Organic nanotubes created from mesogenic lactic acid derivatives
We found a facile route how to prepare nanotubes from rod-like mesogens dissolved in typical organic solvents. For selected types of chiral rod-like molecules, both enantiomers as well as the racemic mixtures, the nanotubes are formed by slow evaporation from a solution, regardless the solvent, concentration or deposition type. Obtained supramolecular assemblies were studied using various experimental techniques and nanotubes of 50-60 nm diameter described. We proposed rolling-up mechanism related to the surface tension difference at the opposite layer surfaces.
Mesoscopic polarization dynamics and two ferroelectric sublattices
in the uniaxial tetragonal tungsten bronze (Sr
The high-frequency dielectric behavior of uniaxial tungsten-bronze strontium barium niobate crystals with various Sr/Ba ratios have been studied in order to thoroughly understand the evolution of the relaxation dynamics across the ferroelectric phase transition. We showed that the dielectric response along the polar axis consists of three relaxations corresponding to polarization mechanisms related to several correlation lengths of mesoscopic order and that they are closly associated with two different ferroelectric subsystems.in the structure .
Changes in spin and lattice dynamics induced by magnetic and structural phase transitions in multiferroic SrMn7O12
Upon cooling, SrMn7O12 undergoes a series of structural and magnetic phase transitions from cubic to rhombohedral symmetry, and to an incommensurately modulated crystal structure, which is connected with charge and orbital ordering of the Mn cations. We report IR, THz, and Raman spectra of SrMn7O12 ceramics reflecting corresponding changes in the phonon selection rules, including new phonons appearing in spin-order-induced ferroelectric phases .
Raman scattering yields cubic crystal grain orientation
The anisotropy of Raman scattering was applied to determine the orientation of individual microcrystal grains, as small as a few µm, of GaV4S8 polycrystalline compound. This was possible by measuring polarised Raman spectra as a function of rotation of the sample along the laser direction. On comparing the resulting set of spectra with a computer simulation for particular symmetries, the orientation of the crystal grains could of the determined with good precision.
Soft mode driven local ferroelectric transition in lead-based relaxors
Analysis of IR and THz spectra using Bruggeman effective medium approach revealed that the mesoscopic structure of Pb(Mg1/3Nb2/3)O3 and Pb(Mg1/3Ta2/3)O3 consists of dynamic randomly oriented uniaxially anisotropic polar nanoregions with harder transverse optical polar modes in the direction along the local dipoles [1,2].
Degenerate (identity) chemical reactions in ferroelastic crystals
The work points to reconsideration of degenerate chemical reactions because of the chemical reactions which take place in a solid phase where the orientation of the substituting molecules is maintained. The work points to the examples of a ferroelastic switching which is accompanied by braking and reestablisment of chemical bonds. The article suggests that the definition of the degenerate chemical reactions might distinguish the reactions where the truly identical molecules are formed from those ones where the resulting molecules are enantiomers or enatiomorphs in case of domains.
Spatio-temporal distribution of relative Ti-O6 displacements in cubic BaTiO3
BaTiO3 is often considered a model ferroelectric material in which the dielectric properties are defined by the displacements of Ti ions with respect to surrounding oxygen atoms. However, despite the decades of a dedicated research, certain controversies have remained as to the description of collective movements of the Ti ions. We approached this problem using nonoscale-oriented X-ray scattering methods and large-scale atomistic simulations . Together these allowed us to show that the Ti dynamics can be exhaustively explained by phonons excited on a timescale of picoseconds.
Electromagnon in the Y-type hexaferrite BaSrCoZnFe11AlO22
We investigated static and dynamic magnetoelectric properties of single crystalline BaSrCoZnFe11AlO22 which is a room-temperature multiferroic with Y-type hexaferrite crystal structure. THz and Raman spectra reveal an electrically active spin wave (electromagnon) below 300 K at ≈1.2 THz. We show that the electromagnon is activated due to the magnetostriction mechanism involving spin vibrations along the hexagonal axis.
Wide Range Dielectric and Infrared Spectroscopy of (Nb+In) co-doped rutile ceramics
The dielectric response of ceramics of co-doped rutile Ti1-x(Nb0.5In0.5)xO2 has been measured via a combination of impedance, high-frequency coaxial, THz transmission, and IR reflectivity spectroscopies spanning 15 decades of frequency between 0.1 Hz and 240 THz . It is argued that the colossal dielectric permittivity reported by other authors can be explained by a combination of thin low-conducting grain boundaries and low-conducting depletion near-electrode layers which give rise to thermally activated dielectric relaxations in higher radiofrequency and low-frequency ranges, respectively.
Physics and applications of charged domain walls
One of the “inner” properties of ferroic walls is their ability to carry some bound charge, which is apt to be screened with free carries. Walls carrying bound charge are termed charged domain walls (CDWs). Remarkably, the screening takes place even in the case where the adjacent domains are insulating. Keeping in mind that the walls are often easily movable, CDWs can be viewed as ultrathin movable conductive sheets embedded into an insulating material.
Electric-field-induced transition from incommensurately to commensurately modulated phase
Antiferroelectric-like polarization hysteresis loops in Ba4Sm2Ti4Nb6O30 and Ba4Eu2Ti4Nb6O30 were explained by electric-field induced structural phase transition from nonpolar incommensurately modulated structure to polar and commensurately modulated phase . This discovery opens new perspective direction of investigation of lead-free materials for possible electric energy storage.
Quantum theory of terahertz conductivity of semiconductor nanostructures
In collaboration with T. Ostatnický of Charles University we participated to the development of the first theory of the quantum conductivity describing the transport in the terahertz spectral range, which does not contain internal contradictions. We have shown that the broken translation symmetry of the nanostructures induces a broadband drift-diffusion current which must be explicitly taken into account.
Přibramite, a new Se-containing mineral from the Czech Republic, characterized by Raman spectroscopy
Minerals containing selenium are interesting and worthy to study due to its inherent photovoltaic effect. The characterization and understanding of these natural minerals is important to be able to make synthetic analogues.
Unusual behaviour under the applied electric field for new cholesteric liquid crystals with extremely short pitch
Unusual behaviour of new cholesteric liquid crystals has been observed under applied electric field. Positive dielectric anisotropy causes reorientation of the long molecular axis along the applied electric field direction. Due to extremely short pitch length a stripe texture has been observed under applied electric field. A model based on disclinations has been proposed and anchoring energy evaluated.
The ferroelectric phase transition of the tetragonal tungsten-bronze SBN-35 unveiled
The structural ferroelectric-paraelectric transition has been definitely observed by electron diffraction tomography in the tetragonal tungsten-bronze (TTB) Sr0.35Ba0.61Nb2O6.04 (SBN-35) from the paraelectric group P4/mbm to the ferroelectric Pmbm. At 625 K, the refined structure shows that the average structure of SBN-35 is tetragonal with an almost negligible orthorhombic distortion [Fig.1].
Departure from BCS response in photo-excited superconducting NbN films observed by terahertz spectroscopy
Ultrashort laser pulses can be used to induce transient exotic states of matter and cause phenomena of a vital interest such as room temperature superconductivity. We have focused on the photo-induced dynamics in niobium nitride (NbN) thin films  under strong excitation.
Domain wall contribution to lattice dynamics and permittivity of BiFeO3
Ferroelectric materials are known for their exceptionally high dielectric permittivity. It turns out, that important part of it originates from a material's complicated microstructure and in particular from interfaces between ferroelectric domains.
First-principles-based Landau-Devonshire potential for BiFeO3
We describe a first-principles-based computational strategy for determination of the Landau-Devonshire potential.
Fast polarization mechanisms in uniaxial tungsten-bronze SBN-81
The high-frequency dielectric response of the uniaxial strontium barium niobate (SrxBa1−xNb2O6) crystals with 81% of Sr (x = 0.81) was studied from 1 kHz to 30 THz along the polar axis in a wide temperature interval [E. Buixaderas et al, Sci. Rep. 7, 18034 (2017)]. Relaxor properties were observed in the complex dielectric response and four main excitations were ascertained below the phonon frequencies. These fast polarization mechanisms take place at THz, GHz and MHz ranges and show different temperature evolution.
New type of dimers composed of bent-core molecules connected through their central cores
Structurally new type of dimers composed of bent-core molecules connected through their central cores has been prepared and studied. The switching behaviour in the applied electric field is reported.
Infrared, terahertz and microwave spectroscopy of the soft and central modes in PMN
Analysis of IR and THz spectra using Bruggeman effective medium approach revealed that the mesoscopic structure of Pb(Mg1/3Nb2/3)O3 (PMN) consists of randomly oriented uniaxially anisotropic polar nanodomains with harder transverse optical polar modes in the direction along the local dipoles.
Existence conditions for ferroaxial materials
All 212 species of structural phase transitions with a macroscopic symmetry breaking were inspected with respect to the simultaneous occurrence of the ferroelastic, ferroelectric, and ferroaxial properties.
Picosecond charge transport in rutile at high carrier densities studied by transient terahertz spectroscopy
In rutile the charge carriers form polarons with high effective mass owing to the strong electron-phonon coupling. In this paper we studied ultrafast terahertz photoconductivity in rutile under strong optical excitation.
Coating of Ti39%Nb substrates by ferroelectric BaTiO3 promotes the bone cell growth
We proved that ferroelectric immersed in a liquid influences electric charge distribution at the surface. Therefore ferroelectric coating on bone implant surface promotes bone cell growth and proliferation, healing should be improved .
Extrinsic influence on the dielectric response of morphotropic PZT ceramics
The dielectric behaviour of the most famous piezoelectric material, Pb(Zr0.52Ti0.48)O3 ceramics, below room temperature is explained by the topology of the sample, and not by intrinsic structural changes.
All-organic liquid crystalline radicals with a spin unit in the outer position of a bent-core system
All-organic paramagnetic liquid crystals offer the advantage of a long-range order of liquid crystalline phases and the magnetic properties of the individual molecules. In such systems, the magnetic properties can be modified by phase transition or the application of external fields. In our work paramagnetic all-organic bent-core liquid crystals having the radical-bearing unit (TEMPO) in the terminal position of an elongating side arm are studied.
Can a tensile tensile strained TiO2 thin film be ferroelectric?
A series of papers about discovery of new ferroelectrics using PFM microscopy has recently been published. On the example of strained TiO2 thin films, which exhibit ferroelectric-like hysteresis loops, we have shown that PFM response can come from electrochemical effects like migration of defects.
Electromagnon in Z-type hexaferrite (Ba0.2Sr0.8)3Co2Fe24O41
THz spectra of Z-type hexaferrite (Ba0.2Sr0.8)3Co2Fe24O41 reveal an electrically active spin wave (electromagnon) at temperatures below 250 K. This excitation is activated due to dynamic magnetoelectric coupling (exchange striction).
Macroscopic heterophase pattern in Pb(Mg1/3Nb2/3)O3-PbTiO3 crystals
Polarization dependence of the Raman scattering intensities demonstrate that lamellar structures observed in (1 - x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT) single crystals are composed of tetragonal-like and rhombohedral-like layers extending over macroscopic (mm) lengths.
Broadband dielectric spectroscopy of lead-free relaxor ferroelectrics
Ba(Zr,Ti)O3 (BZT) and Na1/2Bi1/2TiO3 (NBT) have recently become recently the most studied lead-free relaxor materials for their attractive piezoelectric and interesting physical properties.
Compositional behaviour of phonons in Pb(Zr1-xTix)O3 ceramics
Raman spectra of PZT ceramics were studied systematically in a broad temperature interval (10–600 K) and a broad Ti/Zr concentration range around the morphotropic phase boundary (x = 0.25–0.70) [ E. Buixaderas et al., Phys. Rev. B 91, 014104 (2015) ].
Ultrafast photoconductivity in semiconductor nanostructures
Time-resolved terahertz spectroscopy provides a useful insight into charge carrier motion in nanostructured semiconductors: it provides a contact-free access to local response of charge carriers inside nanostructures averaged over macroscopic volume of a sample. This is a very pertinent quantity for many applications of disordered materials which still have a larger potential than perfectly aligned nanostructures.
Ising lines: Natural topological defects within ferroelectric Bloch walls
Phase-field simulations demonstrate that the polarization order-parameter field in the Ginzburg-Landau-Devonshire model of rhombohedral ferroelectric BaTiO3 allows for an interesting linear defect, stable under simple periodic boundary conditions. This linear defect, here called the Ising line, can be described as an about 2-nm-thick intrinsic paraelectric nanorod acting as a highly mobile borderline between finite portions of Bloch-like domain walls of opposite helicity.
Unique effect of the electric field on a new liquid crystalline lactic acid derivative
We found a unique effect of the applied electric field for new chiral liquid crystalline compound, namely lactic acid derivative. We observed reversible transformation of the planar TGBA texture to the homeotropic one, homogeneously dark in crossed polarizers. The transformation is an analogy of the Frederiks transition known in nematics and only rarely can be found for smectics.
Polar fluctuations of nanoscale size as a source of giant dielectric response of relaxor ferroelectrics
Characteristic frequencies of nanoscale polar fluctuations have been resolved by means of neutron diffuse scattering measurements on a Sr0.61Ba0.39Nb2O6 single crystal in the MHz-GHz frequency region.
Ultra-broadband spectroscopy of dielectric-conductor nanocomposites
The ultra-broadband dielectric spectroscopy is particularly useful for studying dielectric–conductor nanocomposites, particularly near the electrical percolation threshold. Theoretically, we have studied the spectra of effective dielectric response of such composites using several models within the effective medium approximation. It was shown that the divergence of the static permittivity at the percolation threshold is caused by a strong dielectric relaxation, which softens on approaching the threshold with changing the composition, passing through the whole frequency range from infrared down to zero [1-3].
Catching intermediate phases in antiferroelectric single crystals
Ferroelectric and antiferroelectric single crystals show phase transitions that can be evidence by different experimental methods. In this work we investigated single crystals of antiferroelectric PbZrO3 substituted with 1% of Ti (PZT 99/1) by means of optical microscopy, micro-Raman scattering, second harmonic generation and dielectric spectroscopy, on heating to and cooling from its cubic phase.
Preparation of CaMn7O12 single phase ceramics
Variety of multiferroic materials is prepared in our department. One of them was CaMn7O12 which exhibits strong magnetoelectric coupling. Small crystals (~ 0.1 mm) were prepared from high temperature solution of CaCl2 and MnO2, but they contained 9% of undesirable admixtures. An alternative method – modified sol-gel synthesis using water solutions of Ca and Mn nitrates and malic acid – produced pure CaMn7O12 powder with less than 1% admixtures. However, this method was not reproducible.
Molecular tilt induced by gold nanoparticles or clusters of nanoparticles in the liquid crystalline smectic A phase of “de Vries” type of compound
For the first time a tilt of molecules around the gold nanoparticles or their clusters has been observed in the liquid crystalline smectic A phase. We have shown the tilt is a consequence of interaction of the molecules with the surface of nanoparticle leading to a local smectic A-smectic C transition. Nanoparticles attracted to edge dislocations enable to visualize them under an optical polarizing microscope.
Paraelectromagnon in Multiferroic CaMn7O12
We investigated spin and lattice dynamics of CaMn7O12 ceramics using infrared (IR), THz and inelastic neutron scattering (INS) spectroscopies in the temperature range 2 to 590 K, and, at low temperatures, in applied magnetic fields up to 12 T
New approach to preparation of highly tunable microwave dielectrics
Srn+1TinO3n+1 is paraelectric material with layered perovskite structure. We have shown that thin films of Srn+1TinO3n+1 become ferroelectric under tensile strain and their critical temperatures increase to room temperature with rising n. Owing to this, their permittivity and electric tunability increase.
Nanocomposite of superparamagnetic maghemite nanoparticles and ferroelectric liquid crystal
We present a novel type of hybrid nanocomposite prepared by mixing maghemite nanoparticles (MNPs) and a chiral liquid crystalline compound. The multiferroic system exhibits the ferroelectricity as well as superparamagnetic (SPM) properties. The impact of nanoparticles and effect of applied magnetic field on ferroelectric liquid crystalline properties were established. The magnetic behavior is typical for a system of SPM nanoparticles with inter-particle dipolar interactions and surface spin disorder.
Phase transition in ferroelectric domain walls of BaTiO3
The seminal paper by Zhirnov (1958 Zh. Eksp. Teor. Fiz. 35 1175–80) explained why the structure of domain walls in ferroelectrics and ferromagnets is so different. We have recently realized that the antiparallel ferroelectric walls in rhombohedral ferroelectric BaTiO3 can be switched between the Ising-like state (typical for ferroelectrics) and a Bloch-like state (unusual for ferroelectric walls but typical for magnetic ones) [see Figure 1] by a compressive epitaxial stress.
Dielectric response of a phase transition in domain wall
Computer simulations based on Ginzburg-Landau-Devonshire theory was used to study details of a phase-transition between Ising and Bloch type of a domain wall.
Electron localization in CdS nanocrystals
Ultrafast electron dynamics and short-range electron transport in CdS nanocrystals prepared by chemical-bath deposition were investigated in terahertz regime [Z. Mics et al., Phys. Rev. B 83, 155326 (2011)]. Initially high mobility of photogenerated electrons exhibits a decrease within 1 picosecond due to the relaxation of their excess energy. Role of nanocrystal surfaces and nanocrystalline aggregates in electron transport was elucidated.
New experimental method allowed to record dispersion curves of polar lattice modes in magnetoelectric BiFeO3
Bismuth ferrite (BiFeO3) is a prototypical multiferroic system exhibiting extraordinary properties; it is ferroelectric up to TC ~ 1100 K as well as antiferromagnetic up to TN ~ 640 K. Characterisation of polar phonon modes is essential for understanding its dielectric and electromechanical behaviour.
Reentrancy phenomenon of a smectic A phase in chiral liquid crystalline compounds
Unique sequence of phases was discovered in a chiral smectic, namely reentrance of orthogonal paraelectric A phase below the ferroelectric C phase. This behavior was explained by non-monotonous temperature dependence of the principal coefficient in the free energy expansion and microscopically by changes of molecular conformations and intermolecular distances due to occurrence a lateral bulky group in the molecule. This phenomenon is accompanied with anomalies in dielectric, optical and switching properties.
Magnetoelectric multiferroics are materials, which exhibit simultaneously magnetic and ferroelectric order. There is theoretically possible to influence magnetic or ferroelectric domains with electric or magnetic fields, respectively, therefore these materials are intensively studied for their promising potential applications in non-volatile memories. Unfortunately, there are only few multiferroics in nature and most of them work only at low temperatures. We suggested to use large mechanical strain in ultrathin films for preparation of new „artificial“ multiferroics.
Study of linear and nonlinear optical properties of GUHP
Within a common project with I. Němec and M. Fridrichová from Department of Inorganic Chemistry, Faculty of Science, Charles University, focused on development of new advanced nonlinear-optical materials, we studied in detail the optical properties of guanylurea(1+) hydrogen phosphite (GUHP) crystal.
Piezoelectric response of nanotwinned ferroelectric perovskites
Computer simulations based on Ginzburg-Landau-Devonshire theory has benn used to investigate piezoelectric properties of tetragonal BaTiO3 crystals. We have shown that piezoelectric response of twinned BaTiO3 increases with increasing density of 90-degree domain walls.