. Whereas the surface Rayleigh wave exists on all free surfaces of elastic media, the range of existence of the Stoneley waves is rather narrow. It will be shown how the use of auxetics, i.e. the materials with negative Poisson’s ratio [4] enlarges this region. Qualitatively new guided waves arise if the media are separated by an interlayer. The case of a thin membrane will be discussed with an emphasis on long wavelength cut-offs. The systems under consideration admit a number of evanescent waves known as surface resonances or surface leaky waves [5]. [1] R. Stoneley, Proc. Royal Society of London. Series A, 106, 416 (1924) [2] D. Rauch, Marine Science, 16, 623 (1986) [3] V.R. Velasco, Physical Status Solidi A, 60, K61, (1980) [4] R.S. Lakes, Science 235, 1038 (1987) [5] P. Sobieszczyk, M. Gałązka, P. Zieliński, Phase Transitions, 87, 1018 (2014)

Speaker: Mrs Dominika Kuźma (The H. Niewodniczański Institute of Nuclear Physics Polish Academy of Sciences)

19:00 12: Low-energy lattice dynamics of relaxor-like PFN-38%PT by inelastic neutron scattering

Piezoelectric crystals of the Pb(Fe1/2Nb1/2)1-xTixO3 (PFN-xPT) system have drawn much interest in recent years, due to relatively high Curie temperatures and potential multiferroic properties. By substitution of Fe3+/Nb5+ by Ti4+, the magnetic properties are suppressed: the Néel temperature TN decreases rapidly with the Ti content and the Curie temperature TC increases almost linearly. Moreover, at about x ~ 0.12, PFN-xPT possesses a morphotropic phase boundary (MPB) between the monoclinic and tetragonal ferroelectric phases. Recently, a polarized Raman study [1] of the PFN-38%PT single crystal reported significant crystalline anisotropy similar to that of tetragonal PbTiO3. In this contribution, we will present our inelastic-neutron-scattering results of lattice dynamics of the PFN-38%PT single crystal in the cubic and tetragonal phases, mainly with respect to the temperature behaviour of the TO soft mode around the phase transition, and the TA-TO coupled-mode analysis in different Brillouin zones and directions. Further the comparison with the end-members of the PFN-xPT series, pure PbTiO3 [2-5] and Pb(Fe1/2Nb1/2) [6], will be discussed. [1] I. Rafalovskyi, I. Gregora, H. Luo and J. Hlinka, Phase Trans. 87, 1080 (2014). [2] G. Shirane, J. D. Axe, J. Harada and J. P. Remeika, Phys. Rev. B 2, 155 (1970). [3] J. Hlinka, M. Kempa, J. Kulda, P. Bourges, A. Kania and J. Petzelt, Phys. Rev. B 73, 140101 (2006). [4] M. Kempa, J. Hlinka, J. Kulda, P. Bourges, A. Kania and J. Petzelt, Phase Trans. 79, 351 (2006). [5] I. Tomeno, Y. Ishii, Y. Tsunoda and K. Oka, Phys. Rev. B 73, 064116 (2006). [6] C. Stock, S. R. Dunsiger, R. A. Mole, X. Li and H. Luo, Phys. Rev. B 88, 094105 (2013).

Speaker: Dr Martin Kempa (Institute of Physics ASCR, Prague)

19:00 8: Order by disorder or energetic selection of the ground state in the XY pyrochlore antiferromagnet Er2Ti2O7 ? A neutron scattering study.

Examples of materials where an ``order by quantum disorder" mechanism is at play to select a particular ground state are scarce [1,2]. It has been recently proposed that the anti-ferromagnetic pyrochlore Er2Ti2O7 reveals a most convincing case of this mechanism [3,4,5]. Observation of a spin gap at zone centers was interpreted as a definitive proof of this physics [6]. We argue, however, that the magnetic anisotropy provided by the interaction-induced admixing between the CEF ground and excited levels gives an alternative energetic mechanism [7,8]. RPA calculations based on a mean field model taking into account explicitly the CEF anisotropy reproduce well new high resolution inelastic neutron scattering data. Here, the gap originates from the anisotropy rather than quantum fluctuations effects. The present study raises the question of the quantum order by disorder as the sole or even principal mechanism for the selection of the magnetic ground state in this material. [1] C. Lacroix, Introduction to Frustrated Magnetism (Springer-Verlag, Berlin, 2011). [2] J. Villain, R. Bidaux, J.-P. Carton, R. Conte, J. Phys 41, 1263 (1980). [3] J. D. M. Champion et al, Phys. Rev. B, 68, 020401 (R), (2003) [4] M. E. Zhitomirsky et al, Phys. Rev. Lett. 109, 077204 (2012). [5] L. Savary et al, Phys. Rev. Lett. 109, 167201 (2012). [6] K. A. Ross et al, Phys. Rev. Lett. 112 057201 (2014). [7] S. Petit et al, Phys Rev B 90, 060410(R) (2014) [8] P. A. McClarty et al, Journal of Physics: Conference Series 145, 012032 (2009).

Speaker: Dr SYLVAIN PETIT (CEA)

19:00 9: Reflection, refraction, mode conversion and guided waves on surfaces and interfaces of materials with all allowed Poisson ratios

When encountering a mismatch of characteristic impedance a bulk acoustic wave transforms into up to three reflected and refracted waves of different polarizations. The effect is known as mode conversion [1]. The lack of the specularly reflected wave is called total mode conversion because then all the outgoing waves propagate at speeds different than that of the incident one. Conversely, if the only outgoing wave reflects in the specular way one speaks of no-conversion. Discovery of materials with negative Poisson’s ratio [2] enlarged the range of possible impedance mismatch. The conditions for the total mode conversion, for no-conversion and for evanescent partial waves will be presented for half-space elastic media and for interfaces between two different elastic media also separated by a thin membrane. Some frequencies corresponding to these phenomena turn out to coincide with apparently spurious roots of the secular determinant giving, in principle, the frequencies of the surface or interface waves [3,4]. These results will be compared with the anomalies of local densities of states (LDOS). Of particular interest are maxima of and minima of LDOS corresponding to surface resonances and surface antiresonances respectively. Some sharp surface resonances mark the total mode conversion of bulk waves and broader resonances an analogous mode conversion of evanescent waves. The effects of curved surfaces will be also summarized [5]. [1] J. Miklowitz, "The Theory of Elastic Waves and Waveguides", Elsevier North-Holland Inc., New York, 1978, Chapter 3. [2] R.S. Lakes, Science 235, 1038 (1987) [3] R. E. Camley and F. Nizzoli, J. Phys. C: Solid State Phys., 18, 4795 (1985) [4] R. Stoneley, Proc. Royal Society of London. Series A, vol. 106, no. 738, pp. 416–428, 1924. [5] P. Sobieszczyk, M. Gałązka, P. Zieliński, Phase Transitions, 87(10-11), (2014)

Speaker: Mr Paweł Sobieszczyk (Institute of Nuclear Physics PAN)

Tuesday 15 September

08:30 → 10:25 Diffusive dynamics

08:30 Metal borohydrides: from hydrogen storage to electrochemistry

Metal borohydrides (M-BH4) have focused research attention due to large hydrogen content. Extensive basic research efforts were directed toward understanding their dynamical and thermodynamic properties. Unfortunately, till now none of such compounds meets practical requirements for reversible hydrogen storage. The unexpected side effect of such research was discovery of the high ion mobility in pure or ion substituted LiBH4. Even more surprising was the discovery of high sodium mobility in decomposition products of NaBH4 containing large B12H12 anions. All these effects are related to the dynamical properties of systems with small non-spherical ions that will be presented form theoretical perspective during the talk.

Speaker: Prof. Zbigniew Lodziana (Department of structural Research, Instutute of Nuclear Physics)

09:10 Heterogeneous dynamics in a Au-Si liquid investigated with quasielastic neutron scattering

We report on the atomic dynamics in a glass-forming Au81Si19 liquid using quasielastic neutron scattering. At low q-values and high temperatures the self-diffusion of gold is well approximated in the hydrodynamic approach with a simple exponential describing the final decay of the self-correlation function. With increasing q, we observe a systematic stretching of the correlation functions, indicating the existence of dynamical heterogeneities with decreasing observation length scale even at high temperatures in the equilibrium liquid. A dynamic crossover from the hydrodynamic regime to that of heterogeneous dynamics was observed to occur close to the liquidus temperature. There, a pronounced stretching of the correlation functions over the entire investigated q-range is apparent, which suggests the onset of structural arrest. Despite the glass-forming nature of this system, the self-diffusivity obeys an Arrhenius law characteristic of simple liquids, without any apparent signature of a dynamic singularity in the investigated temperature range. This is most likely due to the very fragile nature of this system, in which the microscopic dynamics exhibits characteristics of both simple and glass-forming liquids.

Speaker: Dr Zach Evenson (Institut für Materialphysik im Weltraum, Deutsches Zentrum für Luft- und Raumfahrt, Köln)

09:35 Shining light on atomic hopping processes - diffusion mechanisms in B2 alloys

While diffusion is well understood for pure metals and for solid solutions, diffusion mechanisms in ordered intermetallic alloys are still under debate. Even for the simple case of B2 alloys, very little is known. As nearest-neighbour jumps would destroy the long-range order, effective jumps to further neighbouring shells have to be considered. Such jumps can either take place directly, where the atom has to overcome a large energy barrier, or as a fast sequence of nearest-neighbour jumps. The later can be described by different jump mechanisms among which the six-jump mechanism and the triple defect mechanism are most famous. We use coherent X-rays to find wave-vector dependent relaxation times in different systems. By extending X-ray Photon Correlation Spectroscopy, which is the X-ray analogue of Dynamic Light Scattering, to wide angles [1], it is possible to gain information about diffusion in different materials [2,3,4]. In binary intermetallic alloys it is possible to determine the length and rate of effective atomic jumps. The ratios of jump-rates are compared to jump models. This talk will give a brief introduction to atomic-scale X-ray Photon Correlation Spectroscopy and compare results about diffusion constants, activation energies and jump models as well as the influence of short-range order in the B2 systems Fe-Al and Ag-Mg. [1] M. Leitner, B. Sepiol, L.-M. Stadler, B. Pfau, and G. Vogl , Nature Materials, (2009). [2] M. Leitner, B. Sepiol, L.-M. Stadler, and B. Pfau , Phys. Rev. B, (2012). [3] M. Stana, M. Leitner, M. Ross, and B. Sepiol , J. Phys.: Condens. Matter, (2013). [4] M. Ross, M. Stana, M. Leitner, and B. Sepiol , New J. Phys, (2014).

Speaker: Mr Markus Stana (Universität Wien, Faculty of Physics, Dynamics of Condensed Systems)

10:00 Surface friction investigated by neutron scattering

The study of friction and diffusion processes on molecular length scales requires experimental methods that deliver sub-nanometer spatial resolution at nanosecond time resolution. After decades of research in friction the range of available spectrometers with these characteristics is still too limited. Although, neutron scattering is not a widely used for surfaces, neutron spectroscopy is able to cover the space and time domains that are of interest here. In addition, a wide choice of high surface density materials are available now that can serve as substrates for diffusion studies. These materials have the added benefit, that they are widely applied in energy storage, sensing and filtering. Hence, research on these materials has strong potential technical impact. In this presentation, we will show recent progress in friction research using neutron spectroscopy and diffraction. Our research started with rather small and structurally simple molecules on carbon substrates, such as the aromatic molecule benzene adsorbed on the basal plane surface of graphite(0001) [1,2]. For benzene-graphite(0001) at sub-monolayer coverage, we now established the inter-molecular energy dissipation and the surface friction [2] using a range of neutron time-of-flight spectrometers, neutron spin-echo as well as neutron diffraction. The results led us to a surprisingly simple model: the inter-molecular friction can be quantitatively explained by a model of colliding cogwheels (or rough hard disks) that we had developed on the basis of the long established rough hard sphere model for molecular gases (the equivalent 3D model). The new model explains the collisional friction or viscosity based on universal constants only and makes almost no assumptions about the system. [1] E. Bahn et al., Physical Chemistry Chemical Physics, 2014, 16, 22116. [2] I. Calvo-Almazan, E. Bahn et al., Carbon, 2014, 79, 183.

Speaker: Dr Peter Fouquet (Institut Laue-Langevin)

10:25 → 10:55 Coffee break

10:55 → 12:00 Phonons and magnons

10:55 Computation, the Gibbs Free Energy, and Inelastic Scattering

J.W. Gibbs founded the field of materials science, basing it on the concept of free energy, G = E - TS + PV. For the past century, free energies of materials have been tabulated from measurements of heat capacities, which do not identify the physical sources of enthalpy and entropy. For the past 30 years, it has often been possible to calculate E and PV, but the entropy S has remained more difficult. Most of the entropy S(T) comes from atomic vibrations, i.e., phonons. The phonon entropy is big, small fractional differences are important, and the harmonic approximation is not sufficiently accurate to be useful for materials thermodynamics at elevated temperatures. Ab initio molecular dynamics (AIMD) has proved a versatile approach for calculating the phonon entropy at elevated temperatures, including the parts from phonon-phonon and electron-phonon interactions. The results have enough accuracy so that a calculated S(T) can compete with S(T) from calorimetry. Almost in parallel, inelastic scattering methods have become capable of determining the phonon entropy to similar accuracy. AIMD simulations can also provide Van Hove space-time correlation functions, allowing comparisons to results from inelastic scattering experiments by at the level of individual phonon modes (rather than comparing an integral quantity like S). The entropy of fcc Al metal has been determined by calorimetry, AIMD, many-body perturbation theory, and inelastic neutron scattering. Although fcc Al is modestly anharmonic, these four different results for entropy agree well. Other cases of cubic Ag2O and rutile TiO2 show enormous phonon-phonon anharmonicity, but the AIMD results are still consistent with experimental phonon measurements. With thermal expansion, several modes in rutile TiO2 are known to become unstable in the quasiharmonic approximation. From AIMD simulations it was found that the bottom of the Ti-O interatomic potential flattens with temperature, giving a quartic character that stabilizes the rutile structure. Understanding S(T) is essential for the practical design and use of materials at finite temperatures. Inelastic scattering is a natural partner to ab initio simulations, offering important validations. Likewise, the simulations give deeper insights into the experimental results. The time is ripe for better connections between computational materials science and inelastic scattering research.

Speaker: Prof. Brent Fultz (California Institute of Technology)

11:35 Phonons and magnetism in FeSi

The interactions of spin, lattice and electronic degrees of freedom in materials are at the origin of complex phase diagrams resulting in new emergent phenomena and technical applications. The coupling between lattice and electronic degrees of freedom is well understood, and the interaction between spin and electronic excitations has been investigated intensely, e.g., in the research on high-temperature superconductivity. However, only little is known about the dynamic interactions between spin and lattice excitations, apart from the well-established magneto-elastic coupling. Noncentrosymmetric FeSi is known to undergo a transition from insulating to metallic behavior with increasing temperature, and exhibits strongly temperature dependent phonon energies. Here we show by detailed inelastic neutron scattering measurements that the phonon renormalization in FeSi is in fact linked to its unconventional magnetic properties. In combination with ab-initio calculations, we demonstrate that two different mechanisms cause the unusual behavior: Electronic states mediating conventional electron-phonon coupling are only activated in the presence of strong magnetic fluctuations. Secondly, phonons are damped via a dynamic coupling to the temperature-induced magnetic moment with the highest impact on phonons having strongly varying Fe-Fe distances. Our findings highlight FeSi as one of the rare materials with direct spin-phonon coupling and a prime example for multiple interaction paths.

Speaker: Mr Frank Weber (Karlsruhe Institute of Technology)

12:00 → 13:30 Lunch

13:30 → 14:35 Phonons and magnons

13:30 Ab Initio Computation of Phonon-Phonon and Magnon-Phonon Interactions: Successes and Challenges

A key requirement in developing systematic approaches to explore and predict properties of materials not yet synthesized is the availability of accurate computational tools determining energies not only at T = 0 K but also under realistic finite temperature conditions. A critical step towards this goal is the ability to accurately describe all relevant excitation mechanisms such as phonons, magnons, excited electrons. For many scientifically and technologically important functional as well as structural materials the various excitation mechanisms are not adiabatically decoupled but non-adiabatic couplings e.g. between magnons and phonons or between electrons and photons (electron-phonon interaction) become critical and need to be included for an accurate description of their thermodynamic properties. In the talk we will show how novel sampling strategies in the atomic configuration space together with techniques to address the spin-degrees of freedom including spin-quantization in magnetic materials allow an unbiased and accurate determination of all relevant temperature dependent free energy contributions. While in the past the focus has been mainly on the quasiharmonic contributions (which are computationally most easily to obtain) recent advances provide now for the first time the opportunity to systematically include anharmonic and magnetic contributions all the way up to the melting temperature. The flexibility and the predictive power of these approaches to describe these complex excitation and coupling mechanisms will be discussed for selected materials such as ultra-high strength steels, magnetic shape memory alloys or light-weight alloys.

Speaker: Prof. Jörg Neugebauer (Max-Planck-Institut für Eisenforschung GmbH)

14:10 Pretransitional dynamics in prototype antiferroelectric material PbZrO3

Lead zirconate (PZO) has been recently a subject of intensive studies with its antiferroelectric (AFE) phase transition (PT) being in a spotlight. Since there is no direct link between the paraelectric cubic phase and the low temperature orthorhombic one, the process leading to the emergence of AFE order is not obvious. Results of X-ray inelastic as well as Brillouin scattering experiments has directed Tagantsev et al. to propose a flexoelectric coupling to be the main physical mechanism behind the AFE PT [1]. On the other hand, Hlinka et al. on the basis of infra-red, Raman and THz spectroscopy as well as group theory considerations proposed an alternative picture of the PT mechanism where soft ferroelectric branch is coupled by a trilinear term to two oxygen octahedra tilt modes [2]. We present a shell model molecular dynamics study of PZO. This recently developed model has already been used for explanation of X-ray [3] as well as neutron diffuse scattering data [4]. Very good agreement between modeled and experimental intensities (coming mostly from phonons) assures that the model correctly describes dynamics of the system. The atomistic simulation allows us to study theoretically phonons at finite temperatures (for the first time for PZO). To shed the light on the emergence of the AFE order, we concentrate on the cubic phase and show how the lattice dynamics changes towards the phase transition. This work is supported by the Czech Science Foundation (project no. 13-15110S). The computational part of this research was undertaken on the NCI National Facility in Canberra, Australia. [1] A.K. Tagantsev et al., Nature comms. 4, 2229 (2013), [2] J. Hlinka et al., Phys. Rev. Lett. 112, 197601 (2014), [3] M. Paściak et al., Phase Transitions, 88, 273 (2015), [4] N. Zhang et al., in preparation.

Speaker: Dr Marek Pasciak (Institute of Physics of the Czech Academy of Sciences)

14:35 → 15:00 Theoretical and experimental methods

14:35 Polarized Raman Spectra of Perovskite Relaxor Ferroelectrics

Pseudo-binary solid solutions of perovskites ferroelectrics are often showing either a ferroelectric transitions with a glassy dynamics or enhanced piezoelectric properties. Because of the demonstrated application potential of (1–x)Pb(Zn1/3Nb2/3)O3–xPbTiO3 and related materials, considerable research efforts are still payed to the understanding these phenomena. The IR spectroscopy of such lead-based perovskites is relatively well understood[1], but the assignment of Raman spectra remains a rather difficult task. The Raman activity seems to originate from both the occupational ordering [2] and the ionic off-centering [3]. The weight of these effects is varying from one material to another. Morever, the inherent disorder seems to lift the strict Raman selection rules. Interestingly, the polarized Hyper-Raman scattering spectra obey the standard polarization selection rules rather well [4,5] Nevertheless, Raman scattering in relaxors shows a measurable polarization dependence and it has been argued that specific features of polarised Raman scattering can be even employed for example to probe relaxor to ferroelectric crossover [6] or to distinguish between distinct ferroelectric phases coexisting in materials with compostion close to the so-called MPB boundaries[7]. Here we shall present our recent polarized Raman scattering studies of relaxors. In the spirit of Dyproso symposium, we shall go through the basic concepts, challenges and unpublished results. 1. JH et al, Phase Transitions, 79, 41 (2006). 2. Setter N and Laulicht I, Appl. Spectrosc., 41, 526 (1987). 3. Iwata M, et al, Jpn. J. Appl. Phys., 40, 5819 (2001). 4. A. Al-Zein, et al, Phys. Rev. B, 78, 134113 (2008). 5. A. Al-Zein et al, , Phys. Rev. Lett. 105, 017601(2010). 6. Maier B et al, Phys. Rev. B, 79, 224108 (2009). 7. I. Rafalovskyi, et al, arXiv:1304.1879 (2013).

Speaker: Dr Jiri Hlinka (Institute of Physics AS CR)

15:30 → 18:00 Guided tours

Cathedral and Prince bishops residence;
Research reactor FRM II

18:00 → 19:00 Dinner or snack at FRM II

19:00 → 20:00 Advisory Board Meeting

Wednesday 16 September

08:30 → 10:40 Excitations of strongly correlated electron systems

08:30 Electronic Raman Scattering in Cuprate Superconductors: The Pseudogap Story

The pseudogap phase in cuprates remains hitherto a mysterious state of matter out of which the high-temperature superconductivity emerges. Discovered more than twenty five years ago [1], its indentification is still challenging although extensively studies have been carried out to elucidate its true nature [2,3]. In addition recent investigations in the underdoped side of the cuprate phase diagram have shown inside the pseudogap phase the existence of charge density wave order [4-6] which could be at the origin of the Fermi surface reconstruction in electron and hole pockets detected at high magnetic field by quantum oscillations [7,8] and transport measurements [9]. Instead of clarifying our understanding of the cuprate phase diagram, these recent investigations have revealed its unexpected complexity. In this context our purpose is to reveal by electronic Raman scattering, the signature of the normal state pseudogap and track its doping evolution through the cuprate phase diagram. We will demonstrate there exists a direct connection between Raman and transport measurements on the pseudogap. Although intensively studied in the underdoped regime, relatively less is known about the normal state pseudogap on the overdoped side, where it weakens and eventually disappears at a critical doping pc. Here, combining Raman spectroscopy on Bi-2212 over a large range of finely tuned doping with theoretical calculations, we determine pc=0.22 and we show that it coincides with a Lifshitz transition where the underlying hole-like active Fermi surface becomes electron-like [10]. Interestingly, the superconducting critical temperature Tc is unaffected by this transition. This demonstrates that the microscopic origins of the normal state pseudogap and the superconductivity are distinct. Only the former is tied to the change in the Fermi surface topology. [1] H. Alloul et al., PRL 63 1700, (1989); W. W. Warren et al., Phys. Rev. Lett. 62, 1193 (1989) [2] T. Timusk, T. and B. Statt, Rep. Prog. Phys.62 122, (1999) [3] J. L. Tallon, and J.W. Loram, Physica C 349, 53 (2001). [4] G. Ghiringhelli et al., Science 337 821,2012; M. Le Tacon et al. Nature Physics, 7(9):725,(2014). [5] J. Chang , Nature Phys. 8, 872, (2012) [6] T. Wu et al. Nature 477, 191, (2011) [7] S. Sebastian, Prog. Phys. 75 102501 (2012) [8] F. Laliberte et al. Nature Comm. 2, 432 (2011) [9] L. Taillefer Annu. Rev. Condens. Matter Phys. 1 51, (2010) [10] S. Benhabib, A. Sacuto, M. Civelli, I. Paul et al., Phys. Rev. Lett.114, 147001 (2015)

Speaker: Prof. Alain SACUTO (Université Paris Diderot Paris 7)

09:10 New insights on the cuprates phase diagram from x-ray scattering

I will present an overview of the results obtained from various x-ray scattering experiments on high temperature superconducting cuprates in the last couple of years. I will first focus on Cu L-edge resonant scattering experiments that led us to uncover charge density wave (CDW) correlations competing with superconductivity in the YBCO family, for which a complete temperature and doping dependent phase diagram has been worked out. These investigations have been extended to other families of cuprates (Bi2201, Bi2212 and Hg1201) demonstrating the ubiquity and the universality of the phenomenon. Further information was gained from high resolution inelastic x-ray scattering. The observation of a quasi-elastic ‘central peak’ unraveled the static nature of the CDW correlations, attributed to the pining of CDW nanodomains on defects. Low energy phonons exhibit anomalously large superconductivity induced renormalizations close to the CDW ordering wave vector, providing new insights regarding the long-standing debate of the role of the electron-phonon interaction, a major factor influencing the competition between collective instabilities in correlated-electron materials. Finally I will discuss new results obtained in a heterostructure comprising YBCO and metallic ferromagnet La2/3Ca1/3MnO3, where a stabilization of the CDW phase is concluded and discussed within the scope of tuning the equilibrium conditions of metastable phases via heterostructuring.

Speaker: Dr Matthieu Le Tacon (Max Planck Institute for Solid State Research)

09:50 Magnetic resonant excitations in S-doped iron-chalcogenide superconductors

A sharp magnetic collective mode appearing inside the superconducting energy gap in the spin excitation spectrum is a smoking-gun evidence for phase-reversed electron-pairing symmetry in unconventional superconductors. Such mode has been observed by inelastic neutron scattering in many of iron-based superconductors, in which the phase-reversed s$_\pm$-wave pairing symmetry had been proposed. Yet, it is still under an active debate about the exact pairing symmetry of recently discovered 122-type iron-chalcogenide despite the presence of the spin resonant mode mostly due to the absence of hole Fermi surface at the Brillouin zone center. Here, we present a systematic study of inelastic neutron scattering on the sulfur-doped iron-chalcogenide KxFe2-y(Se1-zSz)2 [z=0,0.25,0.4,0.5] to clarify how S-doping affects to the spin fluctuations and the magnetic resonant excitations. Further, we discuss the physical implication of our inelastic neutron scattering data based on the tight relationship between the magnetic resonant mode and superconducting order parameter symmetry.

Speaker: Jitae Park

10:15 Experimental and theoretical studies of the lattice dynamics in superconducting BaNi2(As1-xPx)2.

We present a combination of Thermal Diffuse Scattering (TDS) and Inelastic X-ray Scattering (IXS) measurements, and Density Functional Perturbation Theory (DFPT) calculations of the lattice dynamics in superconducting BaNi2(As1-xPx)2. At Ts=130 K, BaNi2As2 undergoes a structural phase transition from a tetragonal to a triclinic crystal structure. In addition, superconductivity emerges at Tc=0.7 K. Substitution of the arsenic with phosphorus leads to a suppression of Ts. Finally, at a substitution level of 7%, the structural phase transition is completely suppressed and the Tc jumps from 0.7 to 3.3 K. It is believed that the nature of the superconductivity displayed by this material is of the conventional BCS type. Furthermore, specific heat measurements suggest that a “Giant” phonon softening is responsible for the jump in Tc. In order determine if such a large phonon softening does occur in BaNi2(As1-xPx)2, we have undertaken a combined experimental and theoretical study of the lattice dynamics of this material. TDS measurements on BaNi2As2 have identified locations of significant diffuse scattering. Upon cooling, the scattering from these diffuse regions becomes stronger, collapsing into true Bragg structural reflections below Ts. The IXS measurements and DFPT calculations identify this to be the location of a significant softening of at least one phonon mode. Out results demonstrate that these phonons are strongly coupled to the structural phase transition, and therefore it is a strong candidate for being responsible for the jump in Tc. In order to ascertain if this is correct, we have extended our TDS and IXS measurements to phosphorus doped BaNi2As2. Results from these latest set of measurements will be presented. In addition, our results will be compared to the lattice dynamics to the iron pnictide unconventional superconductors.

Speaker: Dr Thomas Forrest (ESRF - The European Synchrotron)

10:40 → 11:10 Coffee break

11:10 → 12:15 Excitations of strongly correlated electron systems

11:50 Excitonic effects on the optical response of quasi-one-dimensional Ta2NiSe5 and Ta2NiS5

Cooper pairing of fermions is one of the most fundamental and successful concepts of condensed matter physics. The electron pair state is responsible for superconductivity, whereas the condensation of neutral electron-hole composite bosons is predicted to lead to an excitonic insulator state. Unlike superconductivity, the latter state does not exhibit a striking macroscopic manifestation of the quantum phase coherence and has no unequivocal experimental confirmation. Recent ARPES studies have considered quasi one-dimensional Ta2NiSe5 as a candidate for an excitonic insulator, which exhibits flattening of the valence band top below the transition temperature at 325 K [1]. In order to explore the excitonic states we perform a comparative optical study of the closely related ternary chalcogenides Ta2NiSe5 and Ta2NiS5. By means of wide-band spectroscopic ellipsometry we directly measured the complex dielectric function and unambiguously identified the excitonic doublet in both the compounds at low temperatures. Many-body interactions in these systems manifest themselves as a Fano interference of the discrete excitonic states with a band continuum. The determined exciton binding energy decreases with the increase in size of the chalcogenide atom and remains comparable with the optical gap energy. A gradual closing of the optical gap is observed in Ta2NiSe5 as the transition temperature is approached. The optical absorption spectra above the energy scale of the excitonic Fano resonances are dominated by a series of sharp interband transitions. Their steep temperature dependence corroborates strong electron-phonon interaction with a modification of the peak energies and widths proportional to the filling factor of optical phonon modes. The electron-phonon coupling may significantly reduce the effect of the local Coulomb attraction and stabilize the exciton condensation in the system. [1] Y. Wakisaka et al., Phys. Rev. Lett. 103, 026402 (2009).

Speaker: Dr Alexander Boris (Max Planck Institute for Solid State Research)

12:15 → 13:30 Lunch

13:30 → 14:45 Phonons and magnons

13:30 Inelastic scattering study of AgCrSe2

Layered triangular antiferromagnet selenide AgCrSe2 (SG : R3m) has been investigated by means of elastic and inelastic neutron scattering, combined with electrical transport, Seebeck coefficient, magnetisation and heat conductivity (kappa) measurements, in the range 5 to 300 K. Below TN = 55 K [1], long-range magnetic ordering is identified, characterized by long-wavelength incommensurate antiferromagnetic cycloids running along [110], and rotating within the ab plane (k = e e 3/2). No structural transition is observed down to 1.5 K, in contrast with structural parent AgCrS2 [2]. Spin wave modelling of the inelastic scattering spectrum in the magnetically ordered phase leads to ferromagnetic and antiferromagnetic exchange interactions between nearest and next-nearest neighbours, respectively, with a weak antiferromagnetic interplane exchange. The most remarkable features of the excitation spectra are a 3 meV phonon with an anomalous temperature behaviour above 80 K, and the persistence of a magnetic signal up to 150 K (~ 3 TN). The presence of this low energy phonon is in agreement with the extremely low thermal conductivity of AgCrSe2, which is attributed to phonon scattering by Ag+ rattling ; on the other hand, the absence of any magnetic field effect (up to 9T) on kappa suggests the absence of any additional contribution to the heat conduction from magnons [3]. [1] Bongers, P. F. et al., Journal of Physics and Chemistry of Solids 29, 977 (1968) [2] Damay, F., et al., Physical Review B 87, 134413 (2013) [3] Hess, C., Eur. Phys. J. Special topics 151, 73 (2007)

Speaker: Dr Françoise Damay (LLB, France)

13:55 Temperature dependence and linewidths of Al phonon dispersions

The availability of accurate phonon dispersions for all temperatures up to the melting point is essential for the prediction of thermodynamic stabilities of crystal structures. Standard ab initio approaches are based on a quasiharmonic (QH) approximation of the interaction potential, where temperature dependencies enter only via the volume expansion. We have recently developed an extension of the QH approach [1] that uses the asymmetry of the nearest-neighbor potential. The approach allows us to perform computationally efficient molecular dynamics simulations with full ab initio accuracy. The predictive power of the resulting temperature dependencies and phonon linewidths is determined in this study. We have chosen Al for this purpose, since it is isotopically pure, non-magnetic and its electronic structure is known to be adequately described by density-functional theory. Here, we report for the first time measurements of the entire phonon dispersion of Al at temperatures of 293 K, 700 K, 800 K and 900 K, as well as of high symmetry points at additional temperatures between 15 K and 700 K. The experiments were performed at the thermal triple axis neutron spectrometer PUMA at FRM II in Garching, Germany. The presented results give access to the temperature dependent longitudinal and transversal phonon branches in the high symmetry directions including the evaluation of phonon broadening. The combination of the comprehensive theoretical and experimental data allows an evaluation of anharmonic contributions beyond Grüneisen theory to the phonon dispersion and of the temperature dependence of phonon linewidths in metals. [1] A. Glensk, B. Grabowski, T. Hickel, J. Neugebauer, to appear in Phys. Rev. Lett. (2015)

Speaker: Mr Pascal Neibecker (Heinz Maier-Leibnitz Zentrum (MLZ), Technische Universität München, Lichtenbergstr. 1, 85748 Garching, Germany)

14:20 “Half-moon” excitations in the magneto-elastic spin liquid Tb2Ti2O7

Geometrical magnetic frustration is a central concept in condensed matter physics. In this field, rare earth pyrochlore magnets R2Ti2O7 (R is a rare earth) play a prominent role, as they form model systems showing a rich variety of ground states, depending on the balance between dipolar, exchange interactions and crystal field [1]. The Terbium compound Tb2Ti2O7 remains a cooperative paramagnet, or a “quantum spin ice”, with strongly correlated moments still fluctuating at 50 mK [2]. Recent time of flight neutron and triple-axis neutron scattering experiments have recently shed light in this puzzle, revealing a complex “magneto-elastic” ground state [3,4] characterized by a local constraint resulting in “pinch points” [5], analogous to the ice rule in spin ices, and supporting a low energy (bosonic) excitation [4]. Under applied field, a complex antiferromagnetic structure sets in, while the low energy excitations transform into a spin wave like mode whose dynamical structure factor is highly anisotropic, showing “half-moons” in reciprocal space [6]. This peculiar form indeed casts light on the underlying “ice rule” of Tb2Ti2O7. [1] J Gardner, M. Gingras and J. Greedan, Reviews of modern Physics, Vol 82 (2010) [2] J. Gardner et al PRL 82, 1012, (1999) [3] S. Guitteny et al PRL 111, 087201 (2013) [4] T. Fennell et al., PRL 112, 017203 (2014); [5] T. Fennell et al, PRL 109, 017201 (2012) and (2013) [6] S. Petit et al, in preparation

Speaker: Dr SYLVAIN PETIT (CEA)

14:45 → 15:15 Coffee break

15:15 → 16:05 Theoretical and experimental methods

15:15 Dynamic properties of liquids and solids probed by Dynamic Mechanical Analysis

Since structural changes in materials usually couple to strain, acoustic spectroscopy provides a very sensitive tool for the study of dynamic properties of solids and liquids. In principle a frequency range from 0.01 Hz up to GHz or even THz can be covered with different methods, including Dynamic Mechanical Analysis (DMA) , Resonance techniques, Ultrasonics, Brillouin – and neutron scattering. Here we present DMA measurements (0.01 – 100 Hz) for a broad range of materials, including ferroic crystals [1], iron based superconductors [2], nano – confined molecular glass forming liquids [3] and polymers [4] and show the wealth of information (e.g. on domain wall motion dynamics, nematic precursor fluctuations, confinement effects at liquid – glass transitions, etc.) that can be extracted from measurements of their low frequency elastic response. Acknowledgement: The work was supported by the Austrian Science Fund (FWF) P23982-N20. [1] W. Schranz, H. Kabelka, A. Sarras and M. Burock, Appl. Phys. Lett. 101, 141913 (2012). [2] Anna Böhmer, et al. Phys. Rev. Lett. 112 047001 (2014). [3] J. Koppensteiner, W. Schranz and M.A.Carpenter, Phys. Rev. B 81, 024202 (2010). [4] M. Reinecker, V. Soprunyuk, M. Fally, A. Sánchez-Ferrer and W. Schranz, Soft Matter 10 (31), (2014).

Speaker: Prof. Wilfried Schranz (University of Vienna, Faculty of Physics)

15:40 Non-exponential relaxation: multiscale or nonlinear phenomenon?

A distribution of relaxation times results in a relaxation described by formulae more complex than a single decreasing exponential function. A known example is the stretched exponential function [1] often treated as a continuous linear combination of purely exponential decays [2]. An experiment providing the relaxation function and the appropriate impulse response, i.e. the response of the system to the Dirac’s delta-like perturbation would be an evidence of a multiscale origin of the phenomenon. On the other hand, a non-exponential decay may result from a relaxation of a single anharmonic element without any recourse to different time scales. The amplitude-dependent response functions will be presented and the selected experimental data will be analyzed with both methods of description. Criteria will be proposed to distinguish the multiscale and nonlinear [3] mechanisms of non-exponential decay. A sonic effect of reverberation with continuous and discrete distribution of relaxation times will be used to demonstrate how the ordinary exponential and non-exponential regimes affect the intelligibility of speech and music. References [1] R. Metzler, J. Klafter , Journal of Non-Crystalline Solids 305 (2002) 81 [2] see e.g. M.N. Berberan-Santos, E.N. Bodunov, B. Valeur. Chemical Physics 315 (2005), 171 [3] P. Zieliński, Physica B Condensed Matter 316 (2002) 603

Speaker: Mr Marcin Majka (The Henryk Niewodniczański Institute of Nuclear Physics Polish Academy of Sciences)

16:05 → 17:20 Two-dimensional systems

16:05 Theory of Piezoelectricity and Sound Waves in 2D Crystals

Piezoelectricity in three-dimensional (3D) materials is a well known physical phenomenon with many applications ranging from medical imaging (ultrasound) to fuel injection in motorcar engines.In earlier work we have investigated by analytical methods piezoelectricity and phonon dispersions in 2D and in multilayer crystals [1];[2].Here we extend this work by taking into account lattice anharmonicities. Starting from a 2D honeycomb ionic crystal structure with D_3h symmetry we use a Hamiltonian which includes anharmonic couplings between in-plane lattice displacements and out-of-plane flexural modes [3].Using methods of statistical mechanics we derive coupled equations for acoustic , optical and flexural dynamic response functions.The resonances of the correlation functions allow us to study temperature dependent phonon lineshifts and dampings.In the limit of long wavelengths and low frequencies we recover macroscopic equations for sound waves,electrical polarization and flexural motion.There the macroscopic quantities such as sound velocities ,piezoelectric coefficients and bending rigidity are expressed in terms of atomistic parameters.Effects due to two-dimensionality are discussed.As specific materials we refer to hexagonal boron nitride and transition metal dichalcogenides. [1]Theory of elastic and piezoelectric effects in two-dimensional hexagonal boron nitride,K.H.Michel and B.Verberck,Phys.Rev.B 80, 224301 (2009) [2]Phonon dispersions and piezoelectricity in bulk and in multilayers of hexagonal boron nitride,K.H.Michel and B.Verberck,Phys.Rev.B 83, 115328 (2011);eidem Phys.Status Solidi B 248,2720 (2011) [3]Theory of anharmonic phonons in two-dimensional crystals, K.H.Michel,S.Costamagna,and F.M.Peeters,Phys.Rev.B 91,134302(2015)

Speaker: Prof. Karl H. Michel (University of Antwerp)

16:30 High Precision MC/RG Study of Elastic Fluctuations in Solid Membranes

The computation of the critical exponent eta characterizing the universal elastic behavior of crystalline membranes in the flat phase continues to represent challenges to theorists as well as computer simulators that manifest themselves in a considerable spread of numerical results for eta published in the literature. We provide additional insight into this problem [A.T. PRE 91, 022132 (2015)], that results from combining Wilson’s momentum shell renormalization-group method with the power of modern computer simulations based on a recent optimization [A.T. PRB 87, 104112 (2013)] of our Fourier Monte Carlo algorithm. We discuss the ideas and difficulties underlying this combined scheme and present a calculation of the renormalization-group flow of the effective two-dimensional Young modulus for momentum shells of different thickness. Extrapolation to thick shells allows us to produce results in reasonable agreement with those obtained by functional renormalization group or by Fourier Monte Carlo simulations in combination with finite-size scaling. Moreover, our method allows us to obtain for the the first time a numerical estimate for the value of the Wegner exponent omega that determines the leading correction to scaling. This in turn allows us to refine our numerical estimate for eta previously obtained from precise finite-size scaling data [2], and also sheds some light on the possible reasons for the dispersion of previously published numerical estimates for eta. In particular, for the solid case, our numerical estimate for eta is markedly smaller than that derived from other recent simulations, and we find clear evidence against “intrinsic ripples”, whose existence has been repeatedly claimed in the graphene-related literature.

Speaker: Dr Andreas Tröster (Vienna University of Technology, Institute of Material Chemistry, Getreidemarkt 9, A-1060 Wien, Austria)

16:55 Spectroscopical characterisation of high surface area carbons through a multitechnique approach

Andrea Piovano1, Andrea. Lazzarini2, Carlo Lamberti2, Giovanni Agostini3, Giuseppe Leofanti4, Riccardo Pellegrini5, Elena Groppo2 1 Institut Laue-Langevin (ILL), BP 156 X, F-38042 Grenoble Cedex, France 2 Department of Chemistry,University of Turin, Via Giuria 7, I-10125, Turin, Italy 3 European Synchrotron Radiation Facility (ESRF) 6 Rue Jules Horowitz, BP 220 38043 Grenoble Cedex, France 4 Consultant,Via Firenze 43, 20010Canegrate,Milano, Italy 5 Chimet SpA - Catalyst Division, Via di Pescaiola 74, ViciomaggioArezzo, I-52041 Italy High surface area carbons are industrially relevant materials whose properties depend on morphology, texture and surface features. The detailed characterization of functional group on the surface of this class of materials is a fundamental step for understanding their potential in a variety of applications. Unfortunately IR, the widely used laboratory spectroscopy method, is difficult to be applied on carbons due to the intrinsic strong absorption. We present here an multitechnique approach, based on the synergic combination of three vibrational spectroscopies: i) FT-IR diffuse reflectance spectroscopy (DRIFT), that, limiting the strong absorption of the transmitted light is effective in evidencing vibrations with change in the dipole; ii) back-scattering Raman spectroscopy, which is sensitive mostly to carbon bulk vibrational modes; iii) Inelastic Neutron Scattering (INS) that, eliminates the problem of radiation interaction and is sensitive to vibrations involving hydrogen including species, highly aboundant on carbons surface. The three technique are applied to two classes of activated carbons, subjected to specific chemical treatments. The whole set of experimental data, interpreted with the help of DFT calculations, allow us to point out their structural and surface properties, and to clarify some controversial information present in the specialized literature, where conclusions are done on the basis of the data obtained by a single technique.

Speaker: Andrea Piovano (Institut Laue Langevin, Grenoble)

19:30 → 22:30 Conference Dinner at Bräustüberl Weihenstephan

Thursday 17 September

08:30 → 09:35 Materials under high pressure

08:30 (De)formation of mantle minerals: Insights from atomic-scale simulations

Minerals are the building blocks of the Earth. Knowledge of their formation and evolution is needed to understand both the structure and the dynamics of our planet. As only the upper part of the crust is accessible to direct sampling, models of the deep interior of the Earth rely on a combination of geophysical observations, laboratory experiments and simulations. Here, we use various atomic scale simulation methods to shed light on the structure and thermodynamic stability of mantle minerals, defects in their crystal structure and on the mechanisms of diffusion and shear deformation. These simulation techniques are unique in the sense that they provide simultaneous access to atomic structures and to thermodynamic or transport properties. On the other hand, chemical and structural complexity of relevant Earth materials require both substantial computational resources and efficient though still accurate simulation methods. We therefore use a combination of classical and first-principles molecular modeling methods, accelerated dynamics techniques such as metadynamics and sometimes educated guess. A big challenge for future research will be to establish the link between the atomic and the continuum scale, especially for modeling one- or two-dimensional crystal defects and for modeling plastic deformation.

Speaker: Prof. Sandro Jahn (University of Cologne)

09:10 Lattice dynamics and magnetic order in CrAs under pressure

Pressure induced superconductivity in CrAs has been discovered in June 2014, opening a new avenue for searching novel superconductors in Cr and other transition based compounds [1,2]. CrAs belongs to the group of 3d electron systems which can offer stages which induce intriguing superconductivity as realized in cuprates, Fe pnictides, cobalt oxyhydrate, etc. In Cr- and Mn-based systems this behaviour has so far not been observed. The application of external pressure, however, leads to superconductivity in CrAs in the vicinity of antiferromagnetic order. CrAs is paramagnetic at room temperature and shows a first order magnetic phase transition at 265 K to a helimagnetic phase. The magnetic transition is suppressed at higher pressures where superconductivity appears in the paramagnetic phase at low temperature. In order to investigate the origin of superconductivity in CrAs we perform neutron diffraction to study the magnetic structure under pressure [3], inelastic x-ray scattering to determine the phonon dispersion relations and ab initio calculations to address spin-lattice coupling. [1] W. Wu, J. Cheng, K. Matsubayashi, P. Kong, F. Lin, C. Jin, N. Wang, Y. Uwatoko, and J. Luo, Nat.Commun. 5, 5508 (2014). [2] H. Kotegawa, S. Nakahara, H. Tou, and H. Sugawara, J. Phys. Soc. Jpn. 83, 093702 (2014). [3] L. Keller, J. S. White, M. Frontzek, P. Babkevich, M. A. Susner, Z. C. Sims, A. S. Sefat, H. M. Ronnow, and Ch. Rueegg, Phys. Rev. B 91, 020409(R) (2015)

Speaker: Dr Bjoern Wehinger (Department of Quantum Matter Physics, University of Geneva and Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institute)

09:35 → 10:40 Multiferroics and ferroelectrics

09:35 Soft-mode spectroscopy of ferroelectrics and multiferroics

In proper ferroelectrics, the large dielectric anomaly observed at Curie temperature TC is caused by softening of some polar excitation. In displacive ferroelectrics, this excitation is a polar phonon active in far-infrared spectra. In order-disorder ferroelectrics, dielectric relaxation with frequency in the MHz-GHz region drives the ferroelectric phase transition. Many ferroelectrics exhibit crossover from displacive to order-disorder type of phase transition, i.e. some phonon softens on cooling far above TC, but additional relaxation (called central mode) appears close to TC and its relaxation frequency remarkably softens towards TC. As examples we will present phonon and central mode behavior near strain-induced ferroelectric phase transitions in EuTiO3 and Srn+1TinO3n+1 (n=1-6) thin films,[1-3] in relaxor ferroelectric Na0.5Bi0.5TiO3 [4] and multiferroic PbFe1/2Nb1/2O3.[5] In multiferroics, where the ferroelectricity is induced by a spin order, only a small and narrow peak in temperature dependent permittivity appears at TC. We will show that this tiny dielectric anomaly is caused by softening of an electromagnon, whose frequency lies in the microwave region. This electrically active spin excitation can have relaxation character in dielectric spectra (e.g. in MnWO4)[6] or resonance character (e.g. in Sr3Co2Fe24O41 with Z-type hexaferrite structure).[7] 1. J.H. Lee, et al., Nature, 466, 954 (2010). 2. C.H. Lee, et al., Nature, 502, 532 (2013). 3. V. Goian, S. Kamba, et al., Phys. Rev. B 90, 174105 (2014). 4. J. Petzelt, et al., Phase Transitions 87, 953 (2014), ibid. 88, 320 (2015). 5. R. Mackeviciute, V. Goian, et al., J. Appl. Phys. 117, 084101 (2015). 6. D. Nierman, C.P. Grams, P. Becker, et al., Phys. Rev. Lett. 114, 037204 (2015). 7. R. Tang, C. Jiang, J. Jian, et al. Sci., Rep, in press.

Speaker: Dr Stanislav Kamba (Institute of Physics, The Czech Academy of Sciences, Prague)

10:15 The nature of phase transition in GeTe - the parent compound of the phase-change materials

Germanium telluride (GeTe) has attracted an intense renewed interest in the past few years for its relevance and high performance as a phase-change material as such and in combination of the form (GeTe)m(Sb2Te3)n which find useful applications in modern non-volatile data storage devices. Apart from this technological interest, there is a very fundamental issue regarding whether the ferroelectric-to-paraelectric phase transition in GeTe is order-disorder or displacive in its origin. Also, the underlying physics of the crystal volume contraction which assists the structural transformation in GeTe remains far from clear. Our contribution presents results of the high-resolution neutron powder diffraction experiments performed on a spallation neutron source which allow for a better understanding of the structural changes across the rhombohedral-to-cubic phase transition in GeTe. We also report on the phonon dynamics in GeTe which we gain from the inelastic neutron scattering experiments along with the density functional theory calculations that show conclusively the displacive type of the phase transition rather than the order-disorder one suggested by some x-ray absorption fine structure experiments and the pair-distribution function analysis. The structural phase transition in GeTe is shown to be driven by the condensation of exactly three components of the triply degenerate optical transverse soft-phonon mode at the Brillouin zone center. Results of the current work indicate that the local atomic potentials are single-well which strongly supports the displacive nature of the phase change in crystalline GeTe. Our considerations are relevant not only for GeTe but can be applicable to other ferroelectric materials like barium titanate as well as manganites showing colossal magnetoresistance, e.g., lanthanum manganite which are known to exhibit similar phenomena.

Speaker: Dr Urszula D. Wdowik (Institute of Technology, Pedagogical University, Podchorazych 2, PL-30084 Cracow, Poland)

10:40 → 11:10 Coffee break

11:10 → 12:15 Multiferroics and ferroelectrics

11:10 Directional anisotropy of light in multiferroics

Multiferroics are materials which exhibit electric and magnetic order simultaneously. Due to the coupling of electric and magnetic effects, these materials show a strong potential to control electricity and magnetism and, more generally, the properties and propagation of light. One of the most fascinating and counter-intuitive recent results in multiferroics is directional anisotropy, the asymmetry of light propagation with respect to the direction of propagation. The absorption in the material can be different for forward and backward propagation of light, which in extreme case may lead to complete suppression of absorption in one direction. Another remarkable effect in multiferroics is directional birefringence, i.e. different velocities of light for different directions of propagation. As an example, in the multiferroic samarium ferroborate giant directional birefringence can be realized. The effect is easily observed for linear polarization of light in the range of millimeter-wavelengths, and survives down to very low frequencies. The dispersion and absorption close to the electromagnon resonance can be controlled and fully suppressed in one direction. Therefore, samarium ferroborate is a universal tool for optical control: with a magnetic field as an external parameter it allows switching between two functionalities: polarization rotation and directional anisotropy.

Speaker: Prof. Andrei Pimenov (Vienna University of Technology)

11:50 Tiny cause with huge impact: polar instability through strong magneto-electric-elastic coupling in bulk EuTiO3

Multiferroic materials with combined polar, magnetic, and elastic orderings are at the forefront of scientific research in view of their multiple interactive couplings. Even though the phenomenon of multiferroicity has been predicted long ago [1], its realization remains rare since polar order is achieved when a transition metal d0 configuration is combined with highly polarizable anions, whereas magnetic order relies on a finite dn configuration. These two requirements yield a certain incompatibility. Even though a rather large number of materials have been shown to exhibit the desired properties, the coupling between magnetic and polar order is either very weak, or the spontaneous polarization/magnetization appears at low temperature only and remains too small to be of technological interest. Here we propose a new strategy to achieve strong magnetic-polar coupling by deriving the soft mode frequency of EuTiO3 as a function of its lattice parameters which exhibits unusual, yet very small temperature dependencies at high and low temperatures [2, 3]. Specifically we develop a route of how to induce ferroelectric order in bulk EuTiO3 (ETO) by combining experimental results with theoretical concepts. We show that marginal changes in the lattice parameter of the order of 0.01% have a more than 1000% effect on the transverse optic soft mode of ETO and thus easily induce a ferroelectric instability. 1. Smolenskii, G. A., Isupov, V. A., & Agronovskaya, A. I,. Sov. Phys Solid State 1, 149 (1959). 2. Reuvekamp, P. G., Kremer, R. K., Köhler, J., & Bussmann-Holder, A., Phys. Rev. B 90, 094420 (2014). 3. Reuvekamp, P. G., Kremer, R. K., Köhler, J., & Bussmann-Holder, Phys. Rev. B 90, 104105 (2014).

Speaker: Prof. Annette Bussmann-Holder (Max-Planck-Institute for Solid State Research)

12:15 → 13:30 Lunch

13:30 → 15:15 Multiferroics and ferroelectrics

13:30 Multiferroic RMn2O5 (R = Y, Tb, Ho) and isotopically substituted TbMn16/18O3, Dy16/18MnO3: A comprehensive Raman light scattering and neutron scattering investigation

Multiferroic materials demonstrate excellent potential for next-generation multifunctional devices, as they exhibit coexisting ferroelectric and magnetic orders. At present, the underlying physics of the magnetoelectric coupling is not fully understood, and competing theories exist with partly conflicting predictions. Therefore, we have investigated isotopically substituted TbMn16/18O3, Dy16/18MnO3, and RMn2O5 (R = magnetic Tb, Ho and non-magnetic Y) by Raman light scattering and neutron diffraction to elucidate the spin-phonon coupling and crystallographic as well as magnetic phase diagrams in order to shine light on the multiferroic coupling mechanism in both compounds. Raman light scattering allows for the detection subtle changes in phonon energy and lifetime at magnetic and ferroelectric phase transitions through interactions of the lattice vibrations with the electronic systems. This offers valuable information on the mechanisms behind the magnetoelectric properties. For example, our experiments on RMn2O5 did demonstrate that the magnetic and ferroelectric phases in RMn2O5 with magnetic R = Tb, Ho are distinct from the phases in non-magnetic YMn2O5, demonstrating the importance of the rare earth element on the multiferroic properties. Our neutron diffraction investigations did yield additional complementary information in the crystallographic and magnetic structures. Our combined neutron and high resolution X-ray synchrotron investigation on Dy16/18MnO3 did allow for the detection of atomic displacements at the ferroelectric phase transition, possible responsible for the creation of the ferroelectric moment. Opposite to the behavior in other transition metal oxides, oxygen isotope substitution only had a minor effect on the magnetic properties. This provides valuable information on effects behind the magnetoelectric coupling mechanisms in these multiferroic materials.

Speaker: Prof. Clemens Ulrich (University of New South Wales)

13:55 Dynamics of Nanoscale Polarization Fluctuations in a Uniaxial Relaxor

One of the most remarkable properties of relaxor materials is their extraordinarily large dielectric permittivity appearing over a broad temperature interval and attaining its maximum at a temperature Tmax, which varies linearly with the logarithm of the probing frequency. This frequency-dependent Tmax is described by the Vogel-Fulcher (VF) law and originates from a temperature-dependent dielectric relaxation. Such a behaviour has also been observed in tetragonal tungsten bronze crystals such as Sr0.61Ba0.39Nb2O6 (SBN61) [1], an almost ideal uniaxial relaxor with polar fluctuations appearing only along the tetragonal axis. Recently, we have performed a neutron backscattering study on a SBN61 single crystal in the MHz-GHz frequency region [2]. Investigation of diffuse scattering coming from atomic displacements along the tetragonal axis allowed us to resolve the dynamic part of nano-scale polar fluctuations. Typical data are displayed in Fig.1a, showing a clear inelastic component which can be well described by a simple model based on an analysis of dielectric spectra [1,2]. The agreement between the experiment and model data can be better appreciated when the scattering intensity at a fixed energy transfer is plotted as a function of temperature (cf. Fig.1b,c). This comparison demonstrates that the peak positions shift towards higher temperatures with increasing energy transfers, closely following the VF law. Our results provide direct evidence that the ‘critical’ relaxation with its VF-type frequency dependence is associated with dynamic nano-scale polarization fluctuations. Fig.1: Neutron-scattering intensity at Q ~ (0.15,0,1) as a function of (a) energy transfer and (b) temperature compared with (c) a model based on an analysis of dielectric spectra [1,2]. References [1] E. Buixaderas et al, J. Phys. Condens. Matter 17, 653 (2005). [2] P. Ondrejkovic et al, Phys. Rev. Lett. 113, 167601 (2014).

Speaker: Petr Ondrejkovic (Institute of Physics ASCR)

14:20 Broadband Dielectric Spectroscopy of A-site Substituted Relaxor Ceramics

The technique of dielectric spectroscopy which overlaps frequency range from 1 mHz to 5 THz is a powerful tool to investigate disordered materials. Such experiments can be combined with FTIR measurements in order to describe the dynamics of polar nanoregions and optical phonons in materials like relaxor ferroelectrics. In this contribution the broadband dielectric spectroscopy was employed to investigate several solid solution systems based on sodium bismuth titanate (i.e. 0.4Na0.5Bi0.5TiO3-(0.6-x)SrTiO3-xPbTiO3, (0.4-y)Na0.5Bi0.5TiO3-0.6SrTiO3-yPbTiO3 and zNa0.5Bi0.5TiO3-(1-z)Sr0.7Bi0.2TiO3 ). All three systems show rich behavior ranging from glassy state which crossovers to relaxor and even normal ferroelectric phase. The influence of polar nanoregions and phase diagrams of investigated materials will be discussed. In addition, ferroelectric hysteresis, pyroelectric and piezo measurements will be presented.

Speaker: Prof. Juras Banys (Vilnius University)