Synergistic interaction between 2D materials and organic molecules presents an additional dimension for tuning intrinsic properties of such low dimensional materials. Through this presentation I will be discussing our recent findings where we have tried to tune the work function of 2D Ti₃C₂T_x by introducing ultrathin interlayers of organic dipoles with a defined dipole moment value. Interface engineering is achieved through the inclusion of poly(ethylene)amine (PEI 0.1%) and third generation poly(amido-)amine (PAMAM G3), between the Ti₃C₂T_x and c-Si substrate. The work function of the fabricated c-Si/MXene/organic dipoles structures were evaluated from the ultraviolet photo-emission spectroscopy (UPS) measurements. We report a significant reduction in the work function value of Ti₃C₂T_x from 5.8 eV to 4.2 eV for Ti₃C₂T_x/PEI 0.1% and 3.3 eV for Ti₃C₂T_x/PAMAM-G3 heterostructures. Our study introduces an innovative approach for modifying the work function of Ti₃C₂T_x through the incorporation of organic dipoles.

The structuring of water at hydrophilic interfaces is a topic of significant interest due to its implications in various fields, from material science to biological systems. This study investigates the hypothesis of water structuring in exclusion zones (EZs) adjacent to hydrophilic surfaces, specifically focusing on Nafion. Exclusion zones are proposed to be regions near the interface where water exhibits unique ordering properties, leading to the exclusion of solutes and particles. To explore this phenomenon, we employ a range of spectroscopic techniques, such as Raman and IR spectroscopy, to analyze the molecular dynamics and hydrogen-bonding environment of water near Nafion surfaces with the aim of identifying any distinct spectral features or patterns that could indicate the presence and characteristics of EZ-structured water. In this talk, I will present the results of this study, which are expected to provide insights into the nature of interfacial water behavior and the potential mechanisms driving the formation of exclusion zones, thereby contributing to a deeper understanding of water interactions at hydrophilic interfaces.

The interest in nanoscale ferroelectric materials has grown significantly in both scientific and industrial fields due to their unique properties, derived from their ability to change their spontaneous polarization. These properties, combined with distinctive dielectric, ferroelastic, and piezoelectric characteristics, open the door to a wide variety of new functionalities and technological applications.

Recent research on nanoscale ferroelectric systems reveals the presence of novel topological excitations, challenging the notion of uniformity in the ground state. These findings raise new questions about the nature of such structures and their potential practical applications, which could be realized by manipulating different topological states.

In this talk, we present a study of free standing PbTiO3 nanowires of different sizes, using an atomistic second-principles approach combined with the Ginzburg-Landau-Devonshire (GLD) model and the phase-field method. The Atomic-level simulations with a core-shell model revealed a strong size effect on the resulting polar textures. This dependency can be attributed to the influence of surface tension, which manifests laterally through the well-known Laplace pressure and longitudinally along the wire axis.

These effects were incorporated into the GLD functional by adding an additional term to the free energy. Using both simulation techniques and analytical calculations, we constructed a radius-temperature phase diagram where the results from the different methods converged.

Two distinct polar phases were identified: the vortex phase, characterized by the rotation of local polarization around the nanowire’s c-axis, and the uniform c-phase, with polarization aligned along the wire axis. In systems with small radii, up to 20 unit cells, the observed polar phase is the vortex phase. However, for larger radii, the low-temperature structure is the c-phase, which transitions to the vortex phase as the temperature increases. At high temperatures, the system becomes paraelectric for all radius values.

The ability to tune their properties makes one-dimensional ferroelectrics valuable in the development of ferroelectric components for advanced devices, such as multi-level logic units or neuromorphic computing circuits. Specifically, the tailored design of nanowires allows for achieving desirable operational properties, ensuring their implementation in nanoelectronic devices.

Intermetallic materials are an important group of matter at the forefront of current technical development due to their interesting physical properties including mechanic, thermal, transport, magnetic and electronic properties in general. These properties can be influenced by changes of external conditions such as temperature, magnetic field or mechanical pressure. The last mentioned parameter is a unique tool to change the distances between atoms in the studied material by the chemically most pure way and enables view into new fields in the phase space. Besides other mentioned physical variables, variation of pressure faces to a rather complicated situation caused namely by the technical difficulty of high pressure experimental methods. Development of high pressure techniques in the past decades has nevertheless made the application of pressure a regular constituent of characterization procedures and enabled to vary this physical variable for study of phenomena like quantum criticality, valence changes etc. Our laboratory incorporates high pressure experiments in combination with low temperatures and high magnetic fields and enables to follow the studied effects within the phase space. The seminar will be devoted to introductory remarks on the effects of pressure in materials, possible experimental approaches and obtained results on selected compounds.

Hexagonal EuAl₁₂O₁₉ is a quasi-two-dimensional ferromagnet below 1.3 K. Pyroelectric current measurements revealed a weak ferroelectric polarization below T_C = 49 K.  The existence of a ferroelectric phase transition is supported by an anomaly in specific heat and thermal expansion. However, the temperature dependence of permittivity does not show a peak at T_C, but only a change of slope. This could argue in favor of an improper or pseudo-proper ferroelectric phase transition. However, single crystal synchrotron diffraction studies revealed no structural change at T_C and second harmonic generation measurements also showed no signal down to 5 K.  This indicates that EuAl₁₂O₁₉ remains macroscopically centrosymmetric (space group P6₃/mmc) down to low temperatures. We propose to explain the observed behavior by frustrated antiferroelectricity or frustrated antipolar correlations below T_C. An external electric field induces a weak polarization visible in the pyrocurrent, but without the field the sample remains centrosymmetric. Dynamical frustration of antipolar order makes it impossible to see the long-range structural change in XRD and explains the observed strong relaxor ferroelectric-like dielectric dispersion below T_C. Similar frustrated antiferroelectricity was theoretically predicted in the isostructural BaFe₁₂O₁₉ below 4 K [Wang and Xiang, PRX 4, 011035 (2014)], but it was not experimentally observed due to the occurrence of quantum paraelectricity. However, the theory from Wang and Xiang predicts that Al cations are much more polar than Fe and this is the reason, why the antipolar correlations begin to build in EuAl₁₂O₁₉ already at 49 K. Finally, we show that EuAl₁₂O₁₉ exhibits freezing of dielectric relaxation at zero temperature, which would suggest that this system is the electrical analogue of the classical spin liquid known in frustrated magnetic systems.

The perovskite material SrTiO₃ lies close to a quantum critical point separating ferroelectric and paraelectric phases at zero temperature. The presence of a proximate quantum critical point makes these materials highly tunable via, for example, lattice strains, chemical doping, applied electric fields, and isotope substitution. This tunability extends to electron-doped versions of these materials and raises the possibility of bespoke and tunable electronic devices. In this talk, I will review our theoretical modeling of novel phenomena that may emerge at oxide interfaces involving quantum paraelectrics and show that electron doping adds an additional and interesting axis along which these systems may be tuned.

Perovskites are versatile materials with applications in electronics, catalysis or photovoltaics. While bulk properties of perovskites are widely studied, most applications rely on the materials surfaces, which possess distinctly different physical and chemical properties. The investigation of perovskite surfaces is challenging due to the ternary composition of the material and frequent presence of ferroelectricity, which changes the widely accepted rules for the atomic arrangement of surface ions.

The talk will focus on the investigation of perovskite surfaces at the atomic scale by noncontact atomic force microscopy and scanning tunnelling microscopy, mainly focusing on SrTiO₃, KTaO₃ and BaTiO₃. The rules for surface structure will be discussed, as well as the behaviour of charge carriers and relation to surface chemical properties.

Domain structures in ferroelectrics have a decisive effect on their fundamental physical properties and functionality. A possible way of the formation of domain configurations with desired characteristics is the controlled quenching from a high-temperature paraelectric phase to a low-temperature ferroelectric one. This is a stochastic process which significantly depends on initial conditions and even very weak external influences (thermal, electrical, mechanical) due to the high nonequilibrium of the quenched system. Understanding the mechanisms of domain self-assembly opens up the possibility of directed use of the quenching to control domain structure.

In my talk, I will present a stochastic model allowing to describe and predict the process of domain structure formation based on the Landau-Ginsburg-Devonshire theory. I will show how it is used to describe the known experiments on observation the kinetics of domain formation in TGS crystal. Finally, I will outline the challenges we will face on the way to describe other ferroelectric materials.

The high temporal and spectral resolution of 2D electronic spectroscopy (2DES) makes it an ideal tool for the study of exciton states and dynamics in size-disperse quantum systems such as colloidal quantum dots (QDs). I will give an overview of our 2DES work on CdSe QDs with a broad size distribution, including relaxation and trapping, phonon beatings, and size effects on cross peaks and biexciton shifts.

We will follow a historical overview on the solvability of the polynomial equations by radicals. Starting from the Mesopotamian mathematical culture, through vigorous growth of mathematics in Renaissance, work of Newton, Lagrange, Vandermonde. The talk will finish in the 19th century when first Abel and then Galois have shown that it is impossible to solve the quintic equation by using no operations other than addition, subtraction, multiplication, division, and the extraction of roots. An introduction to the Galois theory will be presented with emphasis on the algebraic (or polynomial equations) and the groups associated with them.

Scanning probe microscopy techniques are capable of providing nanoscale, spatially resolved information of a sample’s surface. Far from just topography, when combined with a magnetically coated tip such as in the case of magnetic force microscopy, they are also capable of studying locally resolved magnetic behaviours. The drawbacks, however, are clear – namely the question of how broadly can a locally resolved understanding be applied to the macroscopic properties of a material system? Multiferroics are a class of materials that exhibit more than one form of ferroic ordering – i.e. a spontaneous strain (ferroelasticity), electrical polarisation (ferroelectricity) or magnetisation (ferromagnetism). When more than one ferroic ordering is present, there is the potential for them to interact with each other to different extents, both on a local and global scale. This talk will use the vibrant playground of multiferroic materials to explore the local limitations of scanning probe microscopy, and what tricks we have in the toolbox to attain a more representative understanding from beneath the tip.

Halide perovskites exhibit outstanding opto-electronic properties e.g. long carrier lifetime and low defect densities. They have been attracting attention due to their solar power conversion efficiencies (surpassing sometimes 30%) outperform those of traditional materials such as silicon and therefore may well determine the future of photovoltaics.

It is well known that the dynamic structural instabilities in these materials are ubiquitous - in the Raman spectrum, they even show anharmonic thermal fluctuations resulting in diffuse inelastic scattering that increases towards 0 cm^-1, which is usually the signature of a liquid [1]. This suggests that they could be associated with opto-electronic properties, but unfortunately previous attempts link dynamic structural instabilities to the opto-electronic properties have been incomplete [2, 3]. In our fundamental theoretical work, we introduce the concept of dynamic tilting based on a number of well-documented characteristic experimental signatures [4]. We show how dynamically unstable materials can be stabilized at T > 0, how this can give rise to new crystal symmetries in perovskites, and most importantly, how these dynamical instabilities can explain important properties of highly efficient photovoltaic materials.

References
[1] Gao, L., et al. "Metal cation’s lone-pairs increase octahedral tilting instabilities in halide perovskites." Materials Advances 2 (2021): 4610-4616.
[2] Steele, J.A., et al. "Role of electron–phonon coupling in the thermal evolution of bulk rashba-like spin-split lead halide perovskites exhibiting dual-band photoluminescence." ACS energy Lett. 4 (2019): 2205-2212.
[3] Marronnier, A., et al. "Structural instabilities related to highly anharmonic phonons in halide perovskites." J. Phys. Chem. Lett. 8 (2017): 2659-2665.
[4] Adams, D. J., and Churakov, S. V. "Classification of perovskite structural types with dynamical octahedral tilting." IUCrJ 10 (2023)

Barium titanate (BaTiO₃, BT) and its substituted derivatives have garnered significant attention for their diverse range of applications, owing to their remarkable properties. This presentation discusses the theoretical characterization of pure BT and its substituted variants, which includes the employment of density functional theory (DFT) calculations and the utilization of effective Hamiltonians. The talk addresses the development and parameterization of these effective Hamiltonians and showcases their practical application for computing various properties.

BaZrO₃ perovskite has a tolerance factor close to 1.01 and is considered to have an ideal cubic structure which remains on average cubic down to at least 2 K. The results of first principles calculations are, however, inconclusive, with some authors predicting out-of-phase [ZrO₆] octahedra rotations [1] while others predicting a cubic symmetry [2]. Most recently Levin et al. [3] have confirmed the a⁰a⁰c^– type oxygen octahedral rotations with a coherence length of about 3 nm by total neutron scattering data and electron diffraction. Doping with small amounts of Nb or heating above 80 K causes disappearance of these structural distortions [3]. In contrast, Raman and FIR data on BaZrO₃ indicate local low-symmetry distortions that persist beyond 700 K [4].

Early reports on MW dielectric properties of BaZrO₃ indicated an unusually low Q-factor. In this presentation, I will address possible reasons of high microwave loss in BaZrO₃ ceramics with special focus on low-temperature proton dynamics in this fascinating perovskite.

[1] A. Bilic et al., Ground state structure of BaZrO₃: A comparative first-principles study, Phys. Rev. B 79, 174107 (2009).
[2] A. Perrichon et al., Unraveling the ground-state structure of BaZrO₃ by neutron scattering experiments and first-principles calculations, Chem. Mat. 32, 2824 (2020).
[3] I. Levin et al., Nanoscale-correlated octahedral rotations in BaZrO₃, Phys. Rev. B 104, 214109 (2021).
[4] D. Nuzhnyy et al., Broadband dielectric response of Ba(Zr,Ti)O₃ ceramics: From incipient via relaxor and diffuse up to classical ferroelectric behavior, Phys. Rev. B 86, 014106 (2012).

The ability to convert an electrical field into a mechanical perturbation and vice versa makes piezoelectric materials fundamentally interesting objects of study as well as versatile components for industrial applications. In recent years, research on piezoelectric materials for biomedical applications, as for nerve and bone tissue repair, in vivo sensors or energy harvesting components, has gained significant momentum. However, the boundary conditions that must be met to make these materials work in an in vivo environment are quite different to the ones in their established industrial applications. The main challenge here is that the implanted materials must be biocompatible. This is a concept that goes way beyond simple chemical toxicity but covers all aspects that influence the safe performance of a material at the implant site under the complex conditions that the body imposes. Material and implant design have to be re-thought to match these requirements e.g. in terms of cytotoxicity and structural stability in the presence of body fluids.

We will take an in-depth look at the biocompatibility and applicability of piezoelectric ceramics intended for hard tissue implants. The boundary conditions that the body imposes and how these influence the interplay between implant material and living tissue will be discussed. We will cover studies on cytotoxicity as well as approaches to material design to meet the requirements at the implant site. You will, furthermore, get an overview of some of the steps we have taken to enable piezoelectric BaTiO₃ and (K,Na)NbO₃ ceramics for bone implant applications, e.g. to understand their chemical stability in body simulating fluids and the impact of sterilization routines on their functional performance.

Piezoelectric materials show great promise for a wide range of biomedical applications. Each comes with their specific set of biochemical and mechanical boundary conditions. To transfer piezoelectric materials safely into the biomedical realm, a fundamental understanding of the complex interplay between them and their host environment before and during the implantation period is crucial – it’s a wide field of research!

This project aims to investigate the XYZ spin ½ chain, to extract new properties of this model, through its exact analytical solution. It captures the most generic, anisotropic, magnetic interaction in a one-dimensional system and it constitutes a reference system to better understand quantum magnetism and related phenomena. Starting from this spin chain, defined on a discrete lattice, we will study its odd number of sites sector to analyze frustration (i.e. imposing frustrated boundary conditions) and its implications. This sector allows investigating a part of the model’s Hilbert space usually inaccessible and largely overlooked in the past, which has been recently shown to host different properties compared to the even number of sites case. The standard Bethe ansatz procedure cannot be applied to the XYZ chain with an odd number of sites. Thus, our solution relies on the recent developments of Bethe ansatz techniques, namely the “off-diagonal” Bethe ansatz. This method starts from the TQ relations, which concern the Transfer matrix of the eight-vertex model. These identities are then generalized to the odd N case introducing an inhomogeneous term, and they lead to Bethe equations whose solutions identify the eigenstates of the system. So, it is possible to recover the energy spectrum of the Hamiltonian and compare it to exact diagonalization numerical results. Then, in the continuum limit, the XYZ chain maps in the famous sine-Gordon model, but this mapping is non-trivial. Studying the frustrated boundary condition sector of the chain will allow examining the field theory model, especially the behavior of topological excitations, known for their robustness. The treatment will then focus on out-of-equilibrium settings, which are important also for applications, such as quantum information and technologies.

X-ray diffraction shows that, if quenched to 295 K from high enough temperature, highly disordered Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2O (HEO) forms a single-phase rocksalt structure. Various probes indicate that the the material exhibits a mixture of long range order (oxygen sublattice) and disorder (random cation sublattice). In addition, long range antiferromagnetic order below T_N ≈ 120 K has been demonstrated using neutron scattering despite the fact that only 60% of the ions have moments. In this work, we study the vibrational modes of HEO experimentally using Infrared and Raman spectroscopy and theoretically using the General Utility Lattice Program. New empirical interatomic potentials (EIPs) are developed for the parent binary oxides MgO, CoO, CuO, NiO, and ZnO by fitting to experimental phonon frequencies, dielectric constants, and lattice parameters. The simulated vibrational density-of-states of the parent binary oxides are in agreement with inelastic neutron scattering data and Density Functional Theory calculations and improve upon existing potentials. The EIPs are utilized in HEO by neglecting cation-cation interactions. The simulated infrared and Raman spectra of HEO are in agreement with experimental data. In 3D materials, disorder can produce mode localization as in amorphous Si, where lattice dynamical calculations show that some of the phonon eigenvectors, corresponding to modes above a threshold frequency, are characterized by exponential decay. Furthermore, the onset of phonon localization for mode frequencies above a high-frequency mobility edge in the Vibrational Density of states is accompanied by a participation ratio (PR) < 0.1. We consider the application of these concepts to HEO. The main result is that a smaller percentage of modes are localized in HEO than in a cluster of α-Si with a similar number of atoms. Additionally, we show that the number of localized modes increases if the ordered oxygen sublattice is disturbed by the random substitution of sulfur atoms.

In a magnetoelectric-antiferromagnetic LiCoPO₄ single crystal, we established a single-domain state by illuminating the crystal by light while cooling it through the Néel temperature. The experimental results and symmetry analysis indicate that the thermal gradient and resulting heat flow are responsible for this novel effect. Such a domain selection is a consequence of the magnetoelectric anisotropy in diffusive transport [Phys. Rev. Lett. 94, 016601 (2005)], i.e. nonreciprocal propagation of thermally-activated quasi-particles in different antiferromagnetic domains. The microscopic mechanism behind the observed effect and the dynamics of the process are however not clear at the moment, calling for future theoretical modeling.

Magnetic van der Waals (vdW) materials are subjects of intense interest due to their potential use in spintronic and optoelectronic devices (see [1] and references therein). While the two-dimensional long-range ferromagnetism is destroyed at finite temperatures by thermal fluctuations in an isotropic Heisenberg system, strong magnetocrystalline anisotropy promotes a stable ordering in a 2D-limit. Such long-range ferromagnetism has been experimentally confirmed in atomically thin layers of e.g. CrGeTe₃ [2] or CrI₃ [3]. As the number of known materials with these unique properties was limited, a renaissance of the vdW transition metal trihalides MX₃ (M − d-metal, X-halogen atom) research began.

Most transition-metal trihalides are dimorphic. The chromium-based triad, CrCl₃, CrBr₃, CrI₃, is characterized by the low-temperature trigonal phase while the structure of the high-temperature phase is monoclinic. The structural transition between the two crystallographic phases is of the first-order type with large thermal hysteresis. CrI₃, the most intensively studied trihalide, is an Ising-type ferromagnetic semiconductor with the highest T_C = 61 K. Also its Cr counterpart, CrBr₃ exhibits ferromagnetism whereas CrCl₃ becomes antiferromagnetic.

On the contrary, in the vanadium trihalides, the high-temperature phase has a higher symmetry. In VI₃, rapid progress has led to the subsequent discovery of ferromagnetism in which the unusual magnetocrystalline anisotropy accompanied by two magnetic and structural transitions points to a strong magneto-elastic coupling in these materials [4]. The recent results on VBr₃ prepared in a single-crystal form for the first time, show that the material orders antiferromagnetically with an unusual presence of tricritical point in its phase diagram.

Except transition metal trihalides, also first experimental results reveal the ground state properties of novel actinide trihalides, such as the UI3 antiferromagnet [5].

In the talk, I will briefly introduce some of the recent discoveries of these vdW materials studied by various methods including low-temperature XRD and spectroscopy techniques.

[1] X. Jiang et al., Appl. Phys. Rev. 8, 031305 (2021); K. S. Burch et al., Nature 563, 47 (2018).
[2] C. Gong et al., Nature 546, 265 (2017).
[3] B. Huang et al., Nature 546, 270 (2017).
[4] A. Koriki, M. Kratochvílová, et al., Phys. Rev. 103, 174401 (2021); P. Doležal, M. Kratochvílová et al., Phys. Rev. Matter. 3, 121401(R) (2019) ; J. Valenta, M. Kratochvílová et al., Phys. Rev. Matter. 103, 054424 (2021).
[5] D. Hovancik, M. Kratochvílová, et al., J. Sol. State Chem. 316, 123580 (2022).

ABO₃-type oxides constitute an important class of materials for applications in many areas of modern technology and are of fundamental scientific interest. Despite their chemical simplicity, these perovskite compounds are crystallographically subtle, because they exhibit a wide variety of complex structural instabilities, including ferroelectric, antiferroelectric, and antiferrodistortive distortions. The competition between these various instabilities manifests itself in different ways, depending on the chemical species involved, leading to the unusual variety and richness of microscopic structures. Among the different theoretical methods, the shell model is an effective tool for gaining fundamental insights into the underlying mechanisms behind their microscopic properties while providing a quantitative connection to ab-initio calculations and mesoscale models. Here, we will present the development of a potential for BaTiO₃, as well as its extension, in order to describe the effects of Mg impurities in its composition and to analyze the possibility of a low-temperature non-cubic phase in BaZrO₃. Limitations and possible future directions in the atomic-level simulations are outlined.

High harmonic generation (HHG) in solids reveals information about the nonlinear dynamics of electrons in condensed media exposed to strong nonresonant light fields. There are two mechanisms, which are believed to be the main sources of solid-state HHG. The first source is interband polarization associated with the coherent electron-hole wavepackets excited by the strong driving field via quantum tunneling. The second source is related to anharmonic intraband dynamics of electrons and holes caused by nonparabolicity of the energy bands. Both these mechanisms are strongly connected to the band structure of the studied material because the electron and hole are promoted to high energy bands by the oscillating laser field.

In this talk I will discuss how the Van Hove singularities in the band structure of silicon affect the angular polarization anisotropy of reflected HHG spectra in deep ultraviolet spectral region. In the second part I will introduce attosecond high harmonic interferometry and discuss its application on investigation of attosecond delays of high harmonic emission in silicon.

Despite the unquestionable importance of water in almost all chemical, biological and geological processes, its molecular structure has not yet been fully resolved. Recent experimental studies have shown that interfacial water formed on surfaces of various synthetic materials exhibits unique physicochemical properties such as the long-range exclusion of solutes and microspheres, high refractive index, and charge separation susceptible to incident electromagnetic energy

In this seminar, I will present my project that will be conducted in the Department of Dielectrics and involves experimental investigation of water layers next to different materials, at a molecular level, by employing dielectric, THz and Raman spectroscopic techniques. This project aims to bridge the gap between diverse experimental and theoretical works on unique water properties.

In 2012, the last missing piece of the Standard Model (SM) – the Higgs boson – was finally experimentally detected at the LHC. However, this much celebrated discovery also marked the beginning of what is sometimes called the “nightmare scenario at the LHC”. We are certain that the SM is incomplete as, for example, neutrino oscillations and dark matter indicate, yet high-energy collision experiments have not lead to anything new after the great Higgs discovery.

In this situation, precise low-energy tests of the SM offer one of the viable possibilities how to proceed further in our exploration of fundamental physical laws. For example, one could focus on a very subtle effect known as parity violation. As we will show, this effect arises from Glashow-Weinberg theory of electroweak interactions and in atoms is quantified via a so-called atomic parity non-conservation (PNC) amplitude. The comparison of the theoretical and experimental values of the PNC amplitude constitutes one of the most stringent tests of the SM. By reviewing current theoretical and experimental data, we will see, however, that the theoretical results lag behind the latter. Thus, clearly, a more accurate theoretical determination is necessary.

Scanning near-field optical microscopy (SNOM) is an imaging technique that uses the apex of a very sharp tip for the observation of nanostructures with lateral resolution much beyond the diffraction limit. With this method we can extract the local properties of materials like dielectric permittivity and conductivity. In this work I will describe the principle of operation of scattered-type SNOM operating at THz frequencies and different theoretical models used to determine the local material properties.

Magnetoelectric and multiferroic materials are promising building blocks for energy-efficient memory devices as they allow electric-field control of the magnetic order. In this thesis, I focused on the dynamic magnetoelectric effect, which gives rise to electric-dipole-active magnon excitations and non-reciprocal light absorption. I studied the spin excitations of several magnetoelectric and multiferroic compounds such as Y- and Z-hexaferrites, Ba₂CoGe₂O₇ and LiCoPO₄, by infrared and terahertz spectroscopy. Based on the measured spectra, I determined the selection rules of the spin excitations and found electromagnon as well as magnetoelectric spin excitations. For magnetoelectric spin excitations, I observed non-reciprocal light absorption that I used it to detect antiferromagnetic domains of Ba₂CoGe₂O₇ and LiCoPO₄. Finally, I demonstrated the control of the magnetic order and domain composition by static (in Ba₂CoGe₂O₇) as well as by oscillating electric and magnetic fields [in LiCoPO₄ and possibly in Y-hexaferrite Ba_0.2Sr_1.8Co₂(Fe_0.96Al_0.04)₁₂O₂₂]

The aim of this doctoral thesis is to begin the development of thin magnesium wires for implant applications. It consists of finding the suitable alloy and discussing the possibilities to prepare the wire via direct extrusion. This includes characterization of the microstructure, impurities, mechanical properties, processing conditions, twinning and texture-related anisotropy. A method for effective preparation of thin pure magnesium wires is presented and the effect of processing parameters on the wire properties is studied. An attempt was made to utilize dispersion of quasicrystalline icosahedral phase to obtain better alloys. Due to the difficulties that arise with extensive alloying and subsequent production of thin wires a Mg-0.4Zn alloy was chosen as the initial material. The results of corrosion tests in artificial body fluids made necessary the employment of a coating that slows down the initial degradation processes and makes further surface functionalization possible. A copolymer of L-lactide and ε-caprolactone is used to further improve the corrosion properties of the wires. To overcome the variation in tensile properties connected with occasional MgO impurity, wire ropes are braided. Based on the results, these ropes could be potentially used for sternal fixation in pediatric patients, where the physiologic loads on the sternum are not as severe as for adults. A possibility of a biodegradable sternal fixation would be beneficial to minimize post-sternotomy pain syndrome and suppress complications when multiple open-heart surgeries are required.

In this talk I will show a set of layered materials discovered and studied by means of first-principles calculations. They belong to the family of BiS₂, which has a ‘wrong’ stoichiometry, implying a low-symmetry layered unit cell. In the bulk, several members of this family are promising as photocatalysts for the water splitting reaction. However, their monolayer exhibits a large exciton binding energy, which could be useful for optoelectronic applications, such as light harvesting.

Most of us have probably already heard terms like “topological insulator”, “Weyl semimetal”, “Berry curvature”, “robust surface states”, “nodal-line”, and others in some talk or read them in a publication. Possibly, not everyone is familiar with this field, even though it became very present in the solid state research in the last years. In this seminar series, I will give an introduction into the topic, try to explain the most important concepts and show where could topological materials be useful.

In this informal seminar I will point out a few new considerations (and hints) for accurate determination of THz dielectric / conductivity spectra of thin films.

A brief informal summary of what we have recently learned and what would be great to know about the stability and formation processes of charged domain walls in materials like PbTiO₃ and BaTiO₃.

Most of us have probably already heard terms like “topological insulator”, “Weyl semimetal”, “Berry curvature”, “robust surface states”, “nodal-line”, and others in some talk or read them in a publication. Possibly, not everyone is familiar with this field, even though it became very present in the solid state research in the last years. In this seminar series, I will give an introduction into the topic, try to explain the most important concepts and show where could topological materials be useful.

Most of us have probably already heard terms like “topological insulator”, “Weyl semimetal”, “Berry curvature”, “robust surface states”, “nodal-line”, and others in some talk or read them in a publication. Possibly, not everyone is familiar with this field, even though it became very present in the solid state research in the last years. In this seminar series, I will give an introduction into the topic, try to explain the most important concepts and show where could topological materials be useful.

Most of us have probably already heard terms like “topological insulator”, “Weyl semimetal”, “Berry curvature”, “robust surface states”, “nodal-line”, and others in some talk or read them in a publication. Possibly, not everyone is familiar with this field, even though it became very present in the solid state research in the last years. In this seminar series, I will give an introduction into the topic, try to explain the most important concepts and show where could topological materials be useful.

The possibility to create new materials bottom-up was enhanced via the stacking of atomically thin layers of two-dimensional (2D) materials with van der Waals interactions (vdWIs). Unfortunately, the downside of vdWIs is the, in general, weak electronical and mechanical interaction between the individual layers. This hinders the creation of multiferroic materials working at ambient conditions, which are despite extensive research still very scarce, from stacked layers with vdWIs.

My research aims to create a new building block, the 2D sandwich, to tackle this problem. In this seminar, I will introduce myself and explain why we would need 2D sandwiches, what are 2D sandwiches and how I will bake 2D sandwiches.

Topological polar solitons such as domain walls, vortices, and polar skyrmions in ferroelectrics and related heterostructures have attracted increasing attention owing to their unique functionalities and potential applications in electronic devices. Recent advances in transmission electron microscopy (TEM) enable us to study the atomic structure, composition, properties, and dynamic behaviors of materials and nanostructures at the atomic scale. In this talk, I will show how ferroelectric domains nucleate and evolve under applied field in TEM. The polarization of nanodomains, vortices and other polar solitons can be imaged with atomic resolution. With the novel four-dimensional scanning TEM diffraction imaging (4D STEM) method developed recently, we are able to measure charge density, dipole moment, valence electron distribution in nanocrystals such as topological polar solitons, or at interfaces and single defects with sub-angstrom resolution.

In this talk, I will briefly cover topics I have been interested in during my research. These include bulk and surface electronic structure calculations of topological materials, surface anomalous Hall transport, and ab-initio calculations of the electronic structure of semiconductor heterostructures.

Antiferroelectricity has attracted a growing interest in the last years, notably due to its relevance and potential in applications such as energy storage or cooling via electrocaloric effect, amongst others. However, the very notion of antiferroelectricity is still hard to define (and to agree upon!) and it has different uses across our community.
In this tutorial, I will first highlight some of the problems encountered while trying to define what an antiferroelectric is. I will also describe some fundamental properties of antiferroelectricity and what common pitfalls one can fall in. Here mostly inorganic antiferroelectric will be taken as examples, with a few organic ones too though I won't talk about liquid crystals.
I intend this seminar to be a pedagogical (and hopefully interactive) tutorial, so if you are already an expert in antiferroelectrics, you might not learn much. Also, I probably won’t have time to discuss many application concepts, synthesis or current work, but if you are interested in this, I could arrange a complementary seminar later on.

Ferroelectric domain walls are atomically narrow planes that can behave very differently from the surrounding bulk ferroelectric material. For example, the domain walls in many ferroelectrics can collect and conduct charge carriers despite the insulating nature of the host material. Domain walls can be created, moved, and removed again in a controlled way, thus they can be used to tailor the electronic properties of the ferroelectric. Charge carriers that accumulate at domain walls may lead to metallic or semiconducting charge transport characteristics depending on whether they are delocalized or form self-trapped small polarons. The latter may be detected, for example, as deep levels within the band gap in optical absorption or photoluminescence spectra. First-principles materials modeling can help to interpret experimental findings and make predictions where experimental data are not available. In my talk I will present insights into polaron formation at ferroelectric domain walls in BiFeO₃ gained from first-principles modeling.

The discovery of atomically thin graphene with exceptional photophysics sparked a flurry of research into other two-dimensional (2D) layered materials, including metals, semiconductors, and insulators. The weak interaction between the layers allows for exfoliation and also allows for a wide range of 2D materials to be stacked to create artificial heterojunctions known as van der Waals (vdW) heterostructures. Such heterostructures offer new possibilities to engineer the interactions/coupling between the different material layers exhibiting unique characteristics. We find that large area MoS₂ monolayer sandwiched between two graphene layers makes this heterostructure optically active even much below the band gap of MoS₂. Ultrafast optical pump – THz probe experiment reveals in real-time the transfer of carriers between graphene and MoS₂ monolayer upon photoexcitation with photon energies down to 0.5 eV. It also helps to understand the significant enhancement in the transient THz response from the heterostructure as compared to individual monolayers. We proposed possible mechanism which can account for this phenomenon. Such vdW heterostructures, specially, those having a transition metal dichalcogenide monolayer sandwiched between two graphene layers or vice-versa, would greatly expand the breadth of current photonic and optoelectronic research and applications at wide-band frequencies.

Terahertz (THz) technology has been used for almost two decades for investigations of nanomaterials, including nanocrystals, nanoparticles, nanowires, nanotubes, or 2D crystals [1]. Very recently, 2D MXenes (transition metal carbides structurally analogous to graphene) have shown remarkable impact in material research due to their metallic conductivity, large specific surface area, charge transport properties, and strong interaction with the THz radiation.  Under controlled synthesis conditions, 2D Titanium Carbide MXene (Ti₃C₂T_x) flakes are prepared from the bulk parent Ti₃AlC₂ MAX phase by chemical etching [2]. I will discuss in this talk how the THz properties of this MXene strongly depend on nanoscopic charge carrier transport inside the sheets and on the percolation properties of the fabricated structures.

[1] P. Kužel et al., Terahertz spectroscopy of nanomaterials: a close look at charge-carrier transport, Adv. Opt. Matter. 8, 1900623 (2020).
[2] M. Naguib et al., Two-dimensional nanocrystals produced by exfoliation of Ti3AlC2, Adv. Mater. 23, 4248 (2011).

Direct electronic probing of the band-bending profile in semiconductor nanostructures and its impact on the charge transport is a great challenge, since attachment of probing electrodes inevitably modifies the properties of the semiconductor surface. An efficient tool for such investigations may be the terahertz (THz) spectroscopy enabling contactless probing of the pristine surface.

We used time-resolved THz and multi-THz spectroscopies to assess the ultrafast photoconductivity in an array of aligned GaAs nanobars (prepared by MBE growth and e-beam lithography) at 300 and 20 K. These experiments were complemented by time-resolved THz local-probe measurements (THz-SNOM) within a single nanobar and by quantum-based calculations of the THz conductivity of confined electrons.

Our investigations reveal prominent effects of the band bending close to the nanobar surfaces on the picosecond charge carrier dynamics and on the confinement of electrons.

Caloric effects resulting from the structural response to applied fields have been widely studied in the recent years due to their potential as an alternative to refrigeration technologies such as gas-compression. More specifically, the electrocaloric effect is the temperature change upon application or removal of an electric bias, while mechanocaloric effect (either elasto or barocaloric) is the analogue for the application or removal of mechanical stress. In this seminar, I introduce a perturbative approach to compute and understand both of these effects, with an explicit application to PbTiO₃, as a prototypical ferroelectric [1,2], through Monte Carlo simulations and second-principles methods [3].

References
[1] M. Graf and J. Íñiguez, Commun. Mater., 2, 60 (2021).
[2] D. E. Murillo-Navarro, M. Graf, and J. Íñiguez, Phys. Rev B. 104, 184112 (2021).
[3] J. C. Wojdel, P. Hermet, M. P. Ljungberg, P. Ghosez, and J. Íñiguez, J. Phys.: Condens. Matter. 25, 305401 (2013).

Outline

• Drude weight.
• Orbital magnetization.
• Outline of the geometrical observables beyond band-structure theory (for correlated and/or noncrystalline systems).
• Main concepts in the geometrical theory of the insulating state.

• R. Resta, Geometry and Topology in Electronic Structure Theory, Lecture Notes, www-dft.ts.infn.it/~resta/gtse/draft.pdf
• D. Vanderbilt, Berry Phases in Electronic Structure Theory (Cambridge University Press, Cambridge, 2018)
• R. Resta, Geometry and topology in many-body physics, Lecture Notes, www.cond-mat.de/events/correl20/manuscripts/resta.pdf
• R. Resta, Theory of nonlinear conductivity, longitudinal and transverse, arxiv.org/abs/2111.12617.

Outline

• Modern theory of polarization.
• Historical developments.
• Polarization as a Berry phase.
• The single-point Berry phase.
• Quantization of charge transport in electrolytes.
• Anomalous Hall conductivity, linear and nonlinear.

• R. Resta, Geometry and Topology in Electronic Structure Theory, Lecture Notes, www-dft.ts.infn.it/~resta/gtse/draft.pdf
• D. Vanderbilt, Berry Phases in Electronic Structure Theory (Cambridge University Press, Cambridge, 2018)
• R. Resta, Geometry and topology in many-body physics, Lecture Notes, www.cond-mat.de/events/correl20/manuscripts/resta.pdf
• R. Resta, Theory of nonlinear conductivity, longitudinal and transverse, arxiv.org/abs/2111.12617.

Outline

• Quantum geometry (a.k.a. Berryology): connection, curvature, metric, Berry phase, Chern number.
• Berryology in band structure.
• Synopsis of the geometrical observables, partitioned in two classes: Observables defined modulo 2π (in dimensionless units), and observables exempt from 2π ambiguity.

• R. Resta, Geometry and Topology in Electronic Structure Theory, Lecture Notes, www-dft.ts.infn.it/~resta/gtse/draft.pdf
• D. Vanderbilt, Berry Phases in Electronic Structure Theory (Cambridge University Press, Cambridge, 2018)
• R. Resta, Geometry and topology in many-body physics, Lecture Notes, www.cond-mat.de/events/correl20/manuscripts/resta.pdf
• R. Resta, Theory of nonlinear conductivity, longitudinal and transverse, arxiv.org/abs/2111.12617.

Outline

• What topology is about
• Gauss-Bonnet theorem
• Parallel transport
• Quantization of the surface charge
• Quantum Hall effect
• Flux and flux quantum
• Aharonov-Bohm effect

• R. Resta, Geometry and Topology in Electronic Structure Theory, Lecture Notes, www-dft.ts.infn.it/~resta/gtse/draft.pdf
• D. Vanderbilt, Berry Phases in Electronic Structure Theory (Cambridge University Press, Cambridge, 2018)
• R. Resta, Geometry and topology in many-body physics, Lecture Notes, www.cond-mat.de/events/correl20/manuscripts/resta.pdf
• R. Resta, Theory of nonlinear conductivity, longitudinal and transverse, arxiv.org/abs/2111.12617.

The Born effective-charge tensors are a staple of harmonic lattice dynamics in insulating crystals, and of the ab-initio theory of charge transport in insulating liquids (molten salts and electrolytes). The so-called acoustic sum rule mandates that they sum to zero over the crystal cell (or the supercell). A recent preprint [1] proposes to extend their definition to metals, where it is discovered that they no longer add up to zero; instead, their sum is proportional to the Drude weight. Before presenting a personal view [2] of this interesting result, I will illustrate some fundamental features of the Born effective charges in insulators, and of the Drude weight in metals. The possible relevance of this result in the case of polar metals and/or ferroelectric metals will be tentatively discussed at the end.

References
[1] C. E. Dreyer, S. Coh, and M. Stengel, Nonadiabatic Born effective charges in metals and the Drude weight, arXiv:2103.04425v1 [cond-mat.mtrl-sci] (2021).
[2] R. Resta, Theory of nonlinear dc conductivity, longitudinal and transverse, arXiv:2111.12617 [cond-mat.mtrl-sci] (2021).

Nobel Prize for Physics was awarded this year to three scientists for their groundbreaking contributions to our understanding of complex physical systems. One half of the Prize went to Giorgio Parisi for the discovery of the interplay of disorder and fluctuations in physical systems from atomic to planetary scales. I will introduce G. Parisi as one of the most creative theorists in last decades and delineate his way towards the Nobel Prize via the mean-field theory of spin glasses. Starting with experimental motivation, I will go through the early problems with theoretical models to the Parisi hierarchical construction representing the exact solution of the mean-field Sherrington-Kirkpatrick model of spin glasses. I will end up with the interpretation of the non-measurable order parameters of the Parisi solution and the relevance of the Parisi construction for general complex systems not only in physics.

The Faraday laws of electrolysis state that the charge transported by a solvated ion between two electrodes is an integer multiple of the elementary charge e. Why this happens is far from obvious, because liquids are not assemblies of ions: they are assemblies of atoms, having ionic character only because the neighboring atoms have different ionicity. There is no way of extracting integer charges from a "snapshot" of the electronic charge distribution at a given time. Instead, integer charges manifest themselves only when the nuclei are adiabatically transported over macroscopic distances: a playground where topology--since Thouless' seminal work in the 1980s--has a major role.

[1] R. Resta, “ Faraday law, oxidation numbers, and ionic conductivity: The role of topology”, , arXiv:2104.06026 [cond-mat.mtrl-sci] (J. Chem Phys., in press).

Epitaxial growth of graphene on silicon carbide (SiC) provides a unique opportunity to prepare and study several carbon allotropes. The Si-face of SiC can host four allotropes, namely buffer layer (BL), single-layer graphene (SLG), quasi-freestanding SLG (QFSLG), quasi-freestanding bilayer graphene (QFBLG), while the C-face hosts the multilayer epitaxial graphene (MEG). The graphene on SiC provides several routes for strong light-matter interaction in the far-infrared and terahertz spectral range. The Epitaxial graphene (EG) films are inherently inhomogeneous due to the discontinuous domains, wrinkles, and substrate terraces. Hence, the carrier confinement potentials, which activate localized plasmons, emerge naturally in EG due to these inhomogeneities. In contrast, such confinement effects are often achieved artificially by intentional patterning of graphene films. This seminar will discuss the robust plasmonic response and pump-induced transparency at THz frequencies in various EG allotropes with the help of time-resolved terahertz and multi-THz spectroscopies. The study reveals the evolution of graphene plasmonic response (conductivity spectra) due to optical excitation on a picosecond timescale and its direct correlation with the transient temperature (and chemical potential) of the heated electronic subsystem. The access to carrier temperature also helps us understand the energy dissipation mechanisms in electronic and lattice subsystems relevant in Femto-nanosecond timescales.

References
[1] M. M. Jadidi et al., Nonlinear Terahertz Absorption of Graphene Plasmons, Nano Lett., 16, 2734 (2016).
[2] V. C. Paingad et al., Ultrafast plasmon thermalization in epitaxial graphene probed by time-resolved THz spectroscopy, Adv. Funct. Mater. 31, 2105763 (2021).

Antiferroelectricity is the subject of a renewed interest, mostly due to applications such as energy storage, electrocaloric cooling or microelectronics, but also due to a relative lack of fundamental knowledge of simple model antiferroelectric materials. In this presentation, we will detail the synthesis and the in-plane characterization of canonical antiferroelectric lead zirconate PbZrO₃ and the spectroscopic study of the lattice dynamics of candidate antiferroelectric francisite Cu₃Bi(SeO₃)₂O₂Cl.

Complex topological configurations are a fertile arena to explore novel emergent phenomena and exotic phases in condensed-matter physics. The recent discovery of polarization vortices and the associated complex-phase coexistence and response under applied fields in ferroelectric oxide superlattices, has opened up new vistas to explore topology, emergent phenomena, and approaches for manipulating such features with electric fields [1,2]. Here, by varying epitaxial constraints we report the discovery of room-temperature polar skyrmions in a lead-titanate layer confined by strontium-titanate layers [3]. Phase-field modeling and second-principles calculations reveal that the polar skyrmions have a skyrmion number of +1, and resonant soft X-ray diffraction experiments show circular dichroism confirming chirality. Such nanometer-scale polar skyrmions are the electric analogs of magnetic skyrmions, and could advance ferroelectrics towards new levels of functionality [4–6] Using macroscopic dielectric measurements, we demonstrate that polar skyrmions in (PbTiO₃)_n/(SrTiO₃)_n superlattices are distinguished by a sheath of negative permittivity at the periphery of each skyrmion which enables a strong enhancement of the effective dielectric permittivity as compared to the individual SrTiO₃ and PbTiO₃ layers5,6 and phenomenon could be controlled by electric field and temperature. The production of such a steady-state negative capacitance and large field-tunable response has promise for high- frequency electronic applications [6].

References
[1] A. K. Yadav, et al., Observation of polar vortices in oxide superlattices, Nature 530, 198 – 201 (2016).
[2] A. Damodaran, et al., Phase coexistence and electric-field control of toroidal order in oxide superlattices, Nature Mater. 16, 1003 (2017).
[3] S. Das et al., Observation of room temperature polar skyrmions, Nature 568, 368 – 372 (2019).
[4] Q. Li, et al., Collective excitations of polar vortices, Nature 592, 376 – 380 (2021).
[5] A. K. Yadav, et al., Spatially resolved steady state negative capacitance, Nature 565, 468 – 471 (2019).
[6] S. Das, et al., Local negative permittivity and topological phase-transition in polar skyrmions, Nature Mater. 20, 194 (2021).

The ultimate solution of the long-standing polarization problem was arrived at along the 1990s; polarization theory is nowadays routinely implemented in most electronic-structure codes. Notwithstanding, even the basic principles of the theory remain somewhat obscure to many physicists. I will present the fundamentals of the theory and (time permitting) the logic behind a novel derivation [1] of the main formulas.

[1] R. Resta, “From the dipole of a crystallite to the polarization of a crystal”, J. Chem. Phys. 154, 050901 (2021).

The 2011-discovery of ferroelectricity in a 10-nm HfO₂-ZrO₂ solid solution thin film dramatically revived the dying theoretical and experimental researches on ferroelectrics. This discovery, along with the numerous advantages such as simple structure, strong binding energy between oxygen and transition metal ions, wide electronic bandgap (~ 5.3 – 5.7 eV), and more importantly, the compatibility with current complementary metal oxide semiconductor (CMOS) technologies, aroused the extensive research to achieving reliable properties for various potential applications such as ferroelectric memory, ferroelectric field-effect transistors (FeFETs), energy harvesters, pyroelectric sensors. In this presentation, after a comprehensive discussion on the fundamental aspects of ferroelectricity in HfO₂-based thin film, the latest theoretical and experimental progresses towards a reliable ferroelectric film for memory application will be presented.

Perovskite oxides exhibit a rich variety of structural phases hosting different physical phenomena that generate multiple technological applications. We find that topological phonons – nodal rings, nodal lines, and Weyl points – are ubiquitous in oxide perovskites in terms of structures (tetragonal, orthorhombic, and rhombohedral), compounds (BaTiO₃, PbTiO₃, and SrTiO₃), and external conditions (photoexcitation, strain, and temperature). In particular, in the tetragonal phase of these compounds all types of topological phonons can simultaneously emerge when stabilized by photoexcitation, whereas the tetragonal phase stabilized by thermal fluctuations only hosts a more limited set of topological phonon states. In addition, we find that the photoexcited carrier concentration can be used to tune the topological phonon states and induce topological transitions even without associated structural phase changes. Overall, we propose oxide perovskites as a versatile platform in which to study topological phonons and their manipulation with light [1].

Reference:
[1] Bo Peng et al., Topological phonons in oxide perovskites controlled by light, Science Advances 6, eabd1618 (2020).

Important notice: Guests from outside of the Departments of Dielectrics are kindly asked to contact nemec@fzu.cz for access instructions. Link to the webmeeting access will be sent shortly before the seminar.

GaV₄S₈ belongs to the group of lacunar spinels – materials falling into the group of narrow-gap Mott insulators showing a number of interesting physical phenomena like relativistic spin-orbit effects, pressure-induced superconductivity or two-dimensional topological insulator state [1]. Below approximately 50 K lacunar spinels undergo the Jahn–Teller distortion to a rhombohedral R3m state with C_3v symmetry. In the rhombohedral state GaV₄S₈ is a ferromagnet with Curie temperature about 15 K. Its magnetic moments are formed by the V₄ clusters organized in hexagonal layers having spin ½ each. These magnetic moments interact via the exchange interactions as well as Dzyaloshinskii-Moriya interaction (DMI). Importantly, as a consequence of the polar axial symmetry, GaV₄S₈ has been shown to host Néel-type Skyrmions, which form skyrmion lattice in a certain range of applied magnetic fields and temperatures [2].
Recently, it has been shown by atomistic simulations of 2D Heisenberg model with DMI [3] that thermal fluctuations can disturb the order of the solid-like hexagonal skyrmion lattice and induce its melting towards skyrmion liquid. Since skyrmion lattice is essentially a two dimensional (2D) system, it is natural to expect that, in analogy with system of 2D discs, it will undergo topological phase transition described by the Kosterlitz-Thouless-Halperin-Nelson-Young theory (KTHNY). An important result of this theory is the prediction that the transition from the solid-like phase into the liquid one will happen via a so-called hexatic phase. This is, however, not the case in the numerical simulations [3]. On the other hand, analysis of experimental data [4] indicates the existence of the skyrmion hexatic phase in Cu₂OSeO₃. This opens a general question of the mechanism of skyrmion lattice melting in various materials.
In this seminar we will present our theoretical results analysing the skyrmion lattice melting in GaV₄S₈. By means of an atomistic spin model based on ab initio calculations [2] we performed large scale simulations and analysed the positional and orientational correlations of the skyrmions. In our analysis we focused on the existence of the hexatic phase in GaV₄S₈.

References:
[1] A. Camjayi et al., Phys. Rev. Lett. 113, 086404 (2014).
[2] S. A. Nikolaev and I. V. Solovyev, Phys. Rev. B 99, 100401 (2019).
[3] Y. Nishikawa, K. Hukushima, and W. Krauth, Phys. Rev. B 99, 064435 (2019).
[4] P. Huang et al., Nature Nanotechnology 15, 761 (2020).

Important notice: Guests from outside of the Departments of Dielectrics are kindly asked to contact nemec@fzu.cz for access instructions. Link to the webmeeting access will be sent shortly before the seminar.

Important notice: Due to the current pandemic situation, access to the seminar is restricted to 10 members of the Departments of Dielectrics, preferably those working actively in the subject of the seminar. Guests from outside of the Departments of Dielectrics and other audience is kindly requested to connect via remote access. For the remote access, please contact nemec@fzu.cz. Link to the webmeeting access will be sent shortly before the seminar.

Two nematic liquid crystalline mixtures IRN1 and IRN2, dedicated to advanced electro-optical transducers were developed at the MUT. The IRN1 is an innovative, highly birefringent LC mixture working at VIS and NIR bands. The second LC mixture IRN2 is prepared for working at MWIR range. To obtain IRN2 with low absorption at the edge of MWIR one replaced hydrogen atoms in molecules of IRN1 with heavier ones, preferably with deuterium. To satisfy the requirement for low reflection the components of IRN1 were further selected to reduce the formation of light scattering smectic-like agglomerates.

Ordinary and extraordinary refractive indices of IRN1 and IRN2 in full spectrum range were measured by custom-developed interference method using special “IR cells” with dielectric mirrors. To increase the transmission T of index-matched LC to the level above 97.5% all interfaces of electrooptical transducers were designed as a stack of functional, optically matched layers. The final parameters of prepared electrooptical transducers (reaching an edge of LC technology) and selected applications of developed technology will be presented and discussed. LC mixture IRN1 was used for fabrication of electro-optical transducers dedicated to laser rangefinders for 3D or 5D laser metrology, dense plasma diagnostics and spaceborne laser rangefinder of a landing unit. The second LC mixture IRN2 is prepared for the human breath analyzer where LC shutter works at MWIR range.

I will summarize efforts in the past year to calculate the polarization of skyrmion-host GaV₄Se₈ as it undergoes a hypothetical switching scenario between two of its rhombohedral domain states. Such a “highly distorted” path has not yet been published in the literature and can be helpful to parameterize its Landau energy surface. By using constrained noncollinear magnetism, we find an anomalous jump in the electronic polarization which appears at the central point of this trajectory – when the material is exactly of orthorhombic symmetry. This jump does not arise from changes in the charge density and it can also be shown that by carefully decoupling the ionic degrees of freedom from the trajectory that this discontinuity is solely resulting from the electronic subsystem. In addition to discussing our results, I will also summarize what other teams are publishing with respect to this interesting multiferroic family of materials.

World’s smallest medical robot as the record set on Guinness World Records works on magnetoelectric mechanism in a core-shell heterostructured nanocomposite. Under influence of remotely applied modulated magnetic field, the core and shell of the nanostructure interact with each other to generate high localized electric field. The magnetic body guides and excites nanorobot’s propel. The on demand shell’s electrification gives nanorobot, a capability to act as an ambient electrical field sensor in bio-cellular environment and performs four remotely controlled cellular interactions. Sensing minute electrical energy differences between different cell types, targeted single cell electroporation, usage of electrostatic force to increase nanorobot’s propelling speed, and transport targeted cells to unlimited distance with microscale preciseness in microvascular environment. These phenomenal functions of nanorobot addresses the solution to current technological limits in the treatment methods of diseases such as cancer and Alzheimer’s.
These Nanorobots provide following unique techniques for solving some of the biology’s principle problems:

1. Techniques for fundamental studies of electrical phenomena’s in single cell of different types at nanoscale,
2. Targeted drug delivery (localized treatments without harmful exposure),
3. Transport of surrounding or modified cells for microvascular repair and nerve/neuronal path regeneration,
4. Dynamic and static single cell electroporation studies,
5. Monitoring modified nerves, cells or area.

This talk will summarize experimental and theoretical advances in magnetic and ferroelectric materials at multiple length scales. I will first discuss some recent work on exotic magnetic and polar states observed in BiFeO₃ domain walls [1] and skyrmion states found in PbTiO₃/SrTiO₃ superlattices [2]. Both systems motivate the need for accurate predictive modeling of structural order parameters which need to be carefully coupled to their complex boundary conditions. To that end, I will provide an update on the following theoretical projects that we, at the institute, have been working on:
(i) ab initio calculations of GaV₄Se₈ with the aim of parameterizing its Landau energy surface,
(ii) developing continuum micromagnetic simulations in the Ferret simulation package and
(iii) controlling topological states in ferroelectric PbTiO₈ nanostructures [3].

References
[1] J.-Y. Chauleau et al., Nature Materials doi:10.1038/s41563-019-0516-z (2019).
[2] S. Das et al., Nature 568, 368 (2019).
[3] Y. Tikhonov et al., arXiv preprint arXiv:2001.01790 (2020).
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When a thin ferromagnetic layer is placed on a ferroelectric one, a modulation of magnetic anisotropy can be induced by magnetoelectric coupling between the layers. Namely, ferroelectric domain pattern is imprinted into the ferromagnet and local uniaxial magnetic anisotropy in the ferromagnetic layer rotates between the domains by 90° [1]. As a result, 90° magnetic domain walls are formed at the anisotropy boundaries. Using spin transfer torque effect from an AC spin polarized current, it is possible to drive the pinned magnetic domain wall into high frequency oscillations, which can be used as a tunable source of spin waves [2]. Motivated by a better understanding of static and dynamic properties of the 90° magnetic domain walls, we provide a theoretical study based on 1D analytical model [3]. In accord with micromagnetic simulations, the model predicts active tuning of the domain wall eigenfrequency by a magnetic bias field.

[1] T. H. E. Lahtinen, J. O. Tuomi, and S. van Dijken, Adv. Mat. 23, 3187 (2011).
[2] B. Van de Wiele, S. J. Hämäläinen, P. Baláž, F. Montoncello, and S. van Dijken, Sci. Rep. 6, 21330 (2016).
[3] P. Baláž, S. J. Hämäläinen, and S. van Dijken, Phys. Rev. B 98, 064417 (2018).
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The discussed Wentzel Jamnitzer’s work belongs to works which have been published most frequently throughout the 15-th and 16-th centuries and which depicted Platonic (regular) bodies. It was believed at that time that these bodies comprised the essence of nature of the surrounding world, which was percepted by external senses, or exclusively accessible by intellect. Complexity of the surrounding world could be symbolized by a geometric combination of the Platonic bodies which gave rise to semiregular or even more complicated bodies.

We study the vortex ratchet effects in superconducting asymmetric curved bridges. By transport measurements, we find that the rectified dc voltages are significantly enhanced, and we obtain clearly asymmetric voltage waveforms versus time in one period. The rectification effect can be achieved in a very wide range of temperatures, currents and magnetic fields. Based on the simulations of time-dependent Ginzburg-Landau equations, we demonstrate that the giant ratchet effects are mainly caused by the collective behavior of vortices in this asymmetric structure, which is quite different to the matching effects in conventional vortex ratchet systems.

Progress in microfabrication has allowed fabrication of highly tuneable semiconductor-superconductor heterostructures yielding new platforms to both create and manipulate subgap states. Such bound states, isolated in energy by the superconducting gap, are important building blocks for many schemes of quantum computation. In this talk I will present how bound states arise in Quantum Dot-Superconductor systems, how we can model them and what we can learn from transport measurements. Finally, I will address the complexity in experiments with double quantum dots and the physics it allows us to probe.

Magnetic binary oxides with cubic rocksalt structure are the best candidates in which to experimentally investigate the accuracy of the new mechanisms proposed for the electronic behavior under complex condition. Recently, the effect of strain on the lattice dynamics of magnetic binary oxides attracted a huge attention due to the theoretically predicted emergent phenomena, e.g. colossal dielectric permittivity, ferroelectricity [1] and multiferroicity [2] under epitaxial strain. Moreover, the idea of a purely magnetic order-induced anisotropy in the phonon properties put forth by Massidda et al. [3], demonstrated the substantial contribution of spin-phonon interaction in the lattice dynamics of this group of materials. Since then, there has been increasing interest in spin-phonon coupling within the context of magnetoelectric multiferroics and spintronics, but the mechanisms underlying this phenomenon remained unclear. Luo et al. [4] proposed for the binary oxides that the actual size of the exchange-driven phonon splitting is solely determined in sign and magnitude by the non-dominant exchange J1, while the contributions of the dominant superexchange coupling J2 are canceled. Their idea was experimentally confirmed by Kant et al. [5]. On the other hand, using an ab initio study, Fischer et al. [6] calculated the effect of hydrostatic pressure on the J1 in transition metal monoxides. Consequently, strain can directly affect the lattice dynamics of binary oxide through influence on phonon modes and indirectly on magnetic exchange interaction J1. Here, the recent progress in the pulsed laser deposition and the effect of epitaxial strain on the dielectric properties, lattice dynamics and spin-phonon interaction of three magnetic binary oxides with different chemical stabilities in ambient condition, i.e. NiO, MnO and EuO will be presented.

[1] E. Bousquet , N. A. Spaldin and P. Ghosez, Phys. Rev. Lett. 104 (2010) 037601.
[2] X. Wan, H. C. Ding, S. Y. Savrasov, and C. G. Duan, Sci. Rep. 6 (2016) 22743.
[3] S. Massidda, M. Posternak, A. Baldereschi, and R. Resta, Phys. Rev. Lett. 82 (1999) 430.
[4] W. Luo, P. Zhang, and M. L. Cohen, Sol. State Commun. 142 (2007) 504.
[5] C. Kant, M. Schmidt, Z. Wang, F. Mayr, V. Tsurkan, J. Deisenhofer, and A. Loidl, Phys. Rev. Lett. 108 (2012)177203.
[6] G. Fischer, M. Däne, A. Ernst, P. Bruno, M. Lüders, Z. Szotek, W. Temmerman, and W. Hergert, Phys. Rev. B 80 (2009) 014408.

Perovskite oxides are being increasingly integrated into thin films as functional layers for electrochemical energy conversion, information storage or sensing. Their coupled electro-chemo-mechanical properties determining their performance are often difficult to measure. Here we propose to use vibrational Raman spectroscopy to probe chemo-mechanics by measuring the coupling of the lattice volumetric changes (mechanic) to chemical expansion (chemo). This is done by measuring in situ Raman spectra on an electrochemical cell with a Sr(Ti,Fe)O_3–y working electrode whose oxygen stoichiometry is controlled by an applied bias.

Microtubules are fascinating objects from multiple perspectives. A microtubule is a self-assembling structure with ferroelectric properties [1]. As a discrete electronic device, it is supposed to be the theoretically predicted [2] fourth basic circuit element [3]. Due to convenient geometry, it can serve as a circular waveguide and resonator [4]. Excitation–emission measurements revealed a wide spectrum from kHz up to UV region [5]. Among other functions, a network of microtubules is supposed to serve as the main generator of cellular electromagnetic field [6], providing tissue ordering, information transfer, computer-like brain activity, control of chemical reactions etc.
A microtubule is a nonlinear structure composed of identical tubulin heterodimers containing electric dipoles. We have analysed the space–time coherence using classical dipole theory of generation of near electromagnetic field [7]. We assume that a structure with a spiral and axial periodicity enables interaction of the field components shifted in time by one or more oscillation periods and generation of coherent signals. Supplied energy can be coherently stored in oscillators with a high electrical quality, in the water-containing microtubule inner cavity, and in excited electrons at molecular orbitals and in “semiconduction” and “conduction” bands. The suggested model may be of a general nature, to be possibly applicable to some other helical structures with yet unexplained behaviour.

References:
[1] J. A. Brown and J. A. Tuszyński, Ferroelectrics 220, 141–155 (1999).
[2] L. O. Chua, IEEE Trans. Circuit Theory 18, 507–519 (1971).
[3] A. Adamatzky et al., https://arxiv.org/abs/1810.04981v1 (2018).
[4] F. Jelínek and J. Pokorný, Electro- and Magnetobiol., 20(1), 75–80 (2001).
[5] S. Sahu et al., Biosens. Bioelectron. 47, 141–148 (2013).
[6] J. Pokorný et al., J. Biol. Phys. 23(3), 171–179 (1997).
[7] J. Pokorný et el., Proc. EuMCE, 13–15 May 2019, Prague, Czech Republic.

Entanglement entropies quantify non-locality and operational costs for quantum systems. In ground states of typical condensed matter systems, they obey area and log-area laws. In contrast, subsystem entropies in random and thermal states are extensive, i.e., obey a volume law. For energy eigenstates, one expects a crossover from the groundstate scaling at low energies and small subsystem sizes to the extensive scaling at high energies and large subsystem sizes. We elucidate this crossover. Due to eigenstate thermalization (ETH), the eigenstate entanglement can be related to subsystem entropies in thermodynamic ensembles. For one-dimensional critical systems, the universal crossover function then follows from conformal field theory (CFT). For critical fermions in two dimensions, we obtain a crossover function by employing the 1+1d CFT result for contributions from lines perpendicular to the Fermi surface. Scaling functions for gapped systems additionally depend on a mass parameter. The results are demonstrated numerically for quadratic fermionic systems, finding excellent data collapse to the scaling functions.
Ref.: Qiang Miao and Thomas Barthel, arXiv:1905.07760 (2019).

A striking number of physically and chemically different systems, including monocrystals, glasses and simple liquids, exhibit similar electrical response behaviour at classical frequencies below THz region. It is a purpose of this seminar to show that these similarities in the response can be linked to the similarities in the blocking nature of the interface between the metal electrode and the particular system under investigation. After a short review of the new electrical response analysis, where the interfaces determine the important boundary conditions, the experimental results on monocrystalline silicon, chalcogenide glass Ag_x(AsS₂)_1–x ionic conductor and aqueous chloride solutions will be discussed in terms of bound electrical charges relaxational/polarisation processes in the bulk and relaxational/polarisation processes due to the mobile electrical charges that do not alter the dielectric constant of the material in question.
If time allows, one particular bulk relaxational process seen in the studied glassy materials will be discussed and it will be speculated that this observed relaxational process is due to presence, in glassy materials, of the Two Level Systems (TLS).

[1] Glassy Disordered Systems: Glass Formation and Universal Low-Energy Properties (Michael Klinger, World Scientific 2013).

Self-assembly of nanoparticles into organized structures with various properties has been a topic of research for a long time. One of the ways to efficiently prepare and manipulate such nanosystems is to use liquid crystal molecules and chemically attach them to the nanoparticle surface. The advantage of using mesogenic molecules as surface ligands is to control the distance between the nanoparticles and at the same time preserve liquid crystalline ordering. Furthermore liquid crystalline medium is sensitive to small external stimuli therefore the nanoparticles organization can be dynamically tuned. Especially interesting and most commonly used are metal nanoparticles due to their collective plasmonic properties.
In this talk I am going to present our research efforts in this field. In our study we prepared silver nanoparticles and covered them with liquid crystalline ligands based on lactic acid derivatives. We characterized the prepared systems using various experimental methods and proposed a model for the observed superstructures

The Kondo effect is a strong correlation non-perturbative emergent phenomenon. It was first discovered in the bulk by analyzing magnetic impurities embedded in normal metal. With the progress of nanodevice fabrication a renewed interest in Kondo physics has been rekindled. To obtain reliable quantitative predictions for such experiments, unbiased numerical simulations such as various variants of quantum Monte Carlo (QMC), or numerical renormalization group (NRG) are widely used.
In this talk, I will give a brief introduction into the applications of Renormalization group techniques to continuous phase transitions. Later, a solution of a magnetically tunable Kondo effect by using numerical Renormalization group is shown. The obtained results are carefully compared to the experimental data obtained in a corresponding nanodevice investigated in the group of prof. Van der Zant at TU Delft.

In this talk, I will highlight work done at FZU in the past year regarding the topic of constructing mesoscopic models of ferroelectric and multiferroic materials and the subsequent search for stable skyrmions in their nanostructures. One goal of our group has been to investigate the domain structure of the multiferroic skyrmion-lattice host GaV₄(S,Se)₈. Using density-functional methods, we probe the order parameter subspace of GaV₄Se₈ and parameterize an effective thermodynamic potential useful for domain topology prediction. Additionally for multiferroic BiFeO₃, we also demonstrate that once the energy landscape has been thoroughly explored as was done previously by co-workers, that domain wall profiles can be predicted in agreement with previously published DFT results. In parallel to this effort, the search for skyrmion and skyrmion-like phases in ferroelectric nanoparticles continues. Here, the phase field method is used to explored confined ferroic phases of varying nanoshapes of lead titanate embedded in dielectric media. It is shown that the directionality of an intrinsic vortex-like core can be controlled using an applied electric field or tunable interparticle spacings, and that the arrival of a skyrmion-like vortex phase is universal across the entire superellipsoid shape morphology.

Transition-metal complexes are widely used as photosensitizers and photocatalysts, and in light-emitting devices [Chem. Rev. 117, 10940 (2017)]. We have recently demonstrated first iron complex [FeIII(btz)3]3+ exhibiting room temperature photoluminescence from a charge transfer state and show that it has a 100 ps excited state charge transfer lifetime which is unprecedented for any iron complex [Nature 543, 695 (2017)]. This was characterized as a rare low-spin FeIII d5 complex with emission from a long-lived doublet ligand-to-metal charge transfer (2LMCT) state. Absence of intersystem crossing in this complex avoids significant excited state energy losses as encountered in the prevailing class of d6 transition metal complexes of e.g. FeII and RuII. Most recently, we have also shown [FeII(btz)3]2+ low-spin complex [J. Phys. Chem. Lett. 9, 459 (2018)], a FeII analogue to [FeIII(btz)3]3+. It exhibits strong metal-to-ligand charge transfer (MLCT) absorption bands throughout the visible spectrum, and excitation of these bands gives rise to a 3MLCT state with a 528 ps excited state lifetime in CH3CN solution that is more than one order of magnitude longer compared to the MLCT lifetime of any previously reported FeII complex. Together, these results show that the FeII and FeIII oxidation states of the same Fe(btz)3 complex feature long lived MLCT and LMCT states, respectively. This demonstrates the versatility of iron N-heterocyclic carbene complexes as promising light-harvesters for a broad range of photophysical and photochemical applications, and on-going efforts for further improvements of the excited state dynamics of such systems will be outlined.

Hexa-peri-hexabenzocoronene (HBC) core tethered with three diacetylene (DA)-containing side-chains was designed and their self-assembling behavior in solvent and on substrate was investigated. The polymerizable HBC derivatives are noted as HBC-1,3,5-Ph-DA-Cn, where 1,3,5 refers to the position of DA chain and n is the spacer length of alkyl chain. It is confirmed that the nanostructure of HBC-1,3,5-Ph-DA-Cn can be polymerized in (i) solution and (ii) film by UV-irradiation or thermal treatment. HBC-1,3,5-Ph-DA-C12 exhibits a stable hexagonal columnar phase at 47℃ which was used to discuss self-assembling behavior and the polymerization process. The polymerization reaction has been investigated by powder X-ray diffraction (XRD), infrared spectroscopy (IR), Raman spectroscopy and UV-visible spectroscopy.

[1] Cheng, Y. J.; Yang, S. H.; Hsu, C. S.; Chem. Rev. 109, 5868 (2009).
[2] Enkelmann, I. V. in Polydiacetylenes; ed. Cantow, H. J. Springer-Verlag, Berlin, 1984, p. 91.
[3] Yoon, B.; Lee, S.; Kim, J.-M.; Chem. Soc. Rev. 38, 1958 (2009).

The polar antiferromagnet Ni₃TeO₆ exhibits non-hysteretic colossal magnetoelectric effect [1]. A collinear antiferromagnetic order appears below 53 K, giving rise to spin-induced-polarization. In addition, Ni₃TeO₆ dynamical magnetoelectric coupling [2].
Substitution of Ni with Mn and Co preserves the polar structure of R3 space group symmetry. Spin and lattice excitations were investigated for all compounds Ni₂MnTeO₆ [3], Ni₂CoTeO₆ and NiCo₂TeO₆, and they present spin excitations in the THz range. More specifically, the Co cases present numerous spin excitations, among which also electromagnons.
In an attempt to understand the microscopic mechanisms responsible for the magnetoelectric behavior of the Ni-based Tellurates, we performed Density Functional Theory (DFT) calculations. Since the exact magnetic structure of the above new compounds has not yet been studied experimentally, the main focus has been to determine the magnetic ground state. In addition, spin-polarized phonon calculations by first principles were employed aiming at examining the effect of the spin structure to the lattice vibrations.

[1] Y.S. Oh, et al., Nat Commun 5, 3201 (2014).
[2] S. Skiadopoulou, et al., Phys. Rev. B 95, 184435 (2017).
[3] M. Retuerto, et al. Phys. Rev. B 97, 144418 (2018).

Substitution of one or two cations can change dramatically dielectric, magnetic or mechanical properties of the pure BaFe₁₂O₁₉ hexaferrite, including significant shift of Curie temperature, appearance of spin-lattice coupling and phase transitions. In this talk the dielectric response of a lead substituted barium hexaferrite will be discussed.

Cell-to-cell communication is a prerequisite in multicellular organisms. Coordinated interaction is provided by different mechanisms running in parallel. Just over a decade ago, nanotubular cell-to-cell connections have been discovered, termed tunnelling nanotubes (TNTs) or membrane nanotubes [1, 2]. The shape of TNTs resembles circular tubes with a typical diameter of 50 – 200 nm and a length of 10 – 70 μm which can extend up to 1 mm. TNTs can transport ions, intercellular vesicles, mitochondria, and various cargoes of microparticles, and could mediate electromagnetic signalling between cells.
Cells with nanotubes could form a system of connected resonant cavities with unified electromagnetic oscillations. We calculated circular waveguide cutoff frequencies to be in the 10^15 – 10^16 Hz region. Signals thus can be excited by microtubules with mode frequencies in the 10^16 Hz region. The frequency region suggests a possible interaction of valence and inner electrons in biologically important atoms (e.g. C, N, O) with the cavity modes of the electromagnetic field. Possible signal amplification in TNTs can be described by Manley–Rowe relations [3] or by Fröhlich equation [4].

[1] Rustom A. et al., Science 303, 1007 (2004).
[2] Scholkmann F., Theor. Biol. Med. Model. 13, 16 (2016).
[3] Manley J. M. and Rowe H. E., Proc. IRE 44, 904 (1956).
[4] Fröhlich H., Adv. Electron. El. Phys. 53, 86 (1980).

Far-infrared techniques played a crucial role in the experimental validation of Bardeen–Cooper–Schrieffer (BCS) theory and they continue to provide fundamental information about condensate, quasiparticle and vortex behaviour in both equilibrium and non-equilibrium states of superconductors.

In this talk, we will focus on the response of thin superconducting NbN films. We confirmed that their equilibrium conductivity follows the conventional BCS theory. Application of a dc magnetic field induces vortices in the films, which leads to qualitatively different responses, depending on the mutual orientation of the film, applied magnetic field and probing terahertz electric field.

Excitation by intense femtosecond laser pulses destroys the superconductivity in the films. The following recovery dynamics towards the equilibrium superconducting state occurs via a growth of superconducting islands in the normal-state environment. The spectral weight of the non-equilibrium response of the superconducting phase is found to be shifted compared to a BCS conductivity in thermal equilibrium.

Earlier [10.1038/ncomms12842 (2016)] , we have observed incipient ferroelectric behavior among water molecules localized within nano-size cages formed by ions of crystal lattice of beryl. Emergence of macroscopic polarization among H₂O molecular dipoles is suppressed due to quantum tunneling within hexagonal potential. Like in beryl, in cordierite water molecules are located within nano-cages and can relatively freely rotate around the c-axis experiencing 2-well potential with the depth of ~10 meV. Dielectric spectroscopy reveals typical (anisotropic within ab plane) relaxor-like response of confined water molecules connected with a broad soft relaxation with complicated lineshape. At terahertz frequencies, sharply anisotropic soft-mode response is observed caused by dynamics of water molecular ensemble. We shall present temperature-dependent RF-THz-IR spectra of water-containing cordierite crystal and suggest possible interpretations of the observed phenomena.

I will give a report about topics presented at CEMS Symposium and 9th APCTP Workshop on Multiferroics, scientific meetings held in Japan in November 2017. My talk has the following goals:

• Show the best and latest achievements.
• Classify the described phenomena.
• Provide the corresponding references.

Ferroelectricity on the nanoscale has remained a subject of much fascination in condensed matter physics for the last several decades. It is well-recognized that stability of the ferroelectric state necessitates effective polarization screening, and hence screening mechanism and screening charge dynamics become strongly coupled to ferroelectric phase stability and domain behavior. Previously, the role of the screening charge in macroscopic ferroelectrics was observed in phenomena such as potential retention above Curie temperature, back switching of ferroelectric domains, and chaos and intermittency during domain switching. In the last several years, multiple reports claiming ferroelectricity in ultrathin ferroelectrics based on formation of remanent polarization states, local hysteresis loops, and pressure induced switching were made. However, similar phenomena were reported for traditionally non-ferroelectric materials, creating significant level of uncertainty in the field. We pose that in the nanoscale systems, the ferroelectric state is fundamentally inseparable from electrochemical state of the surface, leading to emergence of coupled electrochemical-ferroelectric states. I will present the results of experimental and theoretical work exploring the basic mechanisms of emergence of these coupled states including the basic theory and phase-field formulation for domain evolution. I further discuss the thermodynamics and thickness evolution of this state using analytical theory and phase-field modelling. These considerations further stimulate the development of the novel SPM modalities addressing time-dependent dynamics and chemical changes during SPM imaging. I will introduce the general data acquisition mode (GMode) of SPM, based on full data capture and subsequent information theory and physics based analysis of the data stream. I will further delineate the applications of in-situ SPM – time of flight secondary ion mass spectrometry (ToF SIMS) to map the changes in surface chemistry during tribological and local electrochemical experiments, including ferroelectric polarization switching and pressure-induced resistance changes in oxides. These analyses reconcile multiple prior studies, and set forward the predictive pathways for new generations of ferroelectric devices and applications.

This research was sponsored by the Division of Materials Sciences and Engineering, BES, DOE, and was conducted at the Center for Nanophase Materials Sciences, sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division.

In physical systems simultaneously breaking time-reversal and spatial inversion symmetries the strength of absorption for two counter-propagating light beams can be different, which phenomenon is termed as non-reciprocal directional dichroism. Until recent experiments on multiferroic materials this effect was generally found to be weak. The multiferroic Ni₃TeO₆ shows strong directional dichroism in its spin excitations even for unpolarized light. The temperature- and magnetic field dependence of these exotic resonances was followed up to the Néel temperature and up to 30 T magnetic field, respectively.

A successful transition of spin-related processes from gigahertz to the terahertz (THz) frequencies is one of the desired goals of the up-to-date spintronics. The intense ultrashort THz pulses are an excellent experimental tool to explore the dynamics of the equilibrium and transient spin order in this frequency range which is, in general, hardly accessible by the conventional electronics.
In this talk, the ultrafast spin to charge current conversion due to the anomalous and spin Hall effects will be discussed in the context of thin semiconducting and metallic heterostructures. On these examples, such as the broadband spintronic THz emission and THz spin/charge accumulation, the ultrafast THz spectroscopy will be presented as a way not only to probe but also to drive spin-polarized currents.

In the last 20 years, the so called universality of conductivity spectra have been studied in disordered ion conductors, such as glasses, polymers etc. [ Z. Phys. Chem. 224, 1891 (2010)]. Recently, we have shown that similar electric conductivity response can be observed in highly ordered proton conductors [ Solid State Ionics 225, 268 (2012); 237, 40 (2013) ]. In this presentation I would like to show the results of studies of electric properties—in particular universal features of conductivity spectra—of proton conductors that experience structural phase transition. The first part of the talk is dedicated to the studies of influence of external pressure on electric properties of (NH₄)₃H(SeO₄)₂ single crystal. The studies were performed in the temperature range 250 K < T < 330 K, that covers 4 phases: I and II – superprotonic, III and IV – ferroelastic, and in the pressure range 0.1 MPa < p < 380 MPa.

In the second part, the studies of Rb₃H(SeO₄)₂ single crystal will be presented. Results indicate existing first and second universalities of conductivity spectra in the aforementioned crystal. The former effect exists in high temperatures, while the latter is observed at sufficiently low temperatures.

We studied charge transfer and dielectric relaxation processes in several materials of biological and organic origin: extracellular matrix of electrogenic bacteria Shewanella oneidensis MR-1, cytochrome c, serum albumin and synthetic melanin. In course of the study we obtained spectra of complex conductivity of materials in frequency range 10^–2 – 10¹⁴ Hz measured in temperature range from 5 K to 300 K. The conductivity spectra were also complemented by heat capacity measurements, element analysis and thermogravimetry. Applying concepts of condensed matter physics, we identify transport mechanisms and a number of energy, time, frequency, spatial and temperature scales in studied materials, which can provide us with deeper insight into the dynamics of proteins and melanin.

In this talk, a new modeling approach is described for simulating the properties of dielectric nano/microstructures with coupled polar and elastic degrees of freedom, as well as the dependence of these properties on the structure size, shape, morphology and applied conditions. The versatility of this approach is exemplified in studying the following systems:
(i) Zn-ZnO and ZnO-TiO₂ semiconducting core-shell nanoparticles and the influence of their size, elastic anisotropy, microstructure and applied pressure on their optical properties;
(ii) Ferroelectric PbTiO₃ and BaTiO₃ nanoparticles embedded in a dielectric medium, and the dependence on their polarization-field topology and transitions on particle shape and size, dielectric medium strength, electric field, as well as other factors;
(iii) Artificial layered-oxide material exhibiting polar Goldstone-like (or phason) excitations and its electrocaloric properties that are tuneable under a wide range of conditions.
The results of these investigations highlight the great promise of functional nano/microstructures for a variety of advanced engineering applications, including electrothermal energy interconversion, non-volatile multibit memories, opto- and low-power-electronics, as well as metamaterials by design.

Magnetic resonance imaging (MRI) is a medical imaging technique based on the nuclear magnetic resonance phenomenon. In this talk, I’ll outline the classical and quantum mechanical approaches to deriving the fundamental equations of NMR and explain some of the interesting physics and engineering that go into producing a final MR image. Time permitting, I’ll talk about applications of these techniques to in-vivo cardiac perfusion MRI.

A- and B-site substitution of barium titanate with homovalent or heterovalent dopants is at the basis of solid solutions that lately found increasing importance for dielectric, piezoelectric, energy storage and microwave applications. High dielectric permittivity—stable over a large temperature range—and large high-field piezoelectric coefficients are typically sought-for figures of merit. Yet, compositional tuning to attain specific properties has been largely based on macroscopic observations and very little is known about the fine material structure on the short range that is necessary to induce those properties. In relaxors, for instance, it is not yet clear whether their peculiar behaviour originates from random electric fields or simple dipolar interactions. Raman spectroscopy, being sensitive to the material’s short range structure, is a very attractive technique to study structure-property relationships in Ba-based ferroelectrics and relaxors. In this presentation, an overview will be given on this technique and on the information it can convey on these systems. It will be shown that the Raman method is complementary to diffraction and macroscopic property measurements, and can add decisive details to understanding the whole picture, especially if coupled with atomistic simulations. We will focus on both A- and B-site substituted BaTiO₃ (with Bi and Zr, respectively) and also A- and B-site co-doped systems (with Bi, Yb and Fe).

Polar and magnetic oxides are fundamentally and technically important, but difficult to prepare. Recently, we were able to synthesize, at high pressure and temperature (HPT) in a Walker-type multi-anvil cell, a number of new compounds, A₂BB′O₆ in the corundum-derived and perovskite structure with unusually small A-site cations [1–5]. At HPT the crystal structures of these A₂BB′O₆ phases allow the incorporation of strong magnetic transition metal ions on all cation sites for magnetic and potentially multiferroic, or magnetoelectric behavior and applications in spintronics. Our aim was to design room-temperature polar ferri- or ferro-magnets by composition modulation of A₂BB′O₆ phases. So far, we have successfully prepared a series of polar and magnetic oxides and systematically investigated the relationship between the crystal, magnetic, and electronic structure and physical properties. The discovery of polar antiferromagnetic LiNbO₃-type (R3c) Mn₂FeMO₆ (M = Nb, Ta) [1] predicted new polar structures with second-order Jahn-Teller effect ions (such as Nb⁵⁺ and Ta⁵⁺, d0) at the B′-site and small ions at the A-site of A₂BB′O₆, which has been confirmed by the preparation of Zn₂FeTaO₆ [2]. In the Ni₃TeO₆-type (R3) ferrimagnetic semiconductor Mn₂FeMoO₆ (T_c ~ 340 K) [3], the polarization of the structure, is found to be stabilized by the spin structure at high pressure, while at ambient pressure, a new spin structure with lower energy state induces an unusually low-temperature (~400 – 550 K) cationic rearrangement, which provides a new way to tune the physical properties at the atomic-scale, under relatively mild conditions, of bulk oxides. In polar ferrimagnetic Mn₂FeWO₆ with Ni₃TeO₆-type structure the charge and size difference between Fe²⁺ and W⁶⁺ leads to a fully ordered Fe/W lattice and several exotic magnetic phases [4]. Other A₂BB′O₆ compounds with perovskite or distorted perovskite structures and interesting magnetic properties were also synthesized at HPT, such as Mn₂FeReO₆ which is half-metallic with large magnetoresistance and orders ferri-magnetically at 520 K [5]. While all of these materials are multiferroic, none studied thus far exhibits ferroelectric switching; the search continues.

[1] M.-R. Li et al., Angew. Chem. Int. Ed. 52, 8406 (2013).
[2] M.-R. Li et al., J. Am. Chem. Soc. 136, 8508 (2014).
[3] M.-R. Li et al., Angew. Chem. Int. Ed. 53, 10774 (2014).
[4] M.-R. Li et al., Adv. Mater. 27, 2177 (2015).
[5] M.-R. Li et al., Angew. Chem. Int. Ed., 54, 1 (2015).
[6] G.-H. Cai et al., Polar Magnets in Double Corundum Oxides, Chem. Mater. (in press).

The talk would present work on a new class of terahertz (THz) waveguides based on structured metal geometries. The waveguides are designed with the core idea that adoption of planar layout in fabrication can lead to exponential growth in device capabilities, analogous to the growth in device capabilities based in electronics. From a functional point of view, the waveguides rely upon propagation of surface waves along the surface of metals. This approach is preferred, since dielectrics tend to be lossy at THz and the loss parameters scales almost quadratically with frequency for most dielectrics. The loss in propagating wave is minimized by utilizing metals, which are highly conducting at THz frequencies. Structuring the metal surface with periodic array of apertures of sub-wavelength dimension allows bound surface wave to propagate as the wave can evanescently decay into the metal. This phenomenon is referred as the coupling of propagating wave to surface plasmon polariton (SPP) like mode at the interface of structured metal surface and air. Thus, these propagating THz waves are simply surface plasmon-polaritons (SPPs). Similarly, complimentary structures that do not perforate the metal, but rather stand on the metal surface also support SPPs. The wavelength of SPPs can be controlled by changing the dimension of these apertures/structures, since the dispersion relationship of the medium depends on the geometrical size. This engineering capability has been exploited in creating all the waveguides presented in this thesis. The devices presented are categorized based on the fabrication technique. Each technique is unique in its own regard and can be selected based on functional needs. A commonly adopted process of laser ablation covers a wide set of waveguides presented here. In one of the waveguides fabricated using ablation technique, the role of disorder is discussed. The waveguide with introduction leads to observance of localized mode with spectral and spatial feature like Anderson localized modes of photons. It is the first report of localized mode at THz frequency. 3D rapid prototyping involving 3D printer is used to create waveguides with complex layout that can allow for multi-plane signal routing. This also is the first demonstration of 3D printing in the development of THz devices. In another approach a unique fabrication technique had to be developed to create waveguides mediums with negative index of refraction (NIM) as they require feature sizes which cannot easily be attained using conventional clean room techniques and 3D printing. This new fabrication approach uses a sacrificial layer technique that is used create an effective medium with negative index of refraction and length on the order of tens of wavelength.

There is a need for the development of comprehensive, multi-scale theoretical tools in the search for better materials. This is essentially at the core of the recent “materials genomics/informatics” initiatives that seek to accelerate materials discovery through the use of computations across length and time scales, supported by experimental work. Such methods will result in customizing, or entirely replacing, existing engineering metallic alloys, polymers, and ceramics which were developed based on trial-and-error approaches in the past century. In this talk we will apply these principles to pyroelectrics and electrocalorics. Pyroelectrics can convert heat into electricity by cycling around thermally- and electrically-induced polarization changes, where the energy density scales with the product of the polarization change and applied field. The challenges in realizing a pyroelectric energy conversion system are multi-scale and multi-faceted, requiring a combination of first principles computations, phenomenological theory, classical thermodynamics, materials synthesis, and eventually system design. We will discuss our successes and challenges with relating modeled to measured material properties.

Solid-state systems with exotic and controllable spin order are being explored as materials for spintronic devices. In this talk I will highlight our recent work on probing spin-orbit coupling in a non-contact manner via terahertz time-domain magneto-spectroscopy of heavy holes in strained germanium quantum wells. I will discuss how strain and asymmetric modulation doping can create a high-mobility 2D heavy-hole gas that remains spin-polarised even to zero magnetic field, via the Rashba interaction, and how this can be probed via terahertz spectroscopy. I will also describe how this high mobility system exhibits the a.c. analogue of the quantum Hall effect even at terahertz frequencies. This suggests the promise of this system for fundamental physical studies as well as in CMOS-compatible electronics.

Metamaterials (MMs) in optics represent a large class of nano-structured artificial media with optical characteristics superior to those exhibited by natural materials. Developing an efficient technology for active control over MM exotic optical response is an essential step towards their practical application. One of the methods to achieve tunable MMs is to functionalise the fabric photonic materials with liquid crystals (LCs), where the MM properties are tuned by changing the LC optical anisotropy with external voltage or light/temperature. In our work we experimentally demonstrated the full potential of electrical tuning of the elastic LC photonic MM, where we combined mechanical properties of electro-mechanical systems with LCs.

Nowadays liquid crystals (LCs) find broad application in a variety of electronic and optical devices. We present a simple all-optical method for measuring both the elastic and dynamic characteristics of nematic liquid crystals. Using the same experimental set-up, we developed the three-step measurement protocol and a corresponding fitting procedure that simultaneously determine the following LC parameters: the splay and bend elastic constants, the rotational viscosity and the combination of Leslie viscosity coefficients. Moreover, the uniformity of LC layer thickness and quality of the alignment on the surfaces can be determined.

Engineering of domains and domain boundaries is quintessential for technological exploitation of numerous functional materials. However, it has only recently realized that the configuration of these domains/domain boundaries can have non-trivial topology. We will discuss a new topological classification scheme of domain/domain boundary configurations with Ising-type or two-dimensional order parameters: Z_m×Z_n domains (m directional variants and n translational antiphases) and Z_l vortices (where l number of domains and that of domain boundaries merge). This classification, with the concept of topological protection and topological charge conservation, has been applied to a wide range of materials such as improper ferroelectric R(Mn,Fe)O₃, antipolar In(Mn,Ga)O₃, hybrid improper ferroelectric (Ca,Sr)₃Ti₂O₇, chiral (and ferromagnetic) Fe_1/3TaS₂, and magnetic-superconducting Sr₂VO₃FeAs. We will also discuss the emergent physical properties of domain boundaries, distinct from those of domains. The presented topological consideration provides a basis in understanding the formation, kinetics, manipulation and property optimization of domains/domain boundaries in quantum materials.

Topological defects in nematic liquid crystals are ubiquitous. The defect studies are important in understanding the fundamental properties of the systems, as well as in practical applications, such as colloidal self-assembly, optical vortex generation and templates for molecular self-assembly. Usually, spatially and temporally stable defects require geometrical frustration imposed by surfaces; otherwise, the system relaxes because of the high cost of the elastic energy. Nematic liquid crystals confined between perfluoropolymer surfaces and a thermal discontinuous anchoring transition will be described and explained.

I will present several alternative algebraic representations of the system of coupled damped harmonic dipoles. I'll try to grasp the physical meaning of it. The theorems will be applied to infrared spectra of real perovskite crystals.

Giant magnetoelectricity in pyroelectric antiferromagnets and slow DW dynamics. Materials with strong magnetoelectric effect are of pressing interest for future spintronics and information storage devices. At the same time, ferromagnetic ferroelectrics are rather rare. Recently very large magnetoelectric effect was measured in AFM pyroelectric Ni₃TeO₆ at the spin flop transition. The effect will be rationalized using Landau theory and model Hamiltonians with parameters extracted from first-principles calculations. The second part of the talk will focus on slow electric field-driven dynamics of ferroelectric domain walls. Understanding this dynamics may be instrumental for efficient manipulation of domain walls in multiferroics.

Stručná rekapitulace objevu kapalných krystalů prof. Reinitzerem v Praze roku 1888 při příležitosti umístění pamětní desky prof. F. Reinitzera na budovu dnešní ČVUT v Husově ulici č. 5.

We studied the dispersion relations of the MSPs and their coupling to bulk and waveguide modes in the three types of THz guiding structures, namely InSb/dielectric planar boundary, InSb/air/metal planar waveguide, and symmetric InSb/air/InSb planar waveguide. In particular we demonstrated the possibilities for tuning the central frequencies of the one-way gap and its bandwidth by varying the amplitude of an external magnetic field, the permittivity and thickness of the dielectric guiding layer. Then we evaluated the dispersion relations of all modes supported by the waveguide structures and analyzed their properties in terms of their propagation constants. By using our in-house 2D numerical technique based on the magnetooptic aperiodic rigorous coupled wave analysis (MOaRCWA), we evaluated the relative spectral transmittances in both forward and backward directions of propagation for the structures considered. Our approach deals with the configurations, in which the magnetized sections are sandwiched between the identical structures but without the magnetic field. We believe that this configuration is closer to a structure that might be adopted in envisaged future applications.

Na tomto netradičním semináři představí sebe a svou vědeckou práci oslovení kandidáti do Rady FZÚ. Po sérii odborných 15-ti minutových příspěvků bude následovat diskuse s kandidáty.

At the ELI Beamlines facility a new generation of high intensity lasers will be used for fundamental and applied science. The research Program for Applications in Molecular, Bio-⁠medical and Material Science develops a range of capabilities for time-⁠resolved experiments using ultrashort pulses ranging from the THz to the hard X-⁠ray range. Laser driven THz sources will be used for photo-⁠electron streaking experiments in AMO science and to initiate bulk modes in e.g. protein crystal that will be studied through high resolution time-⁠resolved X-⁠ray diffraction. The aim of the presentation is to introduce the main research areas under development at the ELI Beamlines facility and find promising topics for future collaborations.

Methods of nonlinear optics are powerful tools for a noninvasive microscopy of biological systems and medical applications. A compact multimodal laser platform with ultrafast tunable chirped pulses is shown to provide sub-10 cm^(–1) spectral resolution in stimulated Raman spectroscopy. This makes it possible to resolve the components of a mixture and recover its quantitative composition. Furthermore, pulse-width optimization enables nonlinear Raman brain imaging: when the spectral bandwidth of laser pulses is accurately matched against the bandwidth of molecular vibrations, the coherent Raman signal is shown to be radically enhanced, resulting in higher sensitivities and higher frame rates.

The list of required parameters for ferroelectric multicomponent mixtures suitable for applications is long. Properties such as: low melting temperature, low viscosity and broad temperature range of liquid crystalline phase existence as well as parameters important for electrooptic modes like tilt angle or helical pitch can be easily optimized. However preparation of multicomponent mixture should be preceded by miscibility studies in bicomponent systems, in order to select appropriate components. The compounds used for preparation of application mixture can differ in phase sequence and it is not necessary that all of them form ferroelectric phase or even liquid crystalline one [1, 2]. The most favourable are mixtures formed by optimization of eutectic composition. In order to prepare multicomponent mixture non-chiral compounds can also be used [3].

The aim of this work was to check how the structure of dopants (from non-chiral pyrimidines and terphenyls through chiral biphenyls to chiral quterphenyls) and their amount influence the phase situation and helical pitch of ferroelectric phase in binary mixtures. As base compounds three ring esters 6F2OBi and 1H6Bi(3F), differing in the type of terminal non-chiral chain and substitution in rigid core, were selected. Base compounds differ also in the dependence of helical pitch upon temperature: 6F2OBi forms only right-handed helix whereas 1H6Bi(3F) forms both left- and right-handed helix. Results of miscibility and helical pitch studies will be presented.

[1] K. Kurp, M. Czerwiński, M. Tykarska, Liquid Crystals 42, 248 (2015)
[2] A. Bubnov, M. Tykarska, V. Hamplova, K. Kurp, Phase Transitions 2016; DOI:10.1080/01411594.2015.1087523
[3] K. Kurp, M. Tykarska, Liquid Crystals 43, 1359 (2016)

This work was supported by the COST Action IC1208.

Oxide materials are the most abundant compound in the earth’s crust and possess a wide range of electrical, optical, and magnetic properties. For instance, insulators, high quality metals, dielectrics, ferroelectrics, piezoelectrics, semiconductors, ferromagnetics, transparent conductors, superconductors, and nonlinear optic materials have all been produced using oxide materials. Oxide materials have enormous potential, particularly as the fundamental building block of a new generation of electronic devices. We create these materials by artificially layering various atoms including oxygen at the single atomic level and discovering novel properties that are likely to find applications in electronic, magnetic, optical and electromechanical devices. I will discuss how our research [1–6] played a role in understanding the fundamental solid state phenomena at the atomic scale and the discovery of new materials so that we can use them to develop new oxide nanoelectronic devices. Atomic layer control of novel oxide heterointerfaces may provide some of the answers that we need to continue the electronics revolution, particularly for nanoscale devices with new functionality that are currently being developed and can be applied to various fields.

1. “Polar Metals by Geometric Design”, Nature 533, 68 (2016)
2. “Emergence of Room-temperature Ferroelectricity at Reduced Dimensions”, Science, 349, 1314 (2015)
3. “Giant piezoelectricity on Si for hyper-active MEMS”, Science, 334, 958 (2011)
4. “Metallic and insulating oxide interfaces controlled by electronic correlations”, Science, 331, 886 (2011)
5. “Creation of a two-dimensional electron gas at an oxide interface grown on silicon” Nature Communications, 1, 94 (2010)
6. “Ferroelastic switching for nanoscale nonvolatile magnetoelectric devices”, Nature Materials, 9, 309 (2010)

School children learn that water has three phases: solid, liquid and vapor. But we have recently uncovered a fourth phase. This phase occurs next to water-loving (hydrophilic) surfaces. It is surprisingly extensive, projecting out from the surface by up to millions of molecular layers. And, its properties differ markedly from those of bulk water.

Of particular significance is the observation that this fourth phase is charged; and, the water just beyond is oppositely charged, creating a battery that can produce electrical current. We found that light charges this battery. Thus, water can receive and process electromagnetic energy drawn from the environment in much the same way as plants. Absorbed electromagnetic (light) energy can then be exploited for performing work, including electrical and mechanical work. Recent experiments confirm the reality of such energy conversion.

This energy-conversion framework seems rich with implication. Not only does it provide an understanding of how water processes solar and other energies, but also it may provide a foundation for simpler understanding natural phenomena ranging from weather and green energy all the way to biological issues such as the origin of life, transport, and osmosis.

The talk will present evidence for the existence of this novel phase of water — how come nobody’s seen it before? — and will consider the potentially broad implications of this phase for health.

The book dealing with this subject is now available http://www.amazon.com/The-Fourth-Phase-Water-Beyond/product-reviews/0962689548.

Viruses are known to be one of the most important risk factors for cancer development in humans. After entering a living cell, a virus harnesses the cell’s machinery for its replication. Some viruses achieve it by inserting their DNA or RNA into that of the host cell. Altering the genome of the host cell can trigger cancer transformation. Our experimental data demonstrate that cancer initiation is commonly linked with lactate dehydrogenase-elevating (LDH) virus, and a novel insight into cancer initiation process with a complex biophysical mechanism is presented.

LDH virus is a silent virus causing no observable morphological changes on the host cell. Its RNAs chronically consume ATP as energy resource. We investigated cell-mediated immunity (CMI) response of T lymphocytes to individual tumour-specific antigens, effective only in the cancer type of the antigen origin, and non-specific LDH-virus antigen which turned out to be effective in all examined cancers. Both antigens were immunologically functional fractions separated by high-pressure gel chromatography at 340 nm. Analysis of experimental data on CMI response related to about 12,000 cases of healthy humans, cancer patients and patients with precancerous cervical lesions disclosed that the specific cancer and the non-specific LDH-virus antigens elicit similar responses. CMI results of patients with precancerous lesions display both healthy and cancer states.

It is known that chemical reactions depend on strong enough electric field which can power catalysis and affect their rate. Therefore, biochemical activity may be affected by near electromagnetic field of coherent electric polar vibrations which are present in the cell. Analysis of the effect of the cellular electromagnetic field on biochemical bonds is based on the reverse vibrational Stark effect. Overlapping of wave functions in biological macromolecules depends on energy of the cellular electromagnetic field which supplies energy to bonding electrons for selective chemical bonds. Decreased cellular field due to parasitic energy consumption of the viral RNAs results in reduced ratio of coherent/random oscillations. Decreased effect of coherent cellular electromagnetic field on bonding electrons in biological macromolecules leads to elevating probability of random reactions including the genome ones. The described mechanism can explain massive occurrence of nonlocal genome somatic mutations in cancer cell.

Interesting ferromagnetic ferroelectric system permitting skyrmionic spin structure phases has also a peculiar ferroelectric transition. We shall briefly report about theoretical and experimental studies of the lattice-dynamics aspects of this phase transition.

Using ab-initio methods (density-functional theory), I search for new functional materials that improve over known ones for example by being thermodynamically more stable or more environment friendly (lead-free). To that end, I perform computational high-throughput materials screening to find new compounds and to predict their crystal structure and their electronic properties, such as their band gap, ferroelectric polarization, effective mass, and magnetic configuration. I am particularly interested in discovering materials for photovoltaic and ferroelectric applications. In order to predict the optical absorption properties (the frequency-dependent dielectric tensor) of potential photovoltaic absorber materials, I use many-body perturbation theory (the GW approximation and for excitonic effects the Bethe-Salpeter equation). In my talk I will focus on perovskites with properties promising for photovoltaic and/or ferroelectric applications found by high-throughput materials screening.

Spintronics has changed the world. Yet many of us would confine its merits only to magnetic data storage. But there is much more room up there: spintronics lays behind many more—and bigger—aspects of our lives. For instance, many processes in automated assembly lines in industry relay on spintronics. And the beauty of it is that the principles that underpin the magnetic read-out in hard drives mimic the way in which one engineer would detect a tuna can before stopping it in order to fill it or label it.

In this talk, we will walk through fundamental advances in spintronics. We will start at the moment in which Lord Kelvin discovered the anisotropic magnetoresistance in 1856. We will accompany Louis Neel in his military service where he managed to render invisible the french navy from the nazi’s magnetic mines. Later, we will visit automated assembly lines at the moment in which the integrated circuits started a revolution in miniaturization. We will review a few inventions related to spintronics that were already envisaged but impossible to realize in the late 70’s. That will bring us to the deployment of anisotropic magnetoresistance, and later giant magnetoresistance, in the read-heads of hard-drives. Finally, we will discuss the role of spintronics in the internet of things, a massive deployment of inter-connected sensors. Spintronics delivers energy-efficient sensors and storage that will help us finding our nearest parking spot or fine tune the real estate prices through precise measurements of geodynamic activity. Did you know that the second cause of lung cancer in the US is radon contamination and this could be anticipated using a magnet and a compass? Would that affect your decision in buying your next home?

The Czech Academy of Sciences participates in a scientific mission in Canary Islands this May. A team of magnetic nanoscientists and geologists will use spin-orbit coupling to monitor the geodynamic activity of a volcanic island. How come 10 nm of permalloy will watch a volcano?

Read more at http://igsresearch.com/spintronics.

Is there any deep reason of using the new (and trendy) words “terahertz” and “multi-terahertz” when the traditional and well-established terms of far-infrared and mid-infrared have been available for decades? The answer is that the terms (multi-)terahertz are closely connected to specific quasi-optical techniques of coherent generation and detection of radiation, which provide some added value compared to “classical” experimental methods.

The standard terahertz range 0.1–3 THz represents a border between the electronics and optics worlds where virtually no efficient radiation sources were available until recently. In the historical context the (pulsed) time-domain terahertz spectroscopy was invented as a technique closely related to the development of femtosecond (optical) lasers. The method of the detection of THz pulses — so called “electro-optic gating” — brings several benefits: high sensitivity at low powers, direct measurement of electric field of the radiation (i.e. phase sensitivity) and sub-picosecond time resolution. It allows one to observe directly the oscillations of terahertz light during various interactions.

Later on, it has been realized that the same principles can be used for the generation and detection of ultrashort mid-infrared (i.e. multi-THz) pulses continuously up to ~ 60 THz and that also high-field (up to 10 MV/cm or 3 T) terahertz nonlinear table-top experiments can be easily carried out.

Finally, related recent and future developments of the THz lab at FZU will be discussed.

We provide a summary of our approach to various quantities in the field of spintronics and magnetization dynamics that can be calculated from first principles:

• related to non-relativistic spin-dependent transport (magnetoresistance, spin-transfer torque [1]),
• to relativistic effects (magnetic anisotropy, anisotropic magnetoresistance, anomalous Hall conductivity [2]),
• phonon related (electron-phonon lifetimes, temperature dependent resistivities [3]),
• affecting magnetization (exchange constants, spin-flip rates [4]).

The last set of quantities can be used for example to study non-equilibrium magnetization dynamics induced by femtosecond lasers. We employ it to calculate for how long times are different subsystems (magnetic, electronic, phononic) not in equilibrium with each other and to see which effects contribute mostly to demagnetization [3,4].

[1] Carva, K; Turek, I, Phys. Rev. B 76, 104409 (2007).
[2] Turek, I.; Kudrnovsky, J.; Carva, K., Phys. Rev. B 86, 174430 (2012).
[3] Carva, K.; Battiato, M.; Oppeneer, P. M., Phys. Rev. Lett. 107, 207201 (2011).
[4] Frietsch, B.; Bowlan, J.; Carley, R.; et al., Nat. Comm. 6, 8262 (2015).

The vanadium ladder compounds XV₂O₅ (X=Na,Ca,Mg) have been drawing attention in the solid state community for about 15 years. They belong to the active research area of complex materials with low-dimensional features, where charge, spin and lattice degrees of freedom strongly interact and lead to extraordinary physical properties. We have investigated the interplay of charge and lattice vibrations using ab-initio calculations based on density functional theory. In order to do so, we have first calculated the eigenfrequencies and eigenvectors of the Г point phonon modes. Then we have calculated the frequency dependent dielectric function and Raman intensities for the Raman active modes. I will present the methodology used in our work and the obtained results, and compare them to experiment. Finally I will discuss how our calculations can contribute to a better understanding of the physics in the investigated material systems.

Recently observed properties of charged ferroelectric domain walls (CDWs) make these structures promising for application in future electronics. It was demonstrated experimentally that the quasi two-dimensional electron gas is present at head-to-head CDWs [1]. The CDWs are therefore good conductors that can be moved by an external electric field. As suggested by theory [2] and experiment [3] CDWs with very low spacing may cause the enhancement of piezoelectric and dielectric properties. However, the problem of controlled regular growth and formation of CDWs is not well understood. Present talk is dedicated to experimental study of CDWs in BaTiO₃ single-crystals. The screening mechanisms, preparation methods, process of formation, identification, stability and stabilization of CDWs will be discussed.

[1] Sluka T., Tagantsev A., Bednyakov P., Setter N., Free-electron gas at charged domain walls in insulating BaTiO₃. Nat. Commun 4, 1808 (2013).
[2] Sluka, T., Tagantsev, A. K., Damjanovic, D., Gureev, M., and Setter, N. Enhanced electromechanical response of ferroelectrics due to charged domain walls. Nat. Commun 3, 748 (2012).
[3] Wada S., Yako K., Kakemoto H., Tsurumi T., Kiguchi T. Enhanced piezoelectric properties of barium titanate single crystals with different engineered-domain sizes. J. Appl. Phys. 98, 014109 (2005).

Antiferro- and ferro-electric ordering is observed in orthogonal smectic phases composed of nonchiral bent-core molecules. These systems are the only proper fluid ferroelectrics confirmed experimentally so far. The delicate balance between the tendencies for local parallel and antiparallel ordering of molecular electric and steric dipoles is analysed and a coclusion is made that it is strongly shifted in favor of ferroelectric ordering in restricted geometries. This is a reason why dipolar ordering is more likely to occur within a smectic layer. We derive model interaction potentials for polar bent core molecules and present the results of the mean-field theory of ferroelectricordering in the orthogonal smectic phase taking into account also the molecular biaxiality.

Until only recently, it was proven challenging to achieve the displacive ferroelectric distortion in the d3 manganites because the [Mn⁴⁺ - O] bonds have not been put under sufficient tension. By advancing elaborate synthesis processes, which are necessary to avoid the more stable hexagonal polymorphs, we were recently able to extended the substitution limit of the large size Ba ion in bulk Sr_1-xBa_xMnO₃ samples to x = 0.45 with two-step “in situ” synthesis in a thermogravimetric furnace in flowing H2/Ar gas followed by oxygen anneal. The achieved perovskite ceramics exhibit ferroelectricity (T_F > 300 K) and G-type antiferromagnetism (T_N ~ 200 K) originating exclusively from the Mn cations (full version).

Dr. Xavier Marti entangles two well-established profiles: a passionate interdisciplinary entrepreneur and an internationally recognized scientist. To connect these two universes, he has taken the leading role at IGSresearch (www.igsresearch). IGSresearch is a small enterprise that comprises the entire supply chain in the microelectronics manufacturing with the mission of delivering prototype devices of scientific groundbreaking ideas. Besides his core business, Dr. Marti has empowered the start-up incubator of University of Barcelona, Penyalab, having created two start-ups with steady income in less than one year.

To realize his interdisciplinary obsession, in the past three years, he has propelled three topics (namely, magnetoresistance [1], piezoelectricity/piezoresistance [2], and geodynamic fault displacement[3]) from academic ideas to business highlights. He is now after developing invisible magnetic ink for the security sector [4]. These actions have been featured several times in the media, including TV and the economic pages of national press.

In this talk, widely open for questions and debate, Xavier will walk along the key steps that were found to be essential to connect fundamental science and technology transfer.

This talk will present recent work on two alternative families of dilute magnetic semiconductor materials. The first half of the talk will present the study of undoped and Cr and V-doped Sb₂Te₃ crystals, showing classical Drude conductivity behavior. The second half of the talk will review the recent attempts to prepare Ge:Mn by ion implantation. Structural, magnetic and temperature dependent infrared characterizations will be discussed.

Domain structure, which is often quite complicated, is known to play an important role in the macroscopic piezoelectric and dielectric response in ferroelectric perovskite oxides. In the seminar I will present results of our recent effort to separate different domain-structure-related contributions to the permittivity from each other. For this sake, we use a combination of simulations and calculations within the framework of the Ginzburg-Landau-Devonshire model as parametrized for the prototypic ferroelectric compound BaTiO₃.

Liquid Crystalline Elastomers (LCEs) are soft materials with actuation and shape memory behaviour. 2H NMR spectroscopy applied to doped LCEs or on selectively deuterated LCEs is a powerful technique in order to get information at a molecular level. In particular, in this seminar, I will show the main results concerning the strustural, local orientational order and dynamic properties of the different components (mesogens and crosslinkers) of the LCEs.

Nanocapacitors are the key component in resistive random access memory (RRAM) for next generation nanometre scaled electronic devices. It is believed that bottom up approach is a cost effective technique compared with the other nanotechnologies. In this work, we report a novel procedure for fabricating high performance nanocapacitors by using oxide nanocubes as colloidal building blocks. The nanocubes of CeO₂, which was synthesised with hydrothermal methodology, were used to build the monolayer and multilayer nanocapacitors through the capillary force assisted self-assembly approach. Such a synthesis results in a large area of high quality ordered structure with several square millimetres due to the narrow size and shape distributions of nanocubes in non-polar organic solvents. The as-fabricated nanocapacitors exhibited excellent resistive switching properties with very large ON/OFF ratios, good reliability and stability. These demonstrate the developed technique is a promising approach for the fabrication of next generation RRAM devices.

International team led by Prof. Tagantsev from EPFL Lausanne [ A.T. et al, Nature Comm 10.1038/ncomms3229] has recently reported that the mystery of antiferroelectricity of lead zirconate is resolved. Which experiments and theories constitute the content of this paper? Is our recent IR and Raman spectroscopic studies in agreement with the picture drawn in this paper? Through a somewhat extended version of my talk "Phonon dispersion of Pb(ZrTi)O₃ unfolded" from DyProSo symposium in Sept 2013, I would like share with you my analysis of these and related questions.

Liquid crystals are the popular materials in material engineering as well as LCD industry. One of the most important problems in using these materials is a long switching time after switching off external fields. The attempt to solve this problem is using Dual-Frequency Nematic Liquid Crystals - DFNLC. They show positive dielectric anisotropy below the so-called cross-over frequency and negative dielectric anisotropy above this frequency. DFNLC mixture is usually formed by a combination of many components (even more than 10), which can be split into two groups: molecules that have a large transverse dipole moment, and molecules having a large longitudinal dipole moment. Thanks to these properties display based on DFNLC exhibits the short switching on- as well as off-time. The presentation will be about dielectric properties of DFNLC and will show how these mixtures are created.

I will present numerical simulations performed using a software toolkit focused on metamaterials. The software features include automated parametric scans and extraction of effective optical constants. Its capabilities will be demonstrated on selected metamaterial structures investigated by our THz laboratory, namely strontium titanate bars and titanium dioxide microspheres combined with a wire medium. The above structures, with proper dimensions, are exhibiting regions of negative effective permittivity and/or permeability.

Multiferroic BiFeO₃ (BFO) has been the subject of wide investigations, given its antiferromagnetic and ferroelectric ordering with large polarization at room temperature. The interest in such material lies particularly on the possible ferroelectric-ferromagnetic coupling, given the fact that the antiferromagnetic planes lye orthogonal to the polarization axis. Removal of constraints from the surrounding material is beneficial for retention properties and nanostructure fabrication is of interest for technological applications.
Here I present the results from research on BFO nanostructures fabricated by Focused Ion Beam milling (FIB). FIB fabrication yields material damage in the form of ion implantation and amorphization. Therefore different FIB fabrication procedures have been employed in order to minimize such occurrence and preserve the functional properties. Ferroelectric properties have been investigated by Piezoresponse Force Microscopy (PFM), which allowed nanoscale spatial resolution and localization of the FIB induced damages, and gave direction for improving the fabrication technique.
Such line of research resulted in a low damage method for direct and rapid fabrication of arrays of BFO nanostructures.

Flexoelectricity is the linear response of polarization to a strain gradient. Because strain gradients break inversion symmetry, flexoelectricity occurs in all insulating crystals. The flexoelectric effect is negligible on conventional length scales, but it can become very strong at the nanoscale where large strain gradients can significantly affect the functional properties of dielectric thin films and superlattices. Previous theories have tended to focus either on the lattice or the electronic (frozen-ion) contribution and have involved some approximations or limitations. I will describe our development of a first-principles theory of the flexoelectric tensor, formulated in such a way that the tensor elements can be computed directly in the context of density-functional calculations on small supercells and present results for a variety of insulating materials.

One of the promising ways of designing new single-phase multiferroic materials is adjusting the temperatures of both ferroelectric and magnetic phase transitions by ion substitutions in the host compound to obtain the maximal magnetoelectric coupling. The aim of the present talk is a brief review of the recent studies of both ferroelectric and magnetic phase transitions in some Fe containing ternary perovskite multiferroics, especially PbFe_0.5B_0.5O₃ (B-Nb, Ta, Sb) and related solid solutions. Possible ways to control the temperature T_N of their antiferromagnetic phase transitions are discussed. In our studies, besides the usual methods, we used the temperature evolution of 57Fe Mössbauer spectrum as a sensitive method of T_N detection. It is believed that in all perovskite ABO₃ multiferroics, magnetic and ferroelectric subsystems are independent. Magnetic properties are provided by B-site (e.g. Mn³⁺ and Fe³⁺) cations while ferroelectric properties are provided by the A-site cations having the so-called dangling bonds (Bi³⁺ and Pb²⁺). In contrast to this assumption, also A-site ion substitutions have been found to effect greatly the T_N value of PbFe_0.5B_0.5O₃. The results obtained are discussed using the models of magnetic superexchange involving Pb²⁺ cations and next-nearest Fe³⁺ neighbours.

Investigation and detailed understanding of microstructure of ferroelectrics is interesting from both fundamental and application-related point of view. In this talk I will address internal properties of ferroelectric domain walls (interfaces between ferroelectric domains) and how they can contribute to the piezoelectric and dielectric response of the material. In particular, recent results on Bloch-type domain walls will be presented. Our tools are analytical calculations as well as numerical simulations in the framework of the so called phase-field approach.

[1] V. Stepkova et al., J. Phys.: Condens Matter 24, 212201 (2012).
[2] M. Taherinejad, D. Vanderbilt et al., Phys. Rev. B, 155138 (2012).

Magnetic semiconductors entwine two of the most successful concepts in both fundamental physics and industry where ferromagnetic materials have played an undismissable role. Only very recently, antiferromagnets have been proposed as alternative material systems [1,2]. Antiferromagnetic spintronics have been demonstrated by the fabrication of tunnel devices [3,4], atomic-size proof-of concepts [5], even devices without auxiliary ferromagnetic layers [6].
We will review the recent progress in the field, and detail on the control of the ohmic resistance of an antiferromagnetic semiconductor by manipulating the magnetic state of a contiguous ferromagnetic layer. We present an oxide-based fully epitaxial heterostructure, its structural characterization and the electrical measurements showing a direct link between state of the ferromagnetic layer and the ohmic resistance of the antiferromagnetic semiconductor.

[1] S. Shick et al., Phys. Rev. B 81, 212409 (2010)
[2] T. Jungwirth et al., Phys. Rev. B 83, 035321 (2011)
[3] B.G. Park et al., Nature Materials 10, 347–351 (2011)
[4] X. Marti et al., Phys. Rev. Lett. 108, 017201 (2012)
[5] S. Loth et al., Science 335, 6065 (2012)
[6] D. Petti et al., submitted

The cubic (or pseudo-cubic) crystalline structure of perovskite ferroelectrics does not lend itself easily to one-dimensional growth. For this reason only rare reports exist on growth and properties of these materials. On the other hand, those rare reports intrigued us to pursue growth in order to study the properties of these nanowires with particular attention to size dependent properties and their potential implication. Reduction in dimensions of ferroelectrics, e.g. ultra-thin layers, results typically in deterioration of properties or at best maintenance of the bulk properties of the material. Is this a general phenomenon? We explored this question in the case of PZT nano-wires.
The main emphasis was on the growth of perovskite nano-wires. Two materials, KNbO₃ and PZT were successfully grown. The growth mechanism(s) were explored and the role of templates understood. The structure of the wires is presented, including the orientation of the spontaneous polarisation relative to the crystallographic orientation of the geometrical axes of the wires. The piezoelectric properties were investigated. The situation in which the polarization axis is inclined to the axis of the wire in a mono-domain nanowire was analysed theoretically. Such a situation corresponds, for example, to a practical case where the nanowire axis of a tetragonal PZT is not [001]. In this configuration, the depolarization field, opposing the spontaneous polarization leads to polarization rotation. In particular diameters and orientations, the appearance of new phases is predicted. Size effect on properties, such as enhancement of T_c is discussed with both positive and negative effect possible. A case where T_c is enhanced is demonstrated experimentally.

After an introduction to metamaterials, I will describe the last achievements of our group. I will focus on high-permittivity structures where the magnetic effective response is due to Mie resonances. Thanks to an original experimental approach, we are able to simultaneously measure the complex transmittance and reflectance of a thin layer consisting of TiO₂ microspheres. Thus its complex permittivity and permeability can be retrieved. Experimental results are in good agreement with simulations. The latter prove that a negative permeability of the metamaterial can be achieved by optimization of structure parameters. I will also talk about our further projects where we aim to tune the resonant response of metamaterials by means of an electric field.

Dielectric permittivity, tunability and loss are dynamical properties of ferroelectrics with many important technological applications. Accurate first-principles based effective Hamiltonian simulations have proven to be an extremely useful tool to investigate these properties of ferroelectrics, since they can: lead to fundamental understanding of GHz/THz range dielectric spectra of materials; aid in optimization of materials for technological applications; resolve controversies, for example, about exact ferroelectric phase diagram of Pb(Zr,Ti)O₃, or the existence of relaxation mode termed central mode (CM) in epitaxially strained SrTiO₃; predict in advance hitherto undiscovered characteristics; and help discover new more practicable materials for applications. Room-temperature ferroelectricity in strained SrTiO₃ has been one of the exciting discoveries in recent times and (Ba,Sr)TiO₃ systems in general have been thoroughly spectroscopically investigated for their attractive room temperature dielectric properties. Molecular dynamics (MD) Simulations incorporating first-principles-based effective Hamiltonians for these systems, were conducted to investigate: effects of epitaxial strain and Ba introduction on the complex dielectric response of SrTiO₃ in the GHz/THz regime. Some important revelations from these investigations will be discussed.

By combining the unique infrared imaging capabilities and nanoscale resolution of the scattering-type scanning near-field optical microscopy (s-SNOM) with bulk sensitivity at variable depth of low-energy muon spin rotation (LE-mSR) we determined the geometry and magnitude of the phase separation in nearly-stoichiometric superconducting Rb₂Fe₄Se₅ single crystals with T_c = 32 K [1]. The paramagnetic domains were found to have a shape of thin metallic sheets parallel to the iron-selenide plane with a characteristic size of only several nanometres out of plane but up to 10μm in plane. The intrinsic nanoscale layering of the metallic and insulating phases is confirmed by the effective-medium in-plane [2] and out-of-plane dielectric response studied by infrared ellipsometry in a combination with time-domain THz transmission spectroscopy. By means of LE-μSR we further show that the antiferromagnetic semiconducting phase occupies ~ 80 % of the sample volume in the bulk and is strongly weakend near the surface. These results have important implications for the interpretation of bulk- and surface-sensitive measurements on Rb₂Fe₄Se₅, and for the understanding of the interplay between superconductivity and antiferromagnetism in this material. In this talk, I will present two important results we recently obtained in our lab. First, the identification of the hole and electron contributions to the carrier mobility of polymer/fullerene solar cell materials previously not possible with time resolved terahertz spectroscopy or other techniques. Second is the dependence of ultrafast THz decay with the excitation density which reveals a very high charge mobility (~0.1 cm2V-1s-1) that prevails for long time (nanosecond) after charge formation.

[1] A. Charnukha et al., Phys. Rev. B 85, 100504R (2012).
[2] A. Charnukha et al. subm. to PRL, Preprint at http://arxiv.org/abs/1202.5446 (2012).

Biologická aktivita buněk závisí na fyzikálních procesech spojených s kooperací struktur mikrotubulů a organel–mitochondrií. Polární vibrační módy mikrotubulů generují koherentní elektromagnetické pole, které zajišťuje korelaci, organizaci (např. tkání) a podmiňuje biologické funkce závislé na silových mechanizmech. Silné statické elektrické pole kolem mitochondrií mění strukturu vody (a tím omezuje tlumení vibračních módů) a posouvá patrně i vibrační módy mikrotubulů do významně nelineární oblasti. Poruchy fyzikálních procesů buněk jsou podstatnou částí transformace vedoucí ke vzniku zhoubných nádorů.

Some materials generate polarization when they are subject to a homogeneous deformation. Such materials are called piezoelectric and find many everyday uses in sensors, actuators and energy harvesters. The exist another type of electromechanical coupling, called flexoelectricity, between strain gradients and polarization. Such coupling is far more general than piezoelectricity, as it affects all dielectrics and not just those with a non-centrosymmetric space group. Though this effect is more general and has been known since the sixties, it was largely ignored for decades because it is generally quite small in magnitude.

This situation, however, has begun to change in the past few years. Firstly, large flexoelectric coefficients have been measured in high-permittivity materials. Second, and perhaps most important, strain gradients can be huge at the nanoscale, so that flexoelectricity can have a considerable impact in nanoscopic devices such as ferroelectric thin films. In fact, the flexoelectric polarization can be bigger than theferroelectric/piezoelectric one in the 10nm scale.

In this talk I will start with a quick overview of the physics and salient features of flexoelectricity, and then move on to our most recent results towards applications of this phenomenon. In particular, I want to emphasize how
(i) flexoelectricity can be used as a means to enhance piezoelectricity at the nanoscale, taking advantage of the large flexoelectric polarizations generated by engineered strain gradients. This could be of potential use in energy harvesting applications [1].
(ii) flexoelectricity can also be used in information technologies as a means to switch ferroelectric polarization in thin film ferroelectric memories using a small mechanical force (1microNewton) delivered by the sharp tip of an Atomic force microscope. This flexoelectric writing mechanism removes the need to apply any voltage to the sample for writing the information state, thus removing problems associated with electric fatigue, leakage and dielectric breakdown [2].

[1] G. Catalan, A. Lubk, A. H. G. Vlooswijk, E. Snoeck, C. Magen, A. Janssens, G. Rispens, G. Rijnders, D. H. A. Blank and B. Noheda; Flexoelectric rotation of polarization in ferroelectric thin films, Nature Materials 10, 963 (2011).
[2] H. Lu, C.-W. Bark, D. Esque de los Ojos, J. Alcala, C. B. Eom, G. Catalan, A. Gruverman, Mechanical Writing of Ferroelectric Polarization, Science 336, 59-61 (2012).

In this talk, I will present two important results we recently obtained in our lab. First, the identification of the hole and electron contributions to the carrier mobility of polymer/fullerene solar cell materials previously not possible with time resolved terahertz spectroscopy or other techniques. Second is the dependence of ultrafast THz decay with the excitation density which reveals a very high charge mobility (~0.1 cm²V^-1s^-1) that prevails for long time (nanosecond) after charge formation.

Transition metal oxides show a variety of crystal structures and physical properties. The fascinating magnetic, electrical, optical and catalytic properties of these metal oxides are central to technological advances in memory devices, electronics/spintronics, sensors, nanomaterials, superconductivity etc. Chemical doping of these oxides with cations of different size and charge is widely used to obtain new and exotic phases, and for fine tuning of physical properties. Optimization of synthesis methods after careful analysis of powder X-ray diffraction patterns and finding correlations between crystal structures and physical properties are major research thrust areas in this field. In this talk, I will mostly discuss the multiferroic properties of Bi_1-xLa_xMnO₃ (x = 0.0, 0.1, and 0.2) samples prepared under 3−6 GPa pressure. Rietveld refinement of powder neutron diffraction data show that BiMnO₃ and Bi_0.9La_0.1MnO₃ adopt a monoclinic C2/c perovskite superstructure whereas Bi_0.8La_0.2MnO₃ has orthorhombic Pnma symmetry. Both structural analysis and Curie−Weiss fits to magnetic susceptibility data show that high spin d4 Mn³⁺ is present with no significant Bi deficiency or Mn⁴⁺ content apparent. La substitution suppresses the magnetic Curie temperature of the monoclinic phase from 105 K for x = 0 to 94 K at x = 0.1, but the x = 0.2 material shows antiferromagnetic order similar to that of LaMnO₃.

Multifunctional ferroic materials whose polarization, magnetization and strain could be controlled by an external influence, e.g., stress or electric/magnetic field, are a hallmark of modern engineering and technology. Both “hard” (complex oxides) and “soft” (polymers), they display a vast range of interesting physical phenomena that are yet to be fully understood and can lead to new and unusual functionalities. Showcasing one of such unexpected phenomena, with the help of first-principles-based computational techniques, we predict that Goldstone-like states (i.e., collective, close to zero frequency excitations of the system, requiring practically no consumption of energy) can be artificially induced in a layered-oxide compound with polarization constrained to a plane. Such excitations had been shown to exist in ferroelectric liquid crystals, which have very weak polarization, and some magnetic structures (as spin waves) but never before in “hard” polar materials. We demonstrate that in the layered-oxide system their presence results in an emergence of a variety of highly useful physical properties that include large, tunable dielectric constants and an ability to easily form vortex polar states in a nanodot geometry. In a similar fashion to the well-known perovskite materials with morphotropic phase boundaries (MPBs), these states emerge as polarization rotations with almost no energy penalty, suggesting that the existence of an MPB is actually yet another manifestation of the Goldstone theorem in solids. Unique functionalities of the proposed template compound with Goldstone-like behavior can be exploited for improved energy generation, storage and conversion, opening up new avenues for disruptive advances in electroactive materials design.

The highest piezoelectric coefficients known to date are obtained in single crystals poled along a non-polar direction, whereby they exhibit a more or less complex ferroelectric domain structure. This discovery had led to the development of "domain engineering", which can be defined as the design of stable domain structures with optimized electromechanical properties. Efficient homogeneization methods are required in order to understand the details of the different contributions to the macroscopic properties. This talk is meant as an overview of this problem, with examples taken from relaxor-based PZN-PT and PMN-PT solid solutions.

In 2004, the use of a first-principles-based effective Hamiltonian [1] led to the prediction of a novel structure in zero-dimensional ferroelectrics, in which the electric dipoles organize themselves to form a vortex [2]. Such structure exhibits the so-called spontaneous toroidal moment, rather than the spontaneous polarization, as its order parameter [2]. Subsequently, other phenomena, all related to vortices, were predicted in ferroelectric nanostructures. Examples of such phenomena are:

• (i) the existence of a new order parameter, denoted as the hypertoroidal moment, that is associated with many complex dipolar structures (such as double-vortex states) [3];
• (ii) the possible control of single and double vortex states by electric fields, via the formation of original intermediate states [4-8];
• (iii) the discovery of a new class of quantum materials (denoted as incipient ferrotoroidics), for which zero-point vibrations wash out the vortex state and yield a complex local structure [9];
• (iv) the existence of chiral patterns of oxygen octahedral tiltings that originate from the coupling of these tiltings with the ferroelectric vortices [10].

A number of recent developments in the field of polymer stabilized liquid crystals will be discussed. These are of academic as well as applied interest, such as the visualisation of director fields in the vicinity of topological defects in liquid crystals, polymer network formation in anisotropic fluids and structuring at the nano-scale, interactions with ferroelectric liquid crystals, all the way to concepts of applicational relevance for example in the field of electronic paper, or novel display concepts based on liquid crystalline Blue Phases.

Piezoelectric materials offer a wider range of applications ranging from actuators over sensors to novel concepts for piezoelectric energy harvesting involving transducers. In that respect, the pseudo-binary solid-solution system Pb[Zr_1-xTi_x]O₃ (PZT) is the material-of-choice, where the desired materials properties for a given device can be tailored by controlling the defect structure in terms of aliovalent doping with transition-metal and rare-earth ions. By using multi-frequency electron paramagnetic resonance (EPR) spectroscopy, local information at the (paramagnetic) dopant sites can be gathered, including information about the dopant site with respect to domain walls or grain boundaries. Recent results allow to monitor the formation of defect complexes between acceptor-type dopants and charge-compensating oxygen vacancies. Furthermore, the defect chemistry emerges from the EPR results and is described for acceptor-doped PZT, as well as for ‘lead-free’ alternatives, such as [Ka0.5Na0.5]NbO3 (KNN) and [Bi_0.5Na_0.5]TiO₃ (BNT). Finally, dynamic properties that include the switching upon external poling and the degradation as function of cyclic loading (electrical fatigue), as well as constant thermodynamic boundary conditions (aging) are studied.

In general our knowledge about friction and diffusion mechanisms on microscopic length scales is still very vague. Nevertheless it has been shown in recent years that microscopic processes determine the macroscopic friction behaviour between surfaces. If we can gain more detailed insight into the microscopic processes, we would be able to find improved solutions for a multitude of scientific and technical questions, such as reduction of friction in bearings, reduction of wear, better control and understanding of liquids in confinements, or we could even dream up motorisation solutions for nano-motors. We have recently started a research programme on the diffusion of carbon based model systems, which is already changing our picture of diffusion at the nanometre scale [1,2]. An adequate description of the diffusion of adsorbed molecules requires experimental and theoretical techniques that, one the one hand, cover the time and spatial dimensions of molecular diffusion, and which, one the other hand, are able to differentiate between different diffusion processes, such as rotations, translations of vibrations. Scanning probe microscopy has seen an impressive development in recent years and was able to deliver friction measurements on molecular length scales [3]. Scanning techniques are in general, however, limited to low temperature measurements, due to their limited frame repetition time. Neutron spectroscopy can deliver the complimentary information needed at temperatures of technological relevance, covering time scales of 10-12–10-6 s, on molecular length scales [4]. Substantial improvements in spectrometer performance allow us to investigate diffusion of molecules on surfaces in intricate detail. The interpretation is underpinned by extensive molecular dynamics (MD) simulations, which can now be performed on rather small computing clusters - even for large systems [1,2].

[1] H. Hedgeland, P. Fouquet et al., Nature Phys. 5, 561 (2009).
[2] P. Fouquet et al., Carbon 47, 2627 (2009).
[3] M. Dienwiebel et al., Phys. Rev. Lett. 92, 126101 (2004).
[4] P. Fouquet et al., Z. Phys. Chem. 224, 61 (2010).

The development of NMR techniques applied in the last ten years to partially oriented systems, and in particular to liquid crystals, is the object of this talk. The evolution of NMR methods (i.e. new NMR pulse sequences) and the improvement of both theoretical models and mathematic tools for the analysis of NMR data specific for partially ordered systems allowed scientists to extend thein research to more and more complex materials, such as dendrimers, polymers and membranes, and to investigate unique phenomena, such as field-induced alignment and confining effects. Furthermore, the fast development of Nanoscience and Biomedicine is offering a rich variety of new “physical chemical” problems related to partially ordered materials. Recent selected research works will be discussed with particular emphasis:
i) the effect of high magnetic fields on the supramolecular structure of chiral liquid crystalline phases, such as the SmC*, TGBA* and ‘de Vries’ type of SmA* phases, by means of solid state NMR and
ii) the influence of the LC environment on the conformational properties of rod-like mesogens studied by high resolution solid state 13C NMR methods.

The control of surface wettability is a major challenge for many practical applications. The biomimetism of natural surfaces allowed for the deeper understanding of our fundamental knowledge of surface wettability. The consequence is the development of many methods to reproduce this phenomenon and to conceive anti-wetting surfaces from water to oil. Thus, some reviews on superhydrophobic materials were reported, showing the interest of the scientific community in this domain. However, the elaboration of superoleophobic surfaces was reported only in few works, mainly based on a fluorinated post-treatment of rough surfaces. Indeed, the lower the surface tension of the liquid probe is, the more difficult it is to prevent spreading out. Here, we report the first example of the elaboration of superoleophobic surfaces by electrodeposition (one-pot build-up method). In order to consider the possible modulation of oleophobic behaviour, the relation between the surface properties and the electrochemical parameters is also presented.

Program:

• Xiaomin Li: Resistance Switching of Transition Metal Oxides for RRAM Application
• Haosu Luo: Piezoelectric performances of lead-free piezoelectric Na_1/2Bi_1/2TiO₃-BaTiO₃ (NBBT) single crystals
• Haosu Luo: Piezoelectric performances of high Tc ternary PIMNT single Crystals

The diverse functional properties available in oxides have been established primarily through measurements on bulk samples. But when it comes to utilizing these properties in microelectronics, thin film synthesis techniques come into play and with them a different set of tricks are available to enhance the properties of oxides. Here I describe the use of reactive molecular-beam epitaxy (MBE) to synthesize multifunctional oxides, including ferroelectrics, ferromagnets, and materials that are both at the same time. The structural perfection of the resulting films is shown to surpass the best single crystals available of these materials; indeed multifunctional oxides with structural quality rivaling semiconductors have been achieved. Standard semiconductor tricks involving epitaxial strain and confined thickness can also be applied to oxide thin films and results of fundamental scientific importance as well as revealing the tremendous potential of utilizing multifunctional oxide thin films to create materials with enhanced performance will be shown.

Transparent ceramic has become a hot-topic due to the impact that these kind of materials will have in several technological areas, but specially for aerospace and solid state laser applications.Different types of transparent and translucent ceramics will be reviewed. Afterwards, the defects that limit the transparency of ceramics will be briefly explained. Consequently, the processing related to transparent ceramics has been specifically designed to avoid or minimize them. Several examples of transparent ceramics fabrication will be presented, with special emphasis in Hot Isostatic Pressing and Spark Plasma Sintering, optical characterization methods, both theoretical and experimental are being to be described.

Artificial BST based superlattices are potential candidates for applications such as storage capacitors and dielectrics for the next generation dynamic random access memories and tunable microwave devices. Using first-principles-based Monte-Carlo and Molecular Dynamics simulations using effective Hamiltonians, we predict the temperature-versus-misfit strain phase diagram, domain evolution, and dielectric response in these systems. Our studies reveal an unusually complex phase diagram involving novel nanodomain phases with anomalous microstructures. The microscopic origins of these anomalous behaviors and their manifestations in the static and dynamic dielectric response will be discussed. This talk will also present results obtained using first principles density functional calculations and inelastic neutron scattering measurements of PbTiO₃, BaTiO₃ and SrTiO₃ carried out at the ILL, Oak Ridge National Lab., and the Intense Pulsed Neutron Source.

As a laser scientist I would like to concentrate in the seminar on the new techniques using `muSR in conjunction with a simultaneous laser excitation of the sample with applications in chemistry, semiconductor physics, spintronics etc. RIKEN muon facility is also unique in the world in the development of laser based technique for slowing down (or cooling) muons from 4.1 MeV to eV level that allows muon implantation near surfaces with nanometre resolution and extension of the muSR technique to studying small samples and thin multilayered films. The applications do not end there: the sub eV energy muon beam would be a perfect source for stringently testing the Standard Model by measuring the Anomalous Magnetic Moment of the Muon (g-2) to unprecedented precision.

We measured the dynamic structure factor of the liquid and glassy phases of the solution LiCl-6H2O by means of inelastic scattering of radiation in the visible, UV and X-ray range from 1 GHz to 10 THz and by means of photon correlation spectroscopy from 0.01 Hz to 20 kHz. The measurements were performed in a broad temperature range between 353 K and 80 K. Our data show that a single relaxation process, which at high temperature has features similar to those of the single relaxation of pure water, starts to differentiate into two relaxations: structural (alpha-) and secondary (beta-), upon cooling below 220 K. On cooling the s ystem at lower temperature, the relaxation strength reveal an uncommon behavior with respect to most glass-forming systems : the beta-relaxation is the continuation of the single process existing at high temperature and an onset occurs for the alpha-relaxation from the beta-process.