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.
Competing synclinic and anticlinic interactions in smectic phases of bent-core mesogens
In collaboration with University of Chemistry and Technology, Prague, and Warsaw University, Vladimíra Novotná and Natalia Podoliak studied new sophisticated molecular systems, which form structurally complex synclinic and anticlinic phases [J. Mater. Chem. C 12,10903 (2024)]. The texture observed in the polarized light microscope and the molecular structure is highlighted in the inside back cover of the 29th issue of Journal of Materials Chemistry C.
Abstract: Recent studies on liquid crystals have focused on structurally new molecular systems forming phases distinct from simple nematic or smectic ones. Sophisticated molecular shapes may reveal structural complexity, combining helicity and polarity. Mirror symmetry-breaking in bent-core molecules can lead to a propensity for synclinic and anticlinic molecular structures within consecutive smectic layers. On the other hand, despite their achiral character, dimers readily adopt helical phases. In this study, we investigate a hybrid molecular structure incorporating both characteristics, namely a rigid bent-core and an attached bulky polar group via a flexible spacer. To perform phase identification, we enrich standard experimental methods with sophisticated resonant soft X-ray scattering technique. Notably, we observe a distinct preference for specific phase types depending on the length of the homologue. Longer homologues exhibit a predisposition towards the formation of tilted smectic phases, which are characterized by complex sequences of synclinic and anticlinic interfaces. Conversely, shorter homologues demonstrate a propensity for helical smectic structures. For intermediate homologues, the frustration is alleviated through the formation of several modulated smectic phases. O n the basis of the presented study, we describe the preconditions for high-level structures in relation to conflicting constraints.
The inside back cover illustrated by a liquid crystal texture of the smectic phase as seen in polarized light microscope [1].
References
[1] J. Svoboda, V. Kozmík, K. Bajzíková, M. Kohout, V. Novotná, N. Podoliak, D. Pociecha, and E. Gorecka,
Competing synclinic and anticlinic interactions in smectic phases of bent-core mesogens
J. Mater. Chem. C 12, 10903 (2024).
Researchers’ Night at the Liquid Crystals lab
Natalia Podoliak and Petr Ondrejkovič participated in the Researchers’ Night with an educational excursion called "Fairy tale about a little Liquid Crystal" aimed for children aged 6-12. A short announcement about this activity was broadcasted by the Czech TV in the main TV news (27.9.2024) and the news program about science named "Věda 24" (29.9.2024).
Natalia and Petr take children to a mysterious world of the laboratory where the little Liquid Crystal was born. This little dab hand, who inherited many special and unique qualities from his father Crystal and mother Liquid, welcomes children to his home. Children and their parents learn about his story and how he discovered a lot of talents and a desire to help people over the time. During the interactive and playful performance, they learn how LCD screens and polarized glasses work, and where you come across liquid crystals in your everyday life.
will learn and see during the excursion (source: Czech TV).
Researchers’ Night was initiated by the European Commission in 2005 and its mission is to show that science is not boring, but on the contrary is a source of interesting and fascinating phenomena. One day a year, universities, research and development centres, science centres, and other places open with free guided tours, popular education presentations, workshops, experiments, science shows, music, and performances, amongst others, taking place. The aim of Researchers’ Night is to dispel myths about scientists as people locked in laboratories and to show the general public that they are “ordinary people” who do work for each of us, they can present it in an engaging way, and they can also have a good time.
More information about the Researchers’ Night at the Institute of Physics of the Czech Academy of Sciences: https://www.nocvedcu.cz/organizace/fyzikalni-ustav-av-cr.
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Vladimíra Novotná - The beauty of liquid crystals
The exhibition is hosted by The small gallery of scientific images at the Faculty of Mathematics and Physics, Charles University, Prague (Ke Karlovu 3, 1st floor). and will last from 5.3. to 6.5.2024. The opening takes place on March 11, 2024 from 5:00 p.m.
The exhibition presents impressive paterns of liquid crystals when viewed in the polarized light of an optical microscope. Most of the photographs were created during the observation of new types of molecules. On cooling from the liquid phase, a nematic phase may appear, its nuclei are colored and different types of defects and arrangements appear. This strongly depends on the surface of the sample, the temperature and the properties of the studied substance.
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Anna Radochová won the Czech Head Award 2023 (České hlavičky) for secondary school students
Anna Radochová received this prestigious award in the category UNIVERSUM for her successful project "Properties of new photosensitive liquid crystals" under supervision of Alexej Bubnov. The ceremony will be broadcasted by the Czech Television on the 18th of November 2023.
Anna’s success started in 2021 with her intention to undertake an internship at the Czech Academy of Sciences within the Open Science framework. Among many announced research projects, she was attracted by Alexej Bubnov’s proposal on liquid crystals mainly because of wide usage of liquid crystals in daily life, like in smartphones, and their broad potential in future applications. As a secondary school student from the Bohumil Hrabal high school in Nymburk, Anna successfully completed her one-year internship under supervision of Alexej Bubnov and Věra Hamplová in the Liquid Crystal group of the Department of Dielectrics and in the Department of Chemistry.
She studied newly synthesized liquid crystals with photosensitive properties by observation of their beautiful textures using polarizing optical microscope and by investigation of their phase transition temperatures by measuring heat capacity. To uncover photosensitivity of these liquid crystals, she measured and analyzed absorption spectra and their structure changes under the UV light. At the end of her Open Science internship, Anna has also presented her results in a traditional national contest for all kinds of high school students called Students` Professional Activities. She got the 5th place in the category Physics with contribution titled “Properties of newly synthesized liquid crystals”.
Recently, the quality of her project titled "Properties of new photosensitive liquid crystals" was recognized by the committee of the Czech Head Award for secondary school students. On October 16, Anna won the 1st place in the category UNIVERSUM which is devoted to experimental and theoretical studies and projects in the field of physics and mathematics, especially with the potential for practical use. The ceremony will be broadcasted by the Czech Television on the 18th of November 2023. Nowadays, Anna continues her studies at the Faculty of Mathematics and Physics of Charles University.
The aim of the Open Science is to provide an access to scientific work for talented students already during their high school studies. Researchers from the Czech Academy of Sciences annually announce internships within the framework of the Open Science.
https://www.fzu.cz/en/popularization/events/open-science.
Charged domain walls in BaTiO3 crystals emerging from superdomain boundaries
Petr Bednyakov and Jirka Hlinka observed a transient domain structure with multiple superdomains whose boundaries transform into charged domain walls in BaTiO3 [Adv. Electron. Mater. 9, 2300005 (2023)]. These superdomains highlighted the inside front cover of the June issue of Advanced Electronic Materials.
Abstract: Previous experiments with barium titanate crystals [1,2] have shown that electric field applied in the vicinity of its ferroelectric phase transition can be used to introduce peculiar ferroelectric domain walls, persisting to the ambient conditions: head-to-head charged walls compensated by the 2D electron gas. The present in situ optical observations [3] allow the documentation of the early stage of this poling process in which the cubic and ferroelectric phases coexist, the latter being broken into multiple martensitic superdomains, separated by superdomain boundaries. It is revealed that the transient superdomains are subsequently converted into the regular ferroelectric domains, while the superdomain boundaries transform into the desired charged domain walls. In order to assign the observed transient domain patterns, to understand the shapes of the observed ferroelectric precipitates and their agglomerates as well as to provide the overall interpretation of the recorded domain formation process, the implications of the mechanical compatibility of the coexisting superdomain states are derived in the framework of the Wechsler–Lieberman–Read theory. These results also suggest that both the electric conductivity and interlinked motion of the superdomain boundaries and phase fronts are involved in the transport of the compensating charge carriers toward the charged domain wall location.
The inside front cover of the June issue of Advanced Electronic Materials illustrated by a transient domain structure with multiple superdomains containing bundles of stripe ferroelastic subdomains in BaTiO3 [3].
References
[1] P. S. Bednyakov, T. Sluka, A. K. Tagantsev, D. Damjanovic, N. Setter,
Formation of charged ferroelectric domain walls with controlled periodicity,
Sci. Rep. 5, 15819 (2015).
[2] P. Bednyakov, T. Sluka, A. Tagantsev, D. Damjanovic, N. Setter,
Free-carrier-compensated charged domain walls produced with super-bandgap illumination in insulating ferroelectrics,
Adv. Mater. 28, 9498 (2016).
[3] P. S. Bednyakov and J. Hlinka,
Charged domain walls in BaTiO3 crystals emerging from superdomain boundaries,
Adv. Electron. Mater. 9, 2300005 (2023).
Antiskyrmions in ferroelectric barium titanate
Our recent molecular dynamics computational study reveals that the bulk crystal of the archetypal ferroelectric perovskite BaTiO3 can host ferroelectric antiskyrmions at zero field. We show that the antiskyrmion has just 2-3 nm in diameter and that it carries a very exotic topological charge of minus two [Phys. Rev. Lett. 133, 066802 (2024)].
The prediction and experimental confirmation of magnetic skyrmions revolutionized the physics of nanoscale magnetism and opened new horizons for spintronics. In spite the inherently shorter and faster correlations of the electric polarization and challengingly smaller correlation lengths, the recent developments in electric skyrmionics follow these innovations. It is only 5 years since the first observation of a ferroelectric skyrmion in thin lead-strontium titanate superlattices [1].
The present molecular dynamics computational study [2)] reveals that the bulk crystal of the archetypal ferroelectric perovskite (barium titanate) can host peculiar 2-3 nm wide standalone polar columns stable till temperatures of several tens of Kelvins. They are spontaneously surrounded by a unique noncollinear polarization pattern (see figure) that has never been described before. We explained how and why this pattern is formed and stabilized. Since its invariant skyrmion topological charge is an integer with a sign opposite to that of the usual skyrmions, they named this soliton as ferroelectric antiskyrmion. They clarify that formation of antiskyrmions consists in breakdown of high-curvature 180-degree ferroelectric walls into triplets of lower-energy 71-degree walls. It is explained that this process is favored by a fortunate combination of the moderate anisotropy of the anharmonic electric susceptibility and the pronounced anisotropy of the polarization correlations in barium titanate crystals. These findings represent a clear milestone in the studies of topological defects in ferroelectrics.
Figure: Polarization pattern of the ferroelectric antiskyrmion in rhombohedral BaTiO3 obtained from atomistic shell-model based computational experiment.
[1] S. Das, Y. L. Tang, Z. Hong, et al., Observation of room-temperature polar skyrmions, Nature 568, 368 (2019).
[2] M.A.P. Gonçalves, M. Paściak, and J. Hlinka, Antiskyrmions in ferroelectric barium titanate, Phys. Rev. Lett. 133, 066802 (2024).
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Ultrafast long-distance expansion of electron-hole plasma in direct bandgap semiconductors
Transport of charge carriers inside crystals is determined by their energy band structure which only permits velocities smaller than ~c/100 in known materials. We demonstrated that ultrafast and long-distance propagation of electron-hole plasma (velocities up to c/10, reaching more than 100 μm) is possible as a quite general result of fundamental electron-photon interaction in direct bandgap semiconductors upon strong pulse photoexcitation with low photon excess energy above the bandgap [PRL 130, 226301 (2023) & Nanophotonics 13, 1859 (2024)].
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.
Figure:
Propagation of plasma as measured by THz pulse probing of photocarriers in GaAs shows that assistance of light can dramatically enhance
the charge propagation rates.
(1) In incoherent regime the plasma propagation is due to stimulated emission and reabsorption of photons;
(2) In coherent regime at low temperature the electron – photon interaction leads to Rabi dynamics and soliton formation and propagation;
(3) for comparison: group velocity of light in GaAs.
[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).
[2] Photon-assisted ultrafast electron–hole plasma expansion in direct band semiconductors, Nanophotonics 13, 1859 (2024).
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Paving the way to a 3-state thermal switch using antiferroelectric Pb(Zr1-xTix)O3
We propose a novel approach of phase-control in Pb(Zr0.95Ti0.05)O3 (PZT95/5) based on the thermal behavior of its phases. Our results show the possibility to thermally switch among three states near room temperature using small temperature gradients by heating-cooling cycles at slow rates. Thus, PZT95/5 ceramics are potential materials for room temperature device applications [Acta Materialia, 119208 (2023), online].
Pb(Zr1-xTix)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(Zr0.95Ti0.05)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.
Figure: Scheme of thermal switching using the three near-room-temperature phases of PZT 95/5.
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(Zr1-xTix)O3,
Acta Materialia, 119208 (2023), online.
Application of HfO2 thin films in non-volatile memories – a review
We review main factors in preparation of HfO2 thin films and their physical properties which are important for applications in high-density resistive random access memories and ferroelectric memories [Small 18, 2107575 (2022)].
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.
Figure: Dependence of crystal structure of HfO2 on the thin film thickness, strain and electrodes.
[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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Multidomain ordered metal–ferroelectric superlattices
By combination of advanced experimental techniques and phase-field simulations, we found that electric dipoles in superlattices, composed of layers of a ferroelectric material separated by thin metallic spacers, form an unusual pattern of nanoscale domains that order in three dimensions. These ferroelectric multidomain ordered superlattices exhibit an outstanding dielectric response and their engineered modulated structural and electronic properties can be controlled using electric field [Nat. Mater. 20, 495 (2021)].
Figure:
Two-dimensional base motif of the ferroelectrically ordered PbTiO3–SrRuO3 superlattices as seen by
(a-d) phase-field simulations and (e,f) transmission electron microscopy:
(a) electric polarization showing ferroelectric domain structure in two PbTiO3 layers separated by SrRuO3 spacers
(b) gradient energy density coming from domain walls and boundaries between layers,
(c-f) in-plane (exx) and out-of-plane (ezz) strain components demonstrating correlations between ferroelectric PbTiO3 layers.
The arrows in (a,b) panels show direction of electric polarization forming characteristic flux-closure patterns.
[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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Ferroelectric and antiferroelectric phases in liquid crystalline compounds with terphenyl in the molecular coere
We designed a new type of antiferroelectric liquid crystalline structure with terphenyl in the molecular core and two lactate units attached to the chiral chain [J. Mol. Liq. 336, 116267 (2021)].
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.
Figure: Texture of the liquid crystalline compound with a terphenyl in the molecular core in antiferroelectric phase without field (left picture) and under applied electric field (right picture). The scale, orientation of polariser (P) and analyser (A) are presented. In the centre, there is a model of the studied molecule in the optimised conformation.
[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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Electric-field-induced multiferroic topological solitons
Nat. Mater. (2024).
Photon-assisted ultrafast electron-hole plasma expansion in direct band semiconductors
Nanophotonics 13, 1859 (2024).
Antiskyrmions in ferroelectric barium titanate
Phys. Rev. Lett. 133, 066802 (2024).
Polar nematic phases with enantiotropic ferro- and antiferroelectric behaviour
J. Mater. Chem. C 12, 7214 (2024).
Competing synclinic and anticlinic interactions in smectic phases of bent-core mesogens.
J. Mater. Chem. C 12, 10903 (2024).
Twisted domains in ferroelectric nematic liquid crystals
J. Mol. Liq. 403, 124873 (2024).
3D graphene straintronics for broadband terahertz modulation
Adv. Electron. Mater., 2300853 (2024).
Dispersion of stable quasicrystal phase by precipitation during extrusion in a Mg-Zn-Al alloy: Microstructure and mechanical properties
Mat. Sci. Eng. A-Struct. 891, 145979 (2024).