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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.


Young researcher brings ERC Starting Grant to our department
It is our pleasure to announce that the Solid-state materials science group led by Přemysl Vaněk will be strengthened by a young researcher Tim Verhagen. The main goal of his ERC Starting Grant is to create unique, however reproducible, multilayered heterostructures with ferroelectric and ferromagnetic properties at room temperature.
To achieve his research goal of developing new multiferroic 2D-sandwich-like materials, Tim has now received a prestigious ERC Starting Grant and a project of the Czech Science Foundation. The grants will allow him to acquire a molecular beam epitaxy apparatus in the new building SOLID21 of the Institute of Physics and to develop his team. In near future, he will be able to produce precise thin multilayers or single-atom layers of multiferroic materials, which might bring him a strong collaboration with other groups of Department of Dielectrics and other departments of the Institute of Physics.
Tim Verhagen obtained bachelor's and master's degree in physics at the Delft University of Technology and PhD at the University of Leiden in the Netherlands. He moved to the Czech Republic in 2014 to work in Jana Kalbáčová Vejpravová's group at the Faculty of Mathematics and Physics of the Charles University and the Institute of Physics of the Czech Academy of Sciences. Since 2020, he has been a researcher at the Institute of Physics of Charles University.
photo: Martin Pinkas, Charles University, Praguesource: https://www.ukforum.cz (show less)

Václav Janovec has passed away
One of the founders of the Department of Dielectrics, outstanding scientist, editor and teacher, enthusiastic promoter of symmetry approaches to ferroic phase transitions and domain boundaries, Prof. Václav Janovec, our beloved friend and colleague Vašek, has passed away on February 16, 2022, at the age of his almost complete 92.

Alexey Bubnov took the 2nd place in the FZU Photo Competition 2021
Alexey Bubnov defended his second place in the FZU Photo Competition 2021. Moreover, he obtained 4th and 10-11th places for his images of liquid crystal textures in polarised light microscope. Futhermore, Maryam Mansoori Kermani and Fedir Borodavka got 5-9th and 10-11th place, respectively.
Best photos coming from Department of Dielectrics in 2021:

Figure 1: Observing liquid crystal textures in polarized light microscope (by Alexey Bububnov, 2nd place).

Figure 2: Observing liquid crystal textures using polarised light microscope (by Alexey Bubnov, 4th place).

Figure 3: The representation of a theoretical model containing a silicon crystal wafer and two water molecule layers, captured using the molecule dynamics visualisation programme. (by Maryam Mansoori Kermani, 5-9th place).

Figure 4: Observing liquid crystal textures using polarised light microscope (by Alexey Bubnov, 10-11th place).

Figure 5: Feroelectric domain structure of the PbTiO3 thin layer (by Fedir Borodavka, 10-11th place).

Picosecond nonlinear optoelectronics in graphene
Petr Kužel and Vaisakh Chelod Paingad in collaboration with colleagues from the Charles University described nonlinear behavior of charge carriers during early times after pulsed optical excitation in epitaxially grown graphene layers. The time evolution of the system is determined by nonlinear electronic response of graphene, which opens the possibility of the increase of the speed of optoelectronic elements [Adv. Funct. Mater. 31, 2105763 (2021)].
Graphene is a single sheet of carbon atoms forming a two-dimensional infinite honeycomb lattice. For example, a macroscopic ordered stack of such layers forms the graphite; however, the behavior of the graphene single atomic layer is dramatically different from that of graphite.
The electronic properties of graphene are determined by the behavior of charge carriers (electrons and holes); their energy band structure is quite different from typical structures of classical semiconductors or metals and resembles much more the energy scheme of photons.
Depending on the position of the so-called Fermi level (which may be shifted, e.g., due to the nature of the surrounding material and dynamically controlled by illumination or by electric current) graphene can be an excellent conductor or a good insulator. Applications of graphene in electronics and optoelectronics rely on various approaches to the Fermi level tuning by electronic, chemical or optical stimuli.

Upper part: scheme of terahertz optoelectronic probing of graphene films on silicon carbide substrate.
Optical pulse (red) excites charge carriers and a delayed ultrashort terahertz pulse (blue, 1 THz = 1012 Hz) probes
the state of these carriers.
By inspecting changes in the terahertz pulse shape, we determine the conductivity spectra Δσ of graphene,
which reflect the distribution of charges within the energy band structure.
Bottom left: typical measured conductivity spectrum containing the plasmon resonance.
Bottom right: ultrafast evolution of the temperature of carriers (Tc) and
of the Fermi level (μ) deduced from the experiments.
Free-standing graphene layer is very fragile and is not very useful for practical applications. Therefore, in this study, the group of Petr Kužel focused on graphene layers epitaxially grown on silicon carbide (SiC) substrates. Properties of films prepared under certain conditions approach those of an ideal free-standing graphene layer. By varying the technological conditions, the properties of graphene can be tuned. However, the substrate surface is not perfectly flat even if the greatest care is devoted to its preparation. Instead, it always consists of a set of nanoscopic terraces.
The researchers studied graphene layers using ultrashort laser pulses and terahertz optoelectronic probing (see Fig. 1): an optical pulse excited charge carriers and a delayed ultrashort terahertz pulse tested the state of these carriers. Measured changes in the terahertz pulse shape provided the conductivity spectra, which reflect the distribution of charges within the energy band structure.
The obtained spectra exhibited the so-called localized plasmon resonance, which expresses the collective motion of charge carriers, and which is related to the existence of terraces on the substrate surface. In the investigated graphene samples the build-up and decay of these plasmons on picosecond time scale (1 ps = 10–12 s) was observed.
In brief, the observed behavior can be described as follows. Before the arrival of the optical laser pulse a significant concentration of equilibrium charge carriers exists in the sample, the graphene film is conducting. Immediately after the optical excitation, the newly generated carriers gain a very high temperature, they exchange energy with equilibrium carriers through elastic scattering, undergo a fast recombination process, and efficiently transfer their energy to the graphene crystal lattice.
During the first picosecond the optically generated carriers practically vanish, and only significantly heated equilibrium carriers remain; the Fermi level exhibits a pronounced decrease with respect to the equilibrium state. The subsequent nonlinear dynamics of plasmons are entirely controlled by the Fermi level of excited carriers through their temperature Tc (see Figure 1). The nonlinear behavior of graphene in this state is a consequence of the unique band structure of graphene which enables very high rate of elastic collisions of carriers. The decay of the nonlinear regime depends on the degree of disorder in the graphene layer. The technological control of the disorder in graphene layers thus allows one to tune the THz response of the material which is important for its optoelectronic applications.
text: Petr Kužel
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Second place for Pavel Baláž in Photogenic Science Competition 2021
A photo of (un)usual window cleaning during home office brought the second place to Pavel Baláž in the category of ‘Scientists and Home Office’. 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 promote science by intriguing photos taken by scientists.

Karel Tesař took the 2nd place in the Falling Walls Lab competition
Karel Tesař was awarded for 'breaking the wall' with his project 'Post sternotomy chest pain' at Falling Walls Lab Wroclaw on 29 September 2021.
The Falling Walls Lab is a world-class pitch competition that brings together a diverse and interdisciplinary pool of students and researchers, and pioneering innovators. One of the preliminaries of the final Falling Walls Lab in Berlin is yearly organized by the University of Wrocław and is devoted to participants from the Central and Eastern Europe. Three minutes is all it takes to succeed in this competition. What matters is an original idea and the ability to present it in a clear way. In Wroclaw, Karel Tesař has captured attention of Polish scientists by his project Post sternotomy chest pain.

(photo by Dominika Hull)

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)

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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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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Predicting solar cell performance from terahertz and microwave spectroscopy
Adv. Energy Mater. 12, 2102776 (2022).
Precipitation of stable icosahedral quasicrystal phase in a Mg-Zn-Al alloy
Acta Mater. 225, 117563 (2022).
Onset of a superconductor-insulator transition in an ultrathin NbN film under in-plane magnetic field studied by terahertz spectroscopy
Phys. Rev. B 105, 014506 (2022).
Single-impurity Anderson model out of equilibrium: A two-particle semianalytic approach
Phys. Rev. B 105, 085122 (2022).
A grease for domain wall motion in HfO2-based ferroelectrics
Nanotechnology 33, 155703 (2022).