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.
Subterahertz collective dynamics of polar vortices
An artist take on the 'whirling' vibrational mode revealed in the polar vortex structure of PbTiO3/SrTiO3 superlattices [Nature 592, 376 (2021)] to highlight the front cover of the April issue of Nature.
The front cover (see Fig. 1) shows collective dynamics of spontaneously formed vortices of electric polarization in ferroelectric PbTiO3 layers of PbTiO3/SrTiO3 superlattices which have been investigated by Marek Paściak, Jirka Hlinka, and Christalle Kadlec in a world-wide collaboration (the Argonne National Laboratory, the Pennsylvania State University, the University of California, Berkeley and other American institutions) [1].
Figure 1: The cover of the April issue of Nature illustrated by subterahertz collective dynamics of polar vortices in a ferroelectric PbTiO3 layer of PbTiO3/SrTiO3 superlattice [1] (cover image by Ellen Weiss/Argonne National Laboratory).
The characteristic ultrafast collective polarization dynamics of vortices was discovered by pump-probe experiments, concretely using a terahertz-field excitation and femtosecond X-ray diffraction measurements. Lattice dynamics calculations with first-principle-based interatomic potentials done in Department of Dielectrics unveiled the atomistic picture of the observed excitations (see Fig. 2). In particular, the most coherent and electric-field susceptible mode at the sub-THz frequencies called a vortexon has been recognized as the transverse oscillation of whole polar vortices. The frequency of the vortexon mode has been experimentally and theoretically found to be tunable by temperature or the substrate strain. Moreover, the mode is behind a phase transition from the state in which the vortices are symmetrical to the one where they are staggered. The experiment and calculations prove that THz pulses can excite the polarization state on the nanometer scale which opens opportunities for electric-field-driven data processing in topological structures with ultrahigh speed and density.
Figure 2: Polarization dynamics in the vortexon mode of the PbTiO3/SrTiO3 superlattice. Each arrow represents a polarization of one perovskite unit cell (distance between neighbouring arrows is ~4 Å). The vortex-hosting PbTiO3 layer in the middle has thickness of 16 unit cells. The animation is the result of all-atom lattice dynamics calculations with first-principle-based interatomic potentials [1].
[1] Q. Li, V.A. Stoica, M. Paściak, Y. Zhu, Y. Yuan, T. Yang, M.R. McCarter, S. Das, A.K. Yadav, S. Park, C. Dai, H.J. Lee, Y. Ahn, S.D. Marks, S. Yu, C. Kadlec, T. Sato, M.C. Hoffmann, M. Chollet, M.E. Kozina, S. Nelson, D. Zhu, D.A. Walko, A.M. Lindenberg, P.G. Evans, L.-Q. Chen, R. Ramesh, L.W. Martin, V. Gopalan, J.W. Freeland, J. Hlinka, and H. Wen, Subterahertz collective dynamics of polar vortices, Nature 592, 376 (2021). (show less)
Perovskites and other Framework Structure Materials
E. Buixaderas in collaboration with Prof. J. Dec contributed with the chapter: “Phonons and relaxations in unfilled tetragonal tungsten-bronzes” to a new book titled "Perovskites and other Framework Structure Materials: new trends and perspectives", published by Collaborating Academics IP, France.
Nowadays mostly studied perovskite materials belong to so-called framework structure materials whose structure is based on units (octahedra, tetrahedra, …) that share some of their corners (or edges) with their neighbors. This book [1] is devoted to the relation between structural organization of such units and unique physical properties of framework structure materials.
One of the important group of framework structure materials long-term studied in the Department of Dielectrics are unfilled tetragonal tungsten-bronze crystals, particularly relaxor ferroelectric strontium barium niobate. The chapter 8 deals mainly with structure relations of NbO6 octahedra, chemical disorder, phonons and dielectric relaxations in strontium barium niobate.
[1] E. Buixaderas and J. Dec Phonons and relaxations in unfilled tetragonal tungsten-bronzes, Chap. 8 in "Perovskites and other Framework Structure Materials: new trends and perspectives", editors M.B. Smirnov and P. Saint-Grégoire, Materials and Devices, Vol 5(1), Publ. by Collaborating Academics IP, France (2021).
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Special Issue on the Contributions of Women in Ferroelectrics Research
Impressive schematic of the crystal structure of the tetragonal tungsten-bronze family made by Elena Buixaderas has been selected for the front cover in the IEEE TUFFC Special Issue on the Contributions of Women in Ferroelectrics Research and Development.
The aim of this Special Issue was to promote and highlight scientific and technological contributions from both established and emerging women scientists and engineers in the field of ferroelectrics. The motivation had come from the fact that women remain underrepresented in science, technology, engineering, and mathematics relative to the total population. For example, in the IEEE Ultrasonics, Ferroelectrics and Frequency Control Society, women remain below 10% of the total society members, suggesting that their underrepresentation persists in ferroelectrics research and development activities as well.
Authors in this Special Issue include women at all career stages—including undergraduate students, recent Ph.D. graduates, and those retired from the profession—the common link being the field of ferroelectrics. The collection of contributed articles in the Special Issue covers multiple aspects of processing, structure, theory, and properties of many different ferroelectric materials and devices. E. Buixaderas and M. Paściak reviewed the local structure, phonons, and polarization dynamics of relaxor-ferroelectric (Sr,Ba)Nb2O6 (SBN) single crystals using scattering and spectroscopic techniques such as pair distribution function experiments [1]. The schematic of the SBN complex crystal structure was selected for the cover (see Figures 1 and 2).
Figure 1: The front cover of the IEEE TUFFC Special Issue highlights artwork and pictures from selected manuscripts.
Figure 2: Schematic of the crystal structure of the tetragonal tungsten-bronze family selected for the cover. The schematic shows three types of channels along the c-axis of the (Sr,Ba)Nb2O6 structure. Triangular channels are empty. Square channels and pentagonal channels are each filled with Sr and Ba. The schematic also shows two types of NbO6 octahedra (dark and light), corresponding to two different crystallographic sites [1].
[1] E. Buixaderas and M. Paściak, Disorder in Strontium Barium Niobate: Local Structure, Phonons, and Polarization Dynamics, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 68, 314 (2021).
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From superlattices to supercrystals
Researchers from Department of Dielectrics, in a world-wide collaboration led by Dr. Pavlo Zubko from University College London, have found that in superlattices, composed of layers of a ferroelectric material separated by thin metallic spacers, electric dipoles form an unusual pattern of nanoscale domains that order in three dimensions to create a ‘domain supercrystal’, exhibiting outstanding dielectric response.
The concept of order is a cornerstone of condensed matter physics. Atoms order to form crystals, which, in turn, play host to other fascinating and technologically useful ordering phenomena. For example, in ferroelectric crystals, tiny displacements of ions create electric dipoles that order cooperatively to produce a net electric polarization. The orientation of this polarization can be reversed by applying an electric field and serves as the basis for the operation of ferroelectric random access memories that exploit the different orientations of the polarization to store the 1 and 0 bits of information.
Thanks to advances in physical vapour deposition techniques, today, it is possible to create artificially ordered materials, such as superlattices, that consist of very thin sheets of different crystalline materials stacked periodically on top of each other. Artificially layered crystals that combine materials with similar structure but different properties in this way often lead to the emergence of new and unexpected behaviour. For example, in superlattices consisting of periodically alternating ferroelectric and dielectric layers, each just a few nanometres thick, electric dipoles often arrange themselves into complex nanoscale patterns of stripes and whorls. Such dipole patterns are extremely responsive to applied electric fields and give rise to unusual dielectric properties that may prove useful in reducing the power consumption of the billions of transistors in our everyday electronics.
Schematic of the polarization arrangement in a domain supercrystal and its corresponding X-ray diffraction pattern [1].
Writing in the journal Nature Materials [1], a collaboration led by Dr. Pavlo Zubko from University College London involving researchers from the United Kingdom, France, Ireland, and Czech Republic, have found that in some superlattices composed of layers of ferroelectric lead titanate (PbTiO3) separated by thin spacers of metallic or insulating oxides, the electric dipoles form an unusual pattern of nanoscale domains—regions with uniform orientation of the dipoles—that order in three dimensions to create a ‘domain supercrystal’. Under the influence of applied electric field, small displacements of the boundaries between the different domains give rise to large changes in the net electric polarization and thereby enhance the dielectric response of the material along all three spatial directions.
Perhaps an even more interesting aspect of the domain supercrystals is their highly inhomogeneous structure (see figure). The complex domain patterns in the ferroelectric layers are accompanied by very strong distortions of the crystalline lattice that, in turn, set up a periodic modulation with very large local curvatures in the metallic or insulating spacers. This curvature modifies the local symmetry of the spacer layers, inducing additional polarity that could lead to interesting changes in their properties. Importantly, the curvature can be tuned by application of electric fields and its periodicity can be engineered on demand by adjusting the thicknesses of the ferroelectric layers, making such superlattices an ideal system for exploring curvature-induced phenomena in a variety of insulating, conducting and magnetic materials.
text: P. Zubko
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).
Researchers' Night in the THz lab
Petr Kužel and his colleagues from the THz science and technology group introduced their research themes in a general public video during the European Researchers' Night.
During the European Researchers' Night, numbers of scientific institutions and infrastructures are animated by popular science activities. The visitors have a unique opportunity to see the labs from inside, to observe some experiments and to listen to lectures on up-to-date topics. Institute of Physics of the Czech Academy of Sciences is one of such sites, and its THz science and technology group is traditionally involved in outreach activities for general public. Its leader Petr Kužel explains specific measures of this year's Researchers' Night: “Undergoing epidemy circumstances did not allow us to organize popular experiments in the THz lab for visitors. Therefore, in a collaboration with our PR division, we created a general public video which introduces the little-known spectral window in the terahertz (THz) range and which gives a flavor of the research and applications with THz radiation.”
General public video about the spectral window in the terahertz and applications with THz radiation (in Czech only, Youtube).
Online art opening of the FZU Photo Competition 2020
Within European Researchers' Night on 27. November 2020 an online art opening of the FZU Photo Competition was performed. Alexey Bubnov got the 2nd place for his image of a beautiful liquid crystal texture resembling the Sun’s corona.
A video created from the best photos presents engaging beauty hidden underneath various microscope objectives. The photo, which was selected as the video thumbnail, presents the beauty of a liquid crystal texture seen by polarized optical microscope, taken by Alexey Bubnov. This picture resembling the Sun’s corona was awarded by the 2nd place.
Video promoting the best photos of the FZU Photo Competition 2020 (European Researchers' Night).
New material for 5G mobile networks
Epitaxial strained thin films of (SrTiO3)n-1(BaTiO3)1SrO were found to be a promising new material for mobile network of the 5th generation [Nature Mater. 19, 176 (2020)].
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.
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.
[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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Ferroelectric phase transition in water molecules localized in mineral cordierite
We discovered that hydrogen bonds are eliminated and the Coulombic interactions dominate in water molecules localized in nano-channels of mineral cordierite. Their dipole moment is perpendicular to the channel axis c 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. Spontaneous polarization measurements yield the saturated value of ~3 nC/cm2 at 0.3 K [Nat. Commun. 11, 3927 (2020)].
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).
Figure 1: Cordierite crystal structure. Investigated water was localized in the free channels along the c-axis.
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.
Figure 2: Ferroelectric phase transition of water in cordierite is induced by a critical slowing down of dielectric relaxation. νp denotes frequency of the maxima in the dielectric loss spectra and τ denotes the relaxation time of the relaxation.
[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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Making EuO multiferroic by epitaxial strain engineering
Optical soft mode driven ferroelectric phase transition was discovered in IR spectra of tensile strained ferromagnetic EuO thin films [Commun. Mater. 1, 74 (2020)].
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.
Figure:
Left: Theoretical strain dependence of the Eu ferroelectric soft mode frequency and
of the spontaneous polarization Ps in (EuO)2/(BaO)2
superlattice obtained from the DFT calculations. Insets show schematic eigenvectors of
the Eu and A2u symmetry polar phonons.
Rigth: Temperature dependence of the static permittivity of the EuO films and EuO layers
in the (EuO)x/(BaO)y superlattices with various tensile strain.
[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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Seemingly anisotropic magnetodielectric effect in isotropic EuTiO3 ceramics
Anisotropic magnetodielectric effect was observed at 0.3 K in the low-frequency dielectric spectra of antiferromagnetic EuTiO3 ceramics [Phys. Rev. B 102, 144402 (2020)].
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.
Figure:
Left: Magnetic field dependence of EuTiO3 permittivity measured for electric field applied perpendicularly to the sample plane, and with the static external magnetic field B in either of two orientations marked in the figure. Inset: Dependence of the permittivity ratio for two orientations of the magnetic field.
Rigth: THz refractive index measured at three magnetic fields applied perpendicularly to the electric vector of THz radiation. Inset: Magnetic field dependence of the magnon frequency measured at 2 and 15 K.
[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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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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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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Subterahertz collective dynamics of polar vortices
Nature 592, 376 (2021).
Metal–ferroelectric supercrystals with periodically curved metallic layers
Nat. Mater. 20, 495 (2021).
Disorder in Strontium Barium Niobate: Local Structure, Phonons, and Polarization Dynamics
IEEE Trans. Ultrason. Ferroelectr. Freq. Control 68, 314 (2021).
Self-assembling behaviour of chiral calamitic monoacrylates targeted for polymer stabilisation of polar smectic phases in chiral liquid crystals
J. Mol. Liq. 331, 115723 (2021).
Spectral and transport properties of a half-filled Anderson impurity coupled to phase-biased superconducting and metallic leads
Phys. Rev. B 103 035419 (2021).
Soft-mode spectroscopy of ferroelectrics and multiferroics: A review
APL Mat. 9, 020704 (2021).