Electron-hole plasma expansion with velocities exceeding c/50 and lasting over 10 ps at 300 K was evidenced by time-resolved terahertz spectroscopy [1]. This regime, in which the carriers are driven over >30 μm is governed by stimulated emission due to low-energy electron-hole pair recombination and reabsorption of the emitted photons outside the plasma volume. At low temperatures a speed of c/10 was observed in the regime where the excitation pulse spectrally overlaps with emitted photons, leading to strong coherent light-matter interaction and optical soliton propagation effects.

Reference:
[1] T. Troha, F. Klimovič, T. Ostatnický, F. Kadlec, P. Kužel, and H. Němec, Ultrafast long-distance electron-hole plasma expansion in GaAs mediated by stimulated emission and reabsorption of photons, Phys. Rev. Lett. 107, 226301 (2023).

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Pb(Zr_1-xTi_x)O₃ with very high content of Zr shows an antiferroelectric ground state and possesses an exceptional property: the coexistence of several built-in structural instabilities at high temperatures. This leads to the possibility of their successive condensation on cooling to trigger a sequence of phase transitions, instead of reaching directly the antiferroelectric state, as in pure PbZrO₃. This peculiar behaviour is more pronounced in compositions near the antiferroelectric morphotropic phase boundary and the tricritical point around room temperature, as Pb(Zr_0.95Ti_0.05)O₃ (PZT 95/5), where three phases are energetically available.

Raman scattering experiments performed out of the thermodynamical equilibrium revealed a complex and hysteretic thermal behaviour of the phase transition dynamics, due to the inhomogeneous microstructure and the coexistence of regions with different phases within the samples. A way to control the thermal development of these different phases is to selectively condense the different instabilities by an external parameter such as temperature, hence creating a sequence of phase transitions instead of a direct phase transition.

Our results, obtained under many different experimental conditions and specific pre-history, suggest that ceramics with composition near PZT 95/5 are potential materials for novel 3-state thermal switches. An innovative approach of phase-control is proposed, based on the thermal behaviour of the intermediate polar states observed and using small temperature gradients by appropriate heating-cooling cycles around room temperature.

Reference:
[1] E. Buixaderas, C. Milesi-Brault, P. Vaněk, J. Kroupa, F. Craciun, F. Cordero, C. Galassi, Peculiar Dynamics of Polar States at the Morphotropic Phase Boundary of Antiferroelectric Pb(Zr_1-xTi_x)O₃, Acta Materialia, 119208 (2023), online.

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Our study delved into a theoretical exploration of charged domain walls in ferroelectric PbTiO₃, which were compensated by randomly distributed immobile charge defects located within a relatively broad slab. To achieve this, we employed a combination of atomistic shell-model simulations and continuous phase-field simulations based on the Ginzburg-Landau-Devonshire model. Our findings showed that domain walls form a zigzag pattern, and we examined their properties across a broad range of compensation-region widths, ranging from a few nanometers to over 100 nm, focusing in particular on understanding the zigzag modulation lengths in terms of material properties of PbTiO₃. The zigzag formation is accompanied by a local ferroelectric-polarization rotation, which we proposed as an efficient mechanism for local charge compensation. Our study provides a new understanding of the behavior of charged domain walls in ferroelectric materials and highlights the significance of the considered compensation charges in their formation. The insights gained from our study may contribute to the development of advanced ferroelectric materials with applications in the field of smart-materials for electronics.

Reference:
[1] P. Marton, M. A. P. Gonçalves, M. Paściak, S. Körbel, V. Chumchal, M. Plešinger, A. Klíč, and J. Hlinka, Zigzag charged domain walls in ferroelectric PbTiO₃, Phys. Rev. B 107, 094102 (2023).

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Ultrathin films of HfO₂ become ferroelectric although bulk crystals are paraelectric. In this review we describe how grain size, thermal stress, dopants, oxygen vacancies, film thickness, annealing process and electrodes influence dielectric properties of HfO₂, which are important for applications in high-density resistive random access memories and ferroelectric memories. We also discuss how to achieve superlative performance with high-speed reliable switching, excellent endurance and retention.

Reference:
[1] W. Banerjee, A. Kashir, and S. Kamba, Hafnium Oxide (HfO₂) – A Multifunctional Oxide: A Review on the Prospect and Challenges of Hafnium Oxide in Resistive Switching and Ferroelectric Memories, Small 18, 2107575 (2022).

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Reference:
[1] M. Hadjimichael, Y. Li, E. Zatterin, G. A. Chahine, M. Conroy, K. Moore, E. N. O’ Connell, P. Ondrejkovic, P. Marton, J. Hlinka, U. Bangert, S. Leake, and P. Zubko, Metal–ferroelectric supercrystals with periodically curved metallic layers, Nat. Mater. 20, 495 (2021).

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For the series of compounds, we studied the mesomorphic properties by various experimental techniques and confirmed the phase identification by x-ray measurements. For selected homologues we proved the antiferroelectric phase with orthoconic properties existing in a wide temperature interval including the room temperatures. Valuable optical properties with the tilt angle about 45 degrees promised a big potential for applications.

Reference:
[1] N. Podoliak, M. Cigl, V. Hamplová, D. Pociecha, and V. Novotná, Multichiral liquid crystals based on terphenyl core laterally substituted by chlorine atom, J. Mol. Liq. 336, 116267 (2021).

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Epitaxial strained thin films of (SrTiO₃)_n-1(BaTiO₃)₁SrO were grown on DyScO₃ substrates using molecular beam epitaxy [1]. The best microwave dielectric properties were discovered in samples with n= 6. Permittivity exhibits huge tuning using electric field and  microwave dielectric loss is anomalously low. Unique properties were confirmed using first-principles calculations and by experimental observation of the soft mode behavior in THz region. These films are ideal for components in 5G networks.

Collaborating institutions: Prof. D.G. Schlom from the Cornell University and other American and German institutions.

Reference:
[1] N.M. Dawley, E.J.Marksz, A.M. Hagerstrom, G.H. Olsen, M.E. Holtz, V. Goian, C. Kadlec, J. Zhang, X. Lu, J.A. Drisko, R. Uecker, S. Ganschow, C.J. Long, J.C. Booth, S. Kamba, C.J. Fennie, D.A. Muller, N.D. Orloff, D.G. Schlomk, Targeted chemical pressure yields tuneable millimetre-wave dielectric, Nature Mater. 19, 176 (2020).

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Water molecules localized in structural crystal nano-channels give a possibility to eliminate hydrogen bonds, dominating among water molecules at short distances up to ~2 Å which prevent ordering of their dipole moments. On larger distances 1-10 nm the Coulombic interactions dominate, which can lead to their ordering. We continued in our previous studies on beryl, where we, on cooling, have detected tendency to ferroelectric ordering, so called incipient ferroelectric behaviour. Mineral cordierite (Mg,Fe)₂Al₄Si₅O₁₈ also comprise structural channels similar to those in beryl, in which the chains of water molecules are ideally suited for the ordering study (Fig. 1).

Crystal structure is orthorhombic, the channels in the c-axis direction are in the distance of 9.9 Å  and the water molecules in the channels are by 4.7 Å from each other. Their dipole moment is perpendicular to the channel axis and our dielectric spectroscopy study have revealed their ordering at ~3 K. The critical relaxation tending to this ordering is polarized along the a-axis direction and lies in the radiofrequency range (Fig. 2). Spontaneous polarization measurements yield the saturated value of ~3 nC/cm² at 0.3 K. Molecular dynamics calculations show the ferroelectric ordering in the plane perpendicular to the channels and antiferroelectric ordering along them.

Reference:
[1] M. A. Belyanchikov, M. Savinov Z. V. Bedran, P. Bednyakov, P. Proschek, J. Prokleska, V. A. Abalmasov, J. Petzelt, E. S. Zhukova, V. G. Thomas, A. Dudka, A. Zhugayevych, A. S. Prokhorov, V. B. Anzin, R. K. Kremer, J. K. H. Fishcer, P. Lunkenheimer, A. Loidl, E. Uykur, M. Dressel, and B. Gorshunov Targeted chemical pressure yields tuneable millimetre-wave dielectric,
Nat. Commun. 11, 3927 (2020)

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The phase transition was predicted already ten years ago in films with 4% strain, but we have observed it only now in films with 6.4% strain. As such strain tends to relax after the epitaxial growth of only a few monolayers, we have achieved it in (EuO)₂/(BaO)₂ superlattices grown epitaxially on LSAT substrates. The observation is supported by a new DFT calculation.

Reference:
[1] V. Goian, R. Held, E. Bousquet, Y. Yuan, A. Melville, H. Zhou, V. Gopalan, P. Ghosez, N. A. Spaldin, D. G. Schlom, and S. Kamba, Making EuO multiferroic by epitaxial strain engineering, Commun. Mater. 1, 74 (2020).

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In addition to the celebrated layer group analysis of domain twins, the present method allows us to investigate tensor properties of domain walls also for the case where order-parameter variables other than the spontaneous ones are active [1].

Reference:
[1] W. Schranz, C. Schuster, A. Tröster, and I. Rychetský, Polarization of domain boundaries in SrTiO₃ studied by layer group and order-parameter symmetry, Phys. Rev. B 102, 184101 (2020).

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The effect was theoretically explained by a demagnetizing field and confirmed by magnetization measurements. THz refractive index revealed also a large anisotropy in external magnetic field. This anisotropy is due to activation of the ferromagnetic resonance only if the magnetic field is applied perpendicularly to the magnetic component of the THz radiation.

Reference:
[1] D. Repček, C. Kadlec, F. Kadlec, M. Savinov, M. Kachlík, J. Drahokoupil, P. Proschek, J. Prokleška, K. Maca, and S. Kamba, Seemingly anisotropic magnetodielectric effect in isotropic EuTiO₃ ceramics, Phys. Rev. B 102, 144402 (2020).

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The magnetism of solids is macroscopic evidence of microscopic quantum dynamics of spins of valence electrons. Magnetic susceptibility of metals with delocalized conduction electrons should theoretically be of Pauli character. However, the transition metals react on the magnetic field via the Curie-Weiss law due to local magnetic moments. We identified the microscopic mechanism combining adequately quantum and thermal fluctuations in metals with strong electron correlations that lead to the genesis of local magnetic moments and the Curie-Weiss susceptibility [1].

Reference:
[1] V. Janiš, A. Klíč, J. Yan, and V. Pokorný, Curie-Weiss susceptibility in strongly correlated electron systems, Phys. Rev. B 102, 205120 (2020).

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The lower-temperature one is characterized by incommensurate modulations while the higher-T intermediate phase is defined by anti-phase rotations of oxygen octahedra. Two kinds of quadrupole splitting indicate that the environment of Sn is locally non-centrosymmetric even in the cubic phase [1].

Reference:
[1] I. Jankowska-Sumara, M. Paściak, M. Kądziołka-Gaweł, M. Podgórna, A. Majchrowski, and K. Roleder, Local properties and phase transitions in Sn doped antiferroelectric PbHfO₃ single crystal, J. Phys.: Condens. Matter 32, 435402 (2020).

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Reference:
[1] V. Novotná, V. Hamplová, L. Lejček, D. Pociecha, M. Cigl, L. Fekete, M. Glogarová, L. Bednárová, P. Majewski, E. Gorecka. Organic nanotubes created from mesogenic derivatives Nanoscale Adv. 1, 2835(2019).

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The archetypical uniaxial tungsten-bronze (Sr_xBa_1−x)Nb₂O₆ (SBN) has been found to show two different ferroelectric subsystems within its structure [Fig 1a]. The uneven shifts of the two Nb atoms create two different polarization regions of dissimilar lengths and volumes: Nb(2) shifts along the polar axis are smaller and less correlated, as a result of the Nb(2)O₆ tilts in the perovskite units. On the contrary the linking octahedra Nb(1)O₆ are isolated and more rigid, and the Nb atom inside is more akin to shift along the c-axis contributing more to the polarization [Fig. 1b]. Because of this, there are at least two correlation lengths associated with the polarization mechanisms.

The high-frequency dielectric behaviour of SBN (from 10⁴ to 10¹³ Hz), investigated in a broad temperature interval for compositions from pure ferroelectric (SBN-35) to relaxor (SBN-81) ones, revealed that the dielectric response along the polar axis consists of three relaxations related to polarization mechanisms with various correlation lengths of mesoscopic order [1].. All of them show dissimilar behaviours with temperature, pointing out to their distinct nature [Fig. 2]. A temperature-dependent central mode at THz frequencies and a relaxation above 10 GHz are accompanied by the slowing down of a relaxation in the MHz range. This complex response reveals coexistence of displacive and order-disorder scenarios in this family.

Reference:
[1] E. Buixaderas, M. Kempa, Š. Svirskas, C. Kadlec, V. Bovtun, M. Savinov, M. Paściak, and J. Dec, Dynamics of mesoscopic polarization in the uniaxial tetragonal tungsten bronze (Sr_xBa_1−x)Nb₂O₆, Phys. Rev. B 100, 184113 (2019).

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The observed phonon activities are compared with the predictions from the factor-group analysis. In the high-temperature phase, more phonons are observed than the number predicted for the cubic symmetry. This is explained by the presence of rhombohedral clusters in the cubic phase. The strongest variations occur in the THz spectra near the two magnetic phase transitions, at T_N1 = 87 K and T_N2 = 63 K. These activate new modes in the spectra, with resonance frequencies and intensities changing with temperature and magnetic field. Below T_N2, we observed a transfer of oscillator strengths from low-frequency phonons to these excitations, which we assign to electromagnons.

Reference:
[1] S. Kamba, V. Goian, F. Kadlec, D. Nuzhnyy, C. Kadlec, J. Vít, F. Borodavka, I. S. Glazkova, A. A. Belik, Changes in spin and lattice dynamics induced by magnetic and structural phase transitions in multiferroic SrMn₇O₁₂, Phys. Rev. B 99, 184108 (2019).

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Reference:
K. Tesar, I. Gregora, P. Beresova, P. Vanek, P. Ondrejkovic, and J. Hlinka, Raman scattering yields cubic crystal grain orientation, Scientific Reports 9, 9385 (2019).

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The lowest-frequency phonon of the E symmetry polarized perpendicular to the local dipole moments undergoes softening towards T* ≈ 400 K, which brings evidence about a local structural phase transition. This softening is also responsible for previously observed high temperature dependence of permittivity, which follows the Curie-Weiss law.

References:
[1] D. Nuzhnyy, J. Petzelt, V. Bovtun, S. Kamba, J. Hlinka, Infrared, terahertz, and microwave spectroscopy of the soft and central modes in Pb(Mg1/3Nb2/3)O3, Phys. Rev. B 96, 174113 (2017).
[2] D. Nuzhnyy et al., Soft mode driven local ferroelectric transition in lead-based relaxors, Appl. Phys. Lett. 114, 182901 (2019).

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Reference:
J. Fábry, Degenerate (identity) chemical reactions in ferro­elastic crystals, Acta Cryst. B75, 287(2019).

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The three-dimensional distribution of the x-ray diffuse scattering intensity of BaTiO₃ has been recorded in a synchrotron experiment and simultaneously computed using molecular dynamics simulations of a shell model. Together, these have allowed the details of the disorder in paraelectric BaTiO₃ to be clarified. The narrow sheets of diffuse scattering, related to the famous anisotropic longitudinal correlations of Ti ions, are shown to be caused by the overdamped anharmonic soft phonon branch. This finding demonstrates that the occurrence of narrow sheets of diffuse scattering agrees with a displacive picture of the cubic phase of this textbook ferroelectric material. The presented methodology allows one to go beyond the harmonic approximation in the analysis of phonons and phonon-related scattering.

Reference:
[1] M. Paściak, T. R. Welberry, J. Kulda, S. Leoni, and J. Hlinka, Dynamic Displacement Disorder of Cubic BaTiO₃, Phys. Rev. Lett. 120, 167601 (2018).

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Reference:
[1] J. Vít, F. Kadlec, C. Kadlec, F. Borodavka, Y.S. Chai, K. Zhai, Y. Sun and S. Kamba, Electromagnon in the Y-type hexaferrite BaSrCoZnFe11AlO22, Phys. Rev. B 97, 134406 (2018).

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Reference:
[1] V. Bovtun, J. Petzelt, M. Kempa, D. Nuzhnyy, M. Savinov, S. Kamba. S. M. M. Yee, D. A. Crandles, Wide range dielectric and infrared spectroscopy of (Nb+In) co-doped rutile ceramics, Phys. Rev. Mat. 2, 075002 (2018).

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The pursuit of nano-electronic applications motivated CDW studies in the 21^st century. CDWs have been documented in many ferroelectrics, including perovskite and non-perovskite oxide materials, polymer compositions, and improper ferroelectrics. A conductivity wall/domain contrast up to 13 orders of magnitude has been recently reported. A number of methods enabling CDW engineering have been developed, and first device prototypes exploiting CDWs appeared.

In the present work, we are aiming at a synoptic presentation of the main concepts behind the understanding of CDWs, combined with a brief and updated review of important findings in the field [1].

Reference:
[1] P. S. Bednyakov, B. I. Sturman, T. Sluka, A. K. Tagantsev, and P. V. Yudin, Physics and applications of charged domain walls, npj Computational Materials 4, 65 (2018).

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Reference
[1] Kun Li, Xiao Li Zhu, Xiao Qing Liu, Xiao Ma, Mao Sen Fu, Jan Kroupa, Stanislav Kamba, and Xiang Ming Chen, Electric-field induced phase transition and pinched P-E hysteresis loops in Pb-free ferroelectrics with tungsten bronze structure, NPG Asia Mat. 10, 71 (2018).

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Usually, all the relevant charge carrier scattering processes in the studied system are not known since their independent experimental determination is difficult. The usual approach based on the Kubo formula introduces a phenomenological charge scattering rate (or relaxation time) accounting for all the scattering processes. This approximation is highly pertinent in the optical range. However, the approach fails at low frequencies in nanocrystals in the regime where the scattering rate is comparable to the probing frequency; e.g., it always yields nonzero conductivity at zero frequency (dc regime) even if the nanocrystals are mutually perfectly isolated.
We have shown [1] that the broken translation symmetry of the nanostructures induces a broadband drift-diffusion current, which is not taken into account in the analysis based on Kubo formula in the relaxation time approximation. The proper introduction of this current removes all the contradictions, fulfills the classical limit in the case of large nanocrystals and it is at the origin of significant reshaping of the conductivity spectra up to terahertz or multiterahertz spectral ranges. It is used for the interpretation of temperature dependent photoconductivity spectra in various nanocrystal systems.

Reference:
[1] T. Ostatnický, V. Pushkarev, H. Němec, and P. Kužel, Quantum theory of terahertz conductivity of semiconductor nanostructures, Phys. Rev. B 97, 085426 (2018).

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In the paper by Sejkora et al [1] several members of the CuSbS₂-CuSbSe₂ join were studied by micro–Raman spectroscopy:

• příbramite CuSbSe₂, which got its name from its bith place, Příbram (Czech Republic)
• chalcostibite CuSbS₂ from Dúbrava (Slovak Republic)
• Se-rich chalcostibite

The main internal and external vibrations of these compounds were identified and related to pyramidal Sb(S,Se)₃ groups, Cu-(S,Se) bonds and external lattice modes. The příbramite mineral only forms polycrystalline aggregates up to 100 μm in size, therefore it can be studied just by specific techniques. Using micro-Raman spectroscopy it was shown that vibrations of SbSe₃ groups, display much higher frequency due to the almost twice mass of selenium compared to sulphur. These results demonstrate the application of Raman spectroscopy for micro-identification of sulphide/selenide minerals and for precise studies of anion isomorphs in different mineral phases.

Reference:
[1] Jiří Sejkora, Elena Buixaderas, Pavel Škácha, Jakub Plášil, Micro-Raman spectroscopy of natural members along CuSbS₂-CuSbSe₂ join , J. Raman Spectroscopy 49, 1364 (2018).

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New lactic acid derivatives have been prepared and studied. We have found that they form cholesteric phase with the helix pitch length in interval 120 – 200 nm within a broad temperature range. Due to the positive dielectric anisotropy and the short pitch, the applied electric field causes reversible optical changes in planar cell, due to reorientation the long molecular axis in the applied electric field and electro-optical effect was observed under polarizing microscope. Presence of short pitch and possibility to effectively affect the optical properties is promising from the point of view of specific applications. A model based on disclinationswas developed to explain the stripe texture under a sufficiently high electric field. The model also allows us to estimate the surface anchoring energy.

References
[1] V. Novotná, V. Hamplová, M. Glogarová, L. Lejček, E. Gorecka, Effect of the applied electric field on new cholesterics with extremely short pitch, Liq. Cryst. 45, 634 (2018),
[2] L. Lejček, V. Novotná, M. Glogarová, A model of field induced stripe texture in the cholesterics with extremely short pitch, Liq. Cryst. 46 (2019), DOI 10.1080/02678292.1550689.

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The combination of structural and broad-band dielectric studies in SBN-35 suggests that ferroelectricity in TTBs in caused by a more complex mechanism than in perovskites. Several excitations were identified related to the multiple mechanisms responsible for the ferroelectric phase transition [Fig.2]:

• Phonons, related to cation displacements along the polar axis,
• An anharmonic excitation located in the THz range (the CM ν_THz), caused by the dynamic disorder of Sr and Ba atoms located at the A2 sites in the pentagonal channels, as supported by the high anisotropic displacements found in the electron diffraction experiment.
• A relaxation in the GHz range, ν₀₁, which slows down to several MHz on cooling and related probably to Nb atoms dynamics.
• A relaxation which appears in the spectra below T_C near 1 GHz and hardens on cooling, consistent with the oscillations of the ferroelectric domain walls.

Results were published in [1].

Reference:
[1] E. Buixaderas, M. Kempa, V. Bovtun, C. Kadlec, M. Savinov, F. Borodavka, P. Vaněk, G. Steciuk, L. Palatinus, and J. Dec, Multiple polarization mechanisms across the ferroelectric phase transition of the tetragonal tungsten-bronze Sr_0.35Ba_0.61Nb₂O_6.04 , Phys. Rev. Materials 2, 124402 (2018).

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NbN is a prototypical BCS superconductor in the ground state. In the strong excitation regime the pump pulse immediately breaks all the Cooper pairs into separate quasiparticles. Subsequently, as the heat is dissipated out of the films towards the substrate, Cooper pairs start to recover. Using time-resolved terahertz spectroscopy, we characterized a series of NbN films with various thicknesses under such strong photoexcitation. Analysis of the photoconductivity spectra reveals that the recovery of Cooper pairs initially proceeds through the emergence of mutually isolated superconducting islands, which subsequently grow with increasing time towards a nearly percolated superconducting network. The superconductivity restoring is faster than the recovery of the superconducting gap to its equilibrium value. Most interestingly, experiments suggest that the profile of the density of states of strongly photoexcited films differs during the recovery process from the ones obtained for the thermal equilibrium at any temperature. This phenomenon is controlled by confinement effects within the films.

Reference:
[1] M. Šindler, C. Kadlec, P. Kužel, K. Ilin, M. Siegel, and H. Němec, Departure from BCS response in photoexcited superconducting NbN films observed by terahertz spectroscopy, Phys. Rev. B 97, 054507 (2018).

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In this study we investigate different types of interfaces in a multiferroic BiFeO₃. By means of atomistic modelling we show that some configurations of interfaces can greatly enhance the permittivity by hosting terahertz-range collective polar fluctuations. The key result is that the permittivity is modified in the THz range. This fundamental finding and its understanding can be used in a design of new materials [ J. Hlinka, M. Pasciak, S. Körbel, and P. Márton, Phys. Rev. Lett. 119, 057604 (2017) ].

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It exploits the configuration space attached to the eigenvectors of the modes frozen in the ground state. This allows to probe the energy surface in the vicinity of the ground state, which is most relevant for the properties of the ordered phase. We apply this procedure to BiFeO₃ in order to determine potential energy associated with strain, polarization, and oxygen octahedra tilt. [ P. Marton, A. Klíč, M. Paściak, and J. Hlinka, Phys. Rev. B 96, 174110 (2017) ].

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A central mode excitation in the THz range, due to anharmonic dynamics of cations, shows critical behaviour towards T~400 K. At lower frequencies, around 10 GHz, another excitation (ν_DW) appears below T_m~330K, which is related to the development of ferroelectric microdomains.
In addition, several relaxations below the phonon frequencies, play an essential role in the dielectric response of the crystal. The main contribution to the permittivity comes from a strong relaxation (ν₀₁) present in the GHz range at high temperatures which slows down on cooling following the Arrhenius law. A second relaxation with lower frequency (ν₀₂) slows down, as well, contributing to the permittivity mainly near T_m. Both these relaxations can be assigned to polar fluctuations, probably flipping and breathing of polar nanodomains.

Altogether, the four mechanisms explain, above the kHz range, the ferroelectric transition in SBN-81 as well as its relaxor character, which differs from the behaviour displayed by SBN-61 and lead-based relaxors.

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We have synthesized and characterized mesomorphic behaviour of a new type of bent-core dimers in which bent-core molecules are connected core-to-core via the alkoxy chain. Interestingly, despite short linkage connecting the mesogenic cores, which should stiffen the molecular structure, the liquid crystalline properties characteristic for bent-core mesogens have been observed. Dimers are more stable in comparison with analogous monomers and the type of mesophases strictly depends on the length of the terminal chains. The shortest studied homologues formed the intercalated smectic A phase, For the dimers with intermediate terminal chains, the columnar phase appears, built of small layer fragments arranged into body centred 2-dimensional crystallographic lattice. The longest homologues exhibit the SmCP phase with antiferroelectric type of switching in the applied electric field [ M. Horčic, J. Svoboda, V. Novotná, D. Pociecha, and E. Gorecka, Chem. Commun. 53, 2721 (2017)].

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The lowest-frequency phonon of the E symmetry polarized perpendicular to the local dipole moments undergoes softening towards T* ≈ 400 K, which gives evidence about a local structural phase transition. This softening is also responsible for previously observed high temperature dependence of permittivity, which follows the Curie-Weiss law with the same critical temperature even without any anomaly in the low-frequency permittivity [D. Nuzhnyy et al, Phys. Rev. B 96, 174113 (2017)].

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For each species, a matrix of representative equilibrium property tensors in both high-symmetry and low-symmetry phases, showing emergence of spontaneous components, were explicitly worked out [http://palata.fzu.cz/species/13x13axial]. Results can serve as a useful tool in search of fundamentally new material properties [J. Hlinka et al., Phys. Rev. Lett. 116, 177602 (2016)].

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In this regime the polaron mobility depends on the polaron density; as a consequence, highly complex transport phenomena occur on a picosecond scale involving fine interplay between diffusion and recombination. We developed a general model to disentangle these phenomena and we determined dynamical properties of electron and hole polarons. At room temperature internal degrees of freedom of polarons are also observed through a dramatic increase of the transient permittivity in photoexcited state
[ V. Zajac, H. Němec, and P. Kužel, Phys. Rev. B 94, 115206 (2016)].

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We synthesized ferroelectric BaTiO₃ films on Ti and Ti39%Nb substrates by hydrothermal method [2, 3]. The bone cell growth is significantly higher on Ti39%Nb substrates coated by BaTiO₃ than that on non-coated Ti39%Nb, glass and reference polystyrene [3].

References
[1] P. Vaněk, Z. Kolská, T. Luxbacher, J.A.L. García, M. Lehocký, M. Vandrovcová, L. Bačáková, and J. Petzelt, Electrical activity of ferroelectric biomaterials and its effects on the adhesion, growth and enzymatic activity of human osteoblast-like cells, J. Phys. D: Appl. Phys. 49, 175403 (2016),
[2] P. Vaněk et al., Biocompatible ferroelectric coatings for bone implants, oral presentation at ISAF/ECAPD/PFM conference, Darmstadt, (2016),
[3] P. Vaněk, M. Vandrovcová et al., to be published.

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The dielectric anomaly found below room temperature has purely extrinsic origin and it is related to two excitations in the GHz range: piezoelectric resonances in grains (PR) with nearly temperature independent frequency and domain wall oscillations (DW) that slows down towards ~ 270 K [ Buixaderas et al, Phys. Rev. B 94, 054315 (2016) ].

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The mesomorphic properties of the materials are ensured by the optimized molecular structure. The paramagnetic nature of the mesogenic materials is investigated by electron paramagnetic resonance, the magnetic properties of the bulk materials are studied by SQUID magnetometry. It is shown that the materials preserve their magnetic properties within the whole temperature range of liquid crystalline behaviour. Moreover, a strong correlation between spin orientation and molecular alignment within different mesophases has been observed [ K. Bajzıkova er al, J. Mater. Chem. C 4, 11540 (2016). ].

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Additional experiments like temperature dependent second harmonic generation, IR spectroscopy of phonons and X-ray diffraction are always necessary for unambiguous confirmation of ferroelectric phase transitions [ S. Skiadopoulou et al, Adv. Funct. Matt. 26, 642 (2016) ].

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Its intensity is highly sensitive to an external magnetic field: its strength drastically drops for magnetic field values above 0.5 Tesla and it completely disappears above 2 Tesla, when the spin structure changes from transverse conical to collinear. This work was published in [ F. Kadlec et al, Phys. Rev. B 94, 024419 (2016) ].

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Lamellar heterostructures, observed in PMN-0.32PT single crystals are formed in samples cooled under bias electric field applied along [001]_pc and then zero-field heated to the vicinity of the so-called depoling temperature T_RT. Similar structures were also encountered at ambient conditions. Spatially resolved polarized Raman scattering techniques confirmed that the stripe pattern is due to the coexistence of the phases attached to the opposite sides of the morphotropic phase boundary in the temperature-composition phase diagram [ I. Rafalovskyi et al., Phys. Rev. B 93, 064110 (2016) ].

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Our dielectric spectra of BZT of various compositions revealed that in addition to the lowest-frequency phonon mode near 1 THz, which is not much temperature dependent (no soft mode), there appear two relaxations in the spectra, the high-frequency one in the 10¹¹ Hz range (assigned to quasi-Debye losses) and the dominant low-frequency one, which obeys the common Arrhenius law for all the relaxor compositions below ~ 200 K (see Fig. 1) [1,2] and accounts for the permittivity vs. temperature maxima characteristic for the relaxor behaviour. Since from first-principles calculations as well as structural characterization it follows that the local Ti-O and Zr-O distances in BZT are independent of composition, the characteristic polar nanoregions (PNRs) should be localised within the chemical nanoclusters of BaTiO₃, where the Ti⁴⁺ ions are off-centred. Their simple Arrhenius thermal activation allowed us the assignment of the relaxations to local hopping of the Ti⁴⁺ ions among the off-centred positions within the Ti-O₆ octahedra. This picture was also confirmed by analysing the relaxation and phonon dynamics using first-principles calculations for the BaZr_1/2Ti_1/2O₃ composition in collaboration with the University of Arkansas, also separately for the Ti and Zr clusters [3]. This picture differs strikingly from the Vogel-Fulcher dynamics in lead-based relaxors, where there is no clear correlation between the PNRs and chemical clusters [4].

In collaboration with Institute Jozef Stefan in Ljubljana, Slovenia, the composition BaZr_0.5Ti_0.5O₃ was particularly studied [5] and from the dielectric behaviour in external electric field it was concluded that the material lies on the crossover between a relaxor and dipolar glass.

In NBT the nature of the well-known dielectric anomaly near T_m ≈ 600 K was analysed for the first time [6], since its dielectric dispersion is not apparent below the standard MHz range. We have shown that the soft mode contributes only weakly to the permittivity, but it couples to two relaxations, from which the lower-frequency one is thermally activated and dominates the dielectric anomaly. At low temperatures only the soft mode contributes to the spectra. At higher temperatures it transfers part of its oscillator strength to both relaxations. Since T_m is specified by domination of the ferroelectric rhombohedral phase in the rhombohedral-tetragonal coexistence region, the permittivity maximum at T_m is caused by a smaller coupling of relaxation with the soft mode in the rhombohedral phase compared to that in the cubic and tetragonal phase. Some of the measured spectra and the analysed frequencies and dielectric strengths are shown in Fig. 2. The whole rather peculiar dynamics up to the soft mode in the THz range was assigned to highly anharmonic motion of the Bi³⁺ ions, which are known to be highly underbonded.

References
[1] D. Nuzhnyy et al., Phys. Rev. B 86, 014106 (2012).
[2] J. Petzelt et al., Ferroelectrics 469, 14 (2014).
[3] D. Wang et al., Nature Communications 5, 5100 (2014).
[4] J. Petzelt et al., Phase Transitions 88, 320 (2015).
[5] C. Filipic et al., Phys. Rev B 93, 224105 (2016).
[6] J. Petzelt et al., Phase Transitions 87, 953 (2014).

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We developed a purposely designed mathematical analysis of the Raman spectra based on their frequency derivatives: the method finds the curvature maxima in concave-down regions of the spectra (CMCD analysis), in order to count peaks and shoulders of the experimental spectra, comparing them to the results of the standard fitting with damped harmonic oscillators. The combination of the two approaches is very helpful to find “hidden” structure in the spectra of disordered materials.

In the case of PZT, the crossover from the tetragonal to the rhombohedral phase is clearly visible in the Raman spectra; however, there are no indications of a systematic splitting of the E-symmetry modes into A’–A’’ doublets related to the monoclinic symmetry in the morphotropic samples. Detailed adjustment of the response function to the spectrum requires to assume additional Raman active modes, but this holds for a much broader concentration range than that of the anticipated monoclinic phase.

The analysis of the phonons found also that the lowest frequency transverse optic mode of E-symmetry (soft mode of the ferroelectric phase transition) is split into two components, a THz frequency anharmonic (central mode-like) component and a resonant component (at frequencies ω ~ 80 cm^-1). A new Raman band appearing in this frequency range at low temperatures is rather associated with the anti-phase tilt vibrations of the oxygen octahedra. These results are in perfect agreement with our previous IR studies on these ceramics [ E. Buixaderas et al., Phys. Rev. B 84, 184302 (2011) ].

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Straightforward fitting of the raw terahertz conductivity spectra by the Drude-Smith model, which was abundantly used in the literature, did not lead to a significant advance in an in-depth understanding of these phenomena. This is mainly because of the depolarization fields which build up in any inhomogeneous system. On the one hand, these fields reflect the sample morphology and their understanding in each particular system may provide new information about the nanostructure connectivity. On the other hand, the effect of unknown depolarization fields can hide or distort fingerprints of the nanoscopic transport.

Recently, we devoted a systematic effort to describe the depolarization fields in photoconductive nanostructures and to disentangle their effect from that of the local carrier response function [1].

1. We proposed a general relation between the effective photoconductivity (measured in the THz experiment) and the microscopic response function. It is based on the general Bergman representation of effective medium theory for a two-component system with a single dominant depolarization factor. We have shown that our model describes both percolated and non-percolated samples and structures with complex percolation pathways [2].
2. We solved the wave equation in an inhomogeneous sample and demonstrated that the transient sheet photoconductivity is directly measured in THz experiments for any spatial profile of the photoconductivity and any thin film sample morphology [1,3].
3. The microscopic response function can be determined by Monte-Carlo calculations of the motion of a single carrier inside nanoparticles [4].

Analysis of experimental data within this framework allows us to uncover the nature of charge carrier transport at nanoscale and to assess the sample morphology in quite arbitrary nanostructured systems.

Based on this approach we proposed microscopic models of the response of partly or completely localized charge carriers in a number of systems including various TiO₂ nano- and microparticle networks [2,5], in Sb-doped SnO₂ nanoparticles [6,7], and in various nanocrystalline silicon systems [3,8], For example, in [9] we investigated a regular array of vertically aligned InP nanowires (Fig. 1). In this structure, photonic waveguiding effects inside nanowires and interferences of the excitation optical beam should have been properly taken into account to describe the sample photoexcitation. We were able to determine high transversal electron mobility in nanowires which, in comparison with the longitudinal mobility, indicates the presence of stacking faults in the growth direction (Figure 1).

References
[1] P. Kuzel and H. Nemec, J. Phys. D: Appl. Phys. 47, 374005 (2014).
[2] H. Nemec et al., IEEE Trans. Terahertz Sci. Technol. 3, 302 (2013).
[3] V. Zajac et al., New J. Phys. 16, 093013 (2014).
[4] H. Nemec et al., Phys. Rev. B 79, 115309 (2009).
[5] S. A. Jensen et al., J. Phys. Chem. C 118, 1191 (2014).
[6] K. Peters et al., Chem. Mater. 27, 1090 (2015).
[7] K. V. Skoromets et al., J. Phys. Chem. C 119, 19485 (2015).
[8] H. H. Němec et al., Phys. Rev. B 91, 195443 (2015).
[9] C. S. Ponseca et al., Phys. Rev. B 90, 085405 (2014).

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These Ising lines play the role of domain boundaries associated with the Ising-to-Bloch domain-wall phase transition. Therefore, the electric field parallel to the Ising line can be used to induce the Ising line motion, which mediates the switching of both polarization and chirality of the Bloch wall without changing the bulk domain polarization. Detailed description of this study can be found in the paper [ V. Stepkova, P. Marton, and J. Hlinka, Phys. Rev. B 92, 094106 (2015)]

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The unusual behavior of the TGBA phase under the electric field can be explained by the specific packing of molecules within the smectic layers, resulting in relatively high layer compressibility, which lowers the energy of structural defects and thus facilitates the structure transformation. The perfectly dark state of studied compounds, induced by the electric field, either stable or reversible, is appealing for specific applications. Positive dielectric anisotropy in both chiral compound and racemic mixture is detected up to the field frequency of about 10 kHz, above this frequency the anisotropy is negative.Such change of the dielectric anisotropysign, known in nematics as the dual frequency effect, might be important for photonics such as adaptive or diffractive optics [ V. Novotna et al., Soft Matter 11, 4649 (2015)].

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Comparison with dielectric measurements proved that these polar fluctuations are a source of giant dielectric response, whose temperature-frequency dependence is specific for a significant group of substances exhibiting a glass-type ferroelectric phase transition, so-called relaxor ferroelectrics [ P. Ondrejkovic et al., Phys. Rev. Lett. 113, 167601 (2014)].

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Experimentally, we have studied e.g. nanocomposites of polymers with carbon nanotubes (CNTs) [4]. A well electrically conducting CNT, due to its specific shape, enables processing composites with extremely low percolation threshold. Within our collaboration with Queen’s University in Belfast, who prepared well-dispersed composites of polyethylene terephthalate (PET) polymer with CNTs, we characterized them by broadband conductivity and dielectric spectroscopy, covering up to 17 orders of magnitude in frequency (10⁻⁴–10¹³ Hz) from room temperature down to 5 K. An extremely low percolation threshold of 0.07 vol% CNT was revealed as appearance of the low-frequency AC conductivity plateau corresponding to the DC conductivity, whose value increased with CNT concentration according to the power law with a high critical exponent of 4.3. Its semiconductor temperature dependence obeying the tunnelling law confirmed the model that each CNT in the percolating clusters is covered by a thin (~ 1 nm) layer of PET [4].

Our results on other nanocomposites of similar type were reviewed in [2,3]. Successful modelling of porosity in high-permittivity ceramics in infrared as well as low-frequency range using the Lichtenecker model was achieved for a classical relaxor PbMg_1/3Nb_2/3O₃ (PMN) [3,5]. In collaboration with Brock University in Ontario, Canada, the giant permittivity in co-doped rutile TiO₂ ceramics was explained as due to interfacial Schottky barrier layers between the electrodes and samples [6].

[1] J. Petzelt et al., Ferroelectrics 526, 171 (2012).
[2] J. Petzelt et al., Phys. Status Solidi A 210, 2259 (2013).
[3] J. Petzelt and D. Nuzhnyy, Phase Trans. 89, 651 (2016).
[4] D. Nuzhnyy et al., Nanotechnology 24, 055707 (2013).
[5] D. Nuzhnyy et al., Phys. Rev. B 89, 214307 (2014).
[6] D. A. Crandles et al., J. Appl. Phys. 119, 154105 (2016).

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Our measurements confirmed that these crystals undergo two phase transitions within the approximate temperature range 488–503 K. Second harmonic generation showed a marked signal between the two anomalies, implying that the intermediate phase is non-centrosymmetric. In situ Raman spectroscopy and the presence of second harmonic signal, excluded the antiferroelectric state of this phase. The intermediate phase of PZT 99/1 crystal is similar to the rhombohedral ferroelectric one with the R3m space group [ E. Buixaderas et al., Phase Transitions 87, 1105 (2014) ].

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Using ethylene glycol instead of malic acid (modified Pechini method) gives reproducible results, no admixtures were detected by powder XRD. The ceramic pellets were prepared by uniaxial pressing and sintering and were used for spectroscopic measurements [ F. Kadlec et al., Phys. Rev. B 90, 054307 (2014)].

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The observed effect proves the concept of existence of a disordered tilt in the smectic A phase of “de Vries” type [ L. Lejcek et al., Phys. Rev. E 89, 012505 (2014)].

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In the two antiferromagnetic phases below T_N1 = 90 K and T_N2 = 48 K, several IR-active excitations emerge below 90 cm^-1. Their frequencies correspond to the maxima in the magnon density of states obtained by INS. At the magnetic phase transitions, these modes display strong anomalies and, for some of them, a transfer of dielectric strength from the higher-frequency phonons is observed. We propose that these modes are electromagnons, i.e. spin waves excited by electric field. Remarkably, at least one of these modes lying near 50 cm^-1 remains IR active in the paramagnetic phase; for this reason, we call it a paraelectromagnon. This mode becomes overdamped above 150 K, but it remains in the spectra at least up to 300 K. In accordance with this observation, quasielastic neutron scattering revealed short-range magnetic correlations persisting within temperatures up to 500 K above T_N1.
Our paper is the first report about paraelectromagnon in literature [ F. Kadlec et al., Phys. Rev. B 90, 054307 ()2014) ].

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Since these films exhibit one-order-of-magnitude better microwave properties than previously-used materials, they have high potential for application in microwave electronics. The paper was published in Nature [ Che-Hui Lee et al., Nature 502, 532 (2013) ].

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The unique SPM properties are preserved in the studied nanocomposite system and the magnetic response was analyzed. We established that the magnetic properties are modified due to the reduction of the dipolar interactions strength between the MNPs. Moreover, we were able to influence the ferroelectric properties by applying the magnetic field. [ V. Novotna et al., RSC Advances 3, 10919 (2013) ].

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Phase-field simulations using a Ginzburg–Landau–Devonshire model allows to explore this strain-induced phase transition within the domain wall in detail [Figure 2]. Strain-tunable chiral properties of ferroelectric Bloch walls promise a range of novel phenomena in epitaxial ferroelectric thin films. Results are published in JPCM [ V. Stepkova et al., J. Phys.: Condens. Matter 24, 212201 (2012)].

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Such transition was predicted to appear for 180 degree domain wall in the rhombohedral ferroelectric phase of BaTiO₃ close to 2GPa compressive stress [ V. Stepkova et al., J. Phys.: Condens. Matter 24, 212201 (2012)]. We confirm the hypothesis that the phase transition is of the first-order and predict corresponding divergent increase of the dielectric permittivity in the vicinity of the transition. In case of dense domain-wall pattern it can represent a substantial part of the dielectric response of the material [ P. Marton et al., Phase Transitions 86, 103 (2012)].

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Despite the fast development of optoelectronic applications of nanocrystalline materials, very little is known about interplay between charge transport and carrier confinement. The absence of a long-range crystal order fundamentally complicates the transport properties. In this work we used time-resolved terahertz spectroscopy to probe nanoscopic transport in nanocrystalline films of cadmium sulfide (CdS) prepared by chemical bath deposition. The sub-picosecond time resolution of the experiment allowed us to resolve the initial stages of the electron transport. The major conclusions are:

• Two length scales (nanocrystals and their clusters) control electron localization at the nanoscale. While interfaces between nanocrystals are highly permeable for electron transport, the interfaces between the clusters efficiently block the motion due to the existence of air voids.

• The efficiency of short-range transport is controlled by energy barriers due to electrostatic interaction between electrons and captured holes. The excess energy of electrons then plays the essential role in the conductive coupling between adjacent nanocrystals. (i) Relaxation of electrons with high excess energy leads to a decrease of their mobility on a subpicosecond time scale. (ii) Filling of conduction-band states by increasing the optical excitation fluence allows us to maintain a high level of conductive coupling even at later times.

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Assigning the BiFeO₃ polar modes probed detected in Raman spectroscopy is not straightforward due to the presence of so-called oblique phonon modes that do not match any pure TO or LO mode frequencies; instead, their frequencies continuously vary with the orientation of the phonon propagation vector. Using a monocrystal, mode assignment can be obtained by analysing a set of Raman spectra taken for different values of the angle between the phonon propagation vector and the optical axis (angular dispersion). To achieve the goal in absence of a de-twinned single crystal sample, we have developed an original technique in collaboration of the Institute of Physics AS CR and The University of Sheffild which enabled for the first time assigning these strongly anisotropic crystal properties using a polycrystalline material. We have taken advantage of the efficiency and spatial resolution of up-to-date micro-Raman spectrometers to collect a large enough set of spectra acquired on different randomly oriented grains of coarse-grain bulk ceramics. The pure TO and LO frequencies needed for angular dispersion determination were obtained from limit values of the phonon frequencies within the whole set of data. For each grain, the angle between the phonon propagation vector and optical axis was than determined from measured frequency of a chosen oblique mode. The obtained angular dispersion of the phonon mode parameters served as a basis of a full mode assignment [ J. Hlinka et al., Phys. Rev. B 83, 020101 (2011)].

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Stabilization of the reentrant phase was determined by mixing of the studied compound with the next homologues, which exhibit normal sequence without reentrancy. In the mixtures a direct transition between A and reentrant A phases was found, which can be understand as an analogy of crossover between liquid-like and gas-like behavior in supercritical fluids (widom-line phenomenon). The reentrant A phase can be easily influenced by electric field and exhibit strong electrooptic effect, which is promising for electrooptic application [ V. Novotná et al., Phys. Rev. E 83, 020701 (2011), N. Podoliak et al., Phys. Rev. E 84, 061704 (2011)].

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We proved that originally antiferromagnetic and paraelectric EuTiO₃ changes in films to strong ferromagnet and ferroelectrics due to strong spin-lattice coupling. Such system should exhibit strong magnetoelectric coupling which can be used in future memories. Paper was published in Nature [ J. H. Lee et al., Nature 466, 954 (2010)].

We also suggested using strong internal electric field in multiferroic Eu_0.5Ba_0.5TiO₃ for the search of permanent electric dipole moment (EDM) of the electron. According to standard model of particles its value should be of order of 10^–40 e.cm. Recently it has been shown that spontaneous violation of charge parity symmetry is much larger then it follows from the standard model, therefore this model needs an extension. New particle theories propose EDM of electrons 8 or 12 orders of magnitude larger then the standard model. The physicists try to measure EDM of electron already 40 years, unfortunately fruitlessly. They reached sensitivity of only 10^–27 e.cm. We have shown that in multiferroic Eu_0.5Ba_0.5TiO₃ the sensitivity should be one order of magnitude higher. We synthesized this material and prepared ceramic samples suitable for this measurement. In case of successful determination of EDM value it will be possible to prove and specify new theories going beyond the standard model. The paper was published in Nature Materials [ K. Z. Rushchanskii et al., Nature Mat. 9, 649 (2010)].

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GUHP presents a novel highly birefringent (i.e. phase matchable) monoclinic non-centrosymmetric material with considerably large optical nonlinearities, wide transparency range and surprisingly high optical damage threshold, at least comparable with the best commonly used NLO crystals as LBO. Favorable properties of GUHP in addition allowed an investigation of several rather rare nonlinear optical effects, as e.g. the second harmonic conical refraction or second harmonic conoscopy (both top pictures) and also the spontaneous noncollinear SHG (bottom left), Fabry-Perot enhanced SHG (middle right) and the two beam noncolinear SHG under various conditions (bottom right).

[1] M. Fridrichová et al., Phase Transit. 83, 761 (2010)
[2] J. Kroupa, J. Opt. 12, 045706 (2010)
[3] J. Kroupa and M. Fridrichová, J. Opt. 13, 035204 (2011)