Dielectric and IR spectroscopy
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THz science and technology
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Light and neutron scattering
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Theory and simulations |
Solid-state materials science
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Liquid crystals |
About us
Main activities of Department of Dielectrics cover experimental and theoretical investigations of high-permittivity insulators like liquid crystals, ferroelectrics, multiferroics, piezoelectrics, semiconductor nanostructures, and low-loss materials.
- Dielectric and IR spectroscopy
- THz science and technology
- Light and neutron scattering
- Theory and simulations
- Solid-state materials science
- Liquid crystals
The most significant fresh scientific results of our deparment are listed in the section Highlights.
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
Department of Dielectrics, Institute of Physics of the Czech Academy of Sciences
Semimetrics Ltd., Kings Langley, England
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
Agx(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).
Bohuslav Březina obituary
Ing. Bohuslav Březina, CSc. passed away on February 4, 2019.
After obtaining his PhD in 1955 he joined the Ceramic Research Institute in Klenčí and Hradec Králové. In 1957 he joined the Institute of Physics of the Czechoslovak Academy of Sciences where he focused on growing single-crystals exhibiting ferroelectric properties. He conducted a truly pioneering research in a dramatically developing field.
The work of Bohuslav Březina and his group greatly contributed to the development of the Department of Dielectrics as experimental research could not be conducted without newly synthesized materials. Observations on newly synthesized compounds stimulated both experimentalists and theoreticians. In such a way his work contributed e.g. to the first international conference on ferroelectric phenomena having been organized in Prague.
He initially grew crystals insoluble in water, prepared from melt, later he started growing water-soluble crystals. Bohuslav Březina’s achievements in this field were applied in industrial production in 1980s; his methods were also patent protected. As an internationally recognized expert in crystal growing, he was invited e.g. to stay at the prestigious Eidgenössische Technische Hochschule in Zürich.
In 1973 a popularising book “Ferroelectrics” (“Feroelektrika”, in Czech) was published by Academia, written by Bohuslav Březina and Petr Glogar. The book is a handy introduction to ferroelectricity, unsurpassed until now.
Bohuslav Březina’s former colleagues will also gratefully remember his kind-hearted personality.
Figure:
(a) Front cover of the book “Ferroelectrics” (“Feroelektrika”, in Czech),
(b-d) Bohuslav Březina's photos of domain structures of a BaTiO3 single crystal grown by the Blattner method:
(b) tetragonal phase at 293 K in crossed polarizers,
(c) coexistence of tetragonal and cubic phases at the phase transition temperature (393 K) in parallel polarizers,
(d) the same in crossed polarizers.
[after: B. Březina and P. Glogar, Feroelektrika, Academia, Praha (1973)].
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 [1]. Together these allowed us to show that the Ti dynamics can be exhaustively explained by phonons excited on a timescale of picoseconds.
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.
Fig. 1: One component of the relative Ti-O6 displacement is mapped within one layer of the material using different time averages (a-c) which clearly shows that chain correlations exist on a timescale of picoseconds. The displacements averaged over the simulation time and all unit cells of the crystal have a cuboidal distribution with shallow minima along diagonal directions.
[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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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 [1]. This discovery opens new perspective direction of investigation of lead-free materials for possible electric energy storage.
Fig. 1: Left – electron diffraction patterns in commensurately and incommensurately modulated phases in Ba4Sm2Ti4Nb6O30. Right – temperature dependence of second harmonic generation signal showing temperature hysteresis near ferroelectric phase transitions in investigated materials.
[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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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.
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.
Fig. 1: Comparison of quantum (solid line) and classical (dashed line) conductivity in GaAs cube-shaped nanocrystals of selected sizes at 300 K and free carrier concentration of 1016 cm-3. Note the excellent agreement of quantum calculations with the classical ones for large nanocrystals (1024 nm).
[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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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.
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
Figure: Raman spectra of the three mineral members along the CuSbS2-CuSbSe2 join, together with their micrographs and the depicted structure for příbramite.
[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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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.
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.
Fig. 1: Texture of the cholesteric liquid crystal in applied electric field. In the inset the chemical formula of the studied compounds is present.
[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 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].
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 νTHz), 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 TC near 1 GHz and hardens on cooling, consistent with the oscillations of the ferroelectric domain walls.
Fig. 2: Temperature dependences of the frequencies of the different excitations found in SBN-35 and their dielectric contributions.
[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 Sr0.35Ba0.61Nb2O6.04 , Phys. Rev. Materials 2, 124402 (2018).
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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 [1] under strong excitation.
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.
Fig. 1: Left panel: Photo-induced dynamics of quasiparticles (red) and Cooper pairs (blue). Right panel: Evolution of the superconducting gap 2Δ with time after photoexcitation. The arrows in indicate the gap widths in equilibrium.
[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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Local structure of relaxor ferroelectric SrxBa1−xNb2O6 from a pair distribution function analysis
Phys. Rev. B 99, 104102 (2019).
Terahertz conductivity and coupling between geometrical and plasmonic resonances in nanostructures
Phys. Rev. B 99, 035407 (2019).
Landau–Devonshire thermodynamic potentials for displacive perovskite ferroelectrics from first principles
J. Mater. Sci. 1573, 4803 (2019).