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.
New material for 5G mobile networks
Stanislav Kamba, Veronica Goian and Christelle Kadlec, in collaboration with Prof. D. G. Schlom from the Cornell University and other American and German colleagues, succeeded in developing a new material for mobile network of the 5th generation. Its outstanding properties were published in journal Nature Materials.
Epitaxial strained thin films of (SrTiO3)n-1(BaTiO3)1SrO were grown on DyScO3 substrates using molecular beam epitaxy. 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. See you more details in Nat. Mater. (2019).
Figure: Schema of crystal structures of investigated (SrTiO3)n-1(BaTiO3)1SrO films and their view in scanning transmission electron microscope. Yellow octahedra depict TiO6 layers, green and red points mark atoms of Sr and Ba.
John Mangeri and Alexey Bubnov succeeded in FZU Photo Competition
John Mangeri won the second place in the FZU Photo Competition for his image of a computational simulation of a BaTiO3 nanoparticle (see Figure 1). Alexey Bubnov got the 3rd-4th place for his image of a beautiful liquid crystal texture seen under polarized light microscope (see Figure 2).
Moreover, Vladimira Novotna and Alexey Bububnov obtained the 6th-12th place for other remarkable liquid crystal textures captured using polarized light microscope (see Figures 3-5) among 125 photos from 33 participants.
Figure 1: Computational simulation of a BaTiO3 nanoparticle: lines of constant polarization flux in an exotic topological state (by John Mangeri).
Figure 2: Liquid crystal textures (by Alexey Bubnov).
Figure 3: Rocks (by Alexey Bubnov).
Figure 4: Colour dance (by Alexey Bubnov).
Figure 5: Liquid crystals (by Vladimira Novotna).
Vladimira Novotna was awarded the 2nd place in Photogenic Science Competition 2019
Photos of liquid crystals under a polarized microscope brought the second place to Vladimira Novotna in the category of ‘Photogenic Science’. The competition is organized by the Czech Academy of Sciences and by the association Science is Alive! (Věda žije! in Czech) and its aim is to evaluate photos related to science.
Selected photos from the celebration:
Figure: The awarded photo of a liquid crystal in polarized light and Vladimira Novotna together with Eva Zazimalova, the president of the Czech Academy of Sciences (photos after the Czech Academy of Sciences).
This talk will summarize experimental and theoretical advances in magnetic and
ferroelectric materials at multiple length scales. I will first discuss some recent work
on exotic magnetic and polar states observed in BiFeO3 domain walls [1] and
skyrmion states found in PbTiO3/SrTiO3 superlattices [2]. Both systems motivate the need
for accurate predictive modeling of structural order parameters which need to be
carefully coupled to their complex boundary conditions. To that end,
I will provide an update on the following theoretical projects that we, at the institute,
have been working on:
(i) ab initio calculations of GaV4Se8 with the aim of parameterizing its Landau energy surface,
(ii) developing continuum micromagnetic simulations in the Ferret simulation package and
(iii) controlling topological states in ferroelectric PbTiO8 nanostructures [3].
References
[1] J.-Y. Chauleau et al., Nature Materials doi:10.1038/s41563-019-0516-z (2019).
[2] S. Das et al., Nature 568, 368 (2019).
[3] Y. Tikhonov et al., arXiv preprint arXiv:2001.01790 (2020).
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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.
Figure: In the background AFM picture shows nanotubes, the diameter of observed nanotubes is 50-60 nm. A scrolling-up mechanism of crystalline molecular layers was proposed based on difference between the surface tension at the opposite surfaces.
[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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Mesoscopic polarization dynamics and two ferroelectric sublattices
in the uniaxial tetragonal tungsten bronze (Srx Ba1−x)Nb2O6
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 [1].
The archetypical uniaxial tungsten-bronze (Sr
Fig. 1: (a) Structure of SBN in the ab plane [dashed red lines mark the perovskite subunit in lighter-coloured octahedra Nb(2)O6, linking oxygen octahedra Nb(1)O6 are shown in blue colour]. (b) Contribution to the spontaneous polarization of Nb(1) and Nb(2).
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.
Fig. 2: (a) Temperature dependences of the frequencies of the main excitations found in SBN crystals. (b) Contribution to the permittivity of the main excitations in SBN crystals. The shaded regions refer to extended error regions found for νGHz and ΔεGHz.
[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
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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 [1].
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 TN1 = 87 K and TN2 = 63 K. These activate new modes in the spectra, with resonance frequencies and intensities changing with temperature and magnetic field. Below TN2, we observed a transfer of oscillator strengths from low-frequency phonons to these excitations, which we assign to electromagnons.
Figure: (a) Temperature dependence of the low-frequency excitations observed in THz and IR spectra of SrMn7O12. The open symbols correspond to spin excitations or phonons activated by breaking of the inversion center, whereas the remaining phonons are marked by solid symbols. (b) Rhombohedral quadruple perovskite crystal structure of SrMn7O12.
[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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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.
Figure:
Grain morphology of the investigated GaV4S8 ceramics.
(a) Scanning electron microscope image of an area of interest with sample coordinate system as an inset;
(b) Inverse pole figure map from EBSD analysis of the area in (a);
(c) positions of four grains chosen for detailed Raman measurements,
indicated projections of elementary cubic cells clarify their crystallographic orientation;
(d) unit triangle with the colour code of the inverse pole figure map;
(e) surface normals of the chosen grains;
(f) Parallel-polarised Raman scattering intensity ratios detected from the four grains,
which allows determination of their crystallographic orientation independent of the EBSD analysis.
Data shows intensity ratio at selected frequencies as a function of the angle ϕ
between the polariser and the reference direction on the sample surface.
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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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].
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.
Figure: (a) Temperature dependence of the overdamped E component of the soft mode in PMN, PMT and PMN-PT characterized by frequency of the peak in dielectric loss. (b) Temperature dependence of the reciprocal static permittivity (perpendicular to local polarization in nanodomains) obtained from the fits of IR and THz spectra.
[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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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.
Figure: (a) The prototypic (paraelastic) phase with a pair of equivalent atoms related by the symmetry operation g with the related symmetry element go. (b) The ferroelastic domain pair is related by the suppressed (lost) symmetry element gs. There is a pseudosymmetry element gr within each domain state which is a remnant of the suppressed (lost) symmetry element go.
J. Fábry, Degenerate (identity) chemical reactions in ferroelastic crystals, Acta Cryst. B75, 287(2019).
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Targeted chemical pressure yields tuneable millimetre-wave dielectric
Nature Materials (2019), doi:10.1038/s41563-019-0564-4
Effect of hydrogen chloride etching on carrier recombination processes of indium phosphide nanowires
Nanoscale 11, 18550 (2019).
Terahertz Spectroscopy of Nanomaterials: a Close Look at Charge-Carrier Transport
Adv. Opt. Mater. 7, 1900623 (2019).
Organic nanotubes created from mesogenic derivatives.
Nanoscale Adv. 1, 2835 (2019).
Efficient green emission from edge states in graphene perforated by nitrogen plasma treatment
2D Mater. 6, 045021 (2019).
Skyrmions in ferroelectric materials, Chapter 4 in Recent Advances in Topological Ferroics and their Dynamics, edited by Robert L. Stamps, Helmut Schultheiß,
in series Solid State Physics, Vol. 70, Academic Press, 2019, pp. 143-169.