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.
Open Science internships at the Institute of Physics open doors to universities worldwide
In 2018, a secondary school student, Nela Sedláčková has undertaken a one-year Open Science internship under supervision of Alexej Bubnov. At present, she is enjoying the studies of Physics at University of St Andrews supported by The Kellner Family Foundation and says: “I am convinced that the Open Science played the key role in my admittance to university in abroad”.
English translation of the full article from the Open Science website:
Nela Sedláčková dreamed to devote herself to science already at high school. She was therefore interested in the possibility of attending Open Science internships at the Czech Academy of Sciences. At that time, she wanted to extend the knowledge and prepare herself for entering a foreign university. Nela successfully entered and completed a highly competitive internship under supervision of Alexej Bubnov in 2018, in the Liquid Crystal group of the Department of Dielectrics, where she studied novel liquid crystalline materials used most frequently in the LCD displays.
How do you remember the Open Science internship?
Thanks to Open Science internship, I had the opportunity to work in a research laboratory,
meet international scientists and present the gained results at the Open Science conference.
High school students do not usually have access to real scientific environment, and
I also referred to my Open Science experience while applying to St Andrews (Note: University of St Andrews in Scotland).
I believe that the Open Science internship played a key role in my acceptance to St Andrews as well as to gain
the scholarship from
The Kellner Family Foundation which financially supports my studies in the UK.
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 anually announce internships within the framework of the Open Science.
https://www.fzu.cz/en/popularization/events/open-science.
How did you actually find out about internships and what made you try out how science is done?
I saw an advertisement of the Open Science project on the Internet and searched through the database of internships. At that time I was in the second year of high school and I was considering to study science at some university abroad. Until then, I had the impression that "real" science was inaccessible to someone who had not yet graduated from college, and the project attracted my attention because it was aimed specifically for the high school students.
How was the internship and cooperation with the supervisor?
The objective of the internship "Study of phase transitions of novel liquid crystals" was to investigate the properties of new liquid crystalline materials, which are nowadays most often used in LCD displays. The supervisor of the internship, Alexej Bubnov, was always willing to answer any of my questions, and our cooperation continues even after the end of the internship. Thanks to him, the results of the internship were published as a regular research article, and I even became a co-author, which does not happen very often for a high school student.
Did the internship help you in choosing your next field of study?
The internship confirmed that I want to study physics with a focus on experiments rather than theory. In the laboratory, I met scientists from abroad and was able to practice and improve my communication skills in English, which, in turn, strengthened my decision to study abroad.
Your dream has come true. So where are you studying now and in which field?
Currently, I study Physics at the
University of St Andrews with the support of The Kellner Family Foundation.
The studies are very fulfilling for me and living abroad has broadened my horizons.
The main difference in the teaching system is in the greater emphasis on independent work and tutorials,
which are classes where a small group of students meets with a professor and the students’ task is asking questions to the professor. The study at St Andrews has met and exceeded my expectations, even though the last year was greatly affected by covid-related limitations.
Do you think about future career in science?
My curiosity continues to draw me into science and I am most interested in plasma physics. At the moment,
I am applying for summer scientific internships and I use my experience from Open Science in my applications.
Scientist and supervisor A. Bubnov about the cooperation with Nela Sedláčková:
Nela Sedláčková is a very hardworking and talented student who did excellent work during and after the Open Science internship.
In 2018, she was chosen for a highly competitive internship "Study of phase transitions of new liquid crystalline materials".
The internship was carried out in the
Liquid Crystal group of the Dielectrics Department
at the Institute of Physics of the Czech Academy of Sciences.
The results we gained by Nela during the internship were presented at an international conference
(XXIII Czech-Polish seminar - Structural and ferroelectric phase transitions),
in the form of a contribution entitled "Self-assembling behavior of new photosensitive cinnamoyl-based monomers aimed for
design of smart macromolecular materials", and received a positive feedback from conference participants.
After completing the internship, the results were also published in the important international scientific journal Liquid Crystals
with a high impact factor (3.6 in 2020 according to Journal Citation Reports):
A. Bubnov, M. Cigl, N. Sedláčková, D. Pociecha, Z. Böhmová, V Hamplová,
Self-assembling behavior of new functional photosensitive cinnamoyl-based reactive mesogens,
Liquid Crystals 47, 2276 (2020). Although the article was published a year ago, it already has 10 citations, which is above the average for this specific field.
I know that Nela Sedláčková is currently a student at the School of Physics and Astronomy, at the University of St Andrews in Great Britain, and that she received a quite prestigious scholarship from the Kellner Family Foundation in 2021. I wish Nela the best of luck. I really hope that one day she will return to the Czech Republic and will work at the Czech Academy of Sciences, which would be of great benefit for us.
authors: Michaela Marková (Open Science) in collaboration with Nela Sedláčková and Alexej Bubnov (Department of Dielectrics)
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Silver medal for young scientists
Team of secondary school students (nicknamed NAFTA), consisting of M. Švanda (captain), T. Čajan, B. Růžičková, P. Štencl and M. Yurchenko, representing the Czech Republic in the 35th International Young Physicists Tournament was awarded a silver medal!
This year the tournament (sometimes referred to as the “Physics World Cup”) took place in Timisoara, Romania from 15th to 25th July 2022. The NAFTA team finished at the 7th place among 25 teams from all over the world. This ranking thus builds on the previous success of the Czech teams in this challenging competition. During the year-lasting preparation for this international competition the team was also strongly supported by feedback from Hynek Němec, Dalibor Repček, and Martin Kempa from the Departments of Dielectrics who were also involved as jury members in the national round at the Faculty of Nuclear Sciences and Physical Engineering at CTU, Prague.
The key to the success in this tournament is a teamwork. The success of the Czech team has to be particularly appreciated in the context of the epidemy circumstances which severely limited cooperation possibilities and which reduced the number of participants in the Czech Republic. In the next year there are planned accompanying events (introductory workshop, training course VYDRA, final symposium with the possibility to present achieved results, discussions with experts, etc.) which all aim to support new participants. We believe that the success acknowledged by the silver medal will motivate to participate the next generation of young physicists in this competition!
About the tournament:
The preparation of the Young Physicists Tournament participants takes almost a year during
which the students have to solve 17 open-ended inquiry problems. Similarly as in the real-world scientific research,
they have to inspect problems from different points of view, to look for relevant information in literature,
to analyze data, to defend results obtained, etc. The competition itself consists in a model scientific discussion
about the achieved results, in which the teams alternate in three different roles:
presenting their solution of the problem, acting as an opponent, and evaluating solutions of the other teams.
The performance of the team in each role is evaluated by a jury which consists of real experts.
The tournament is thus a demanding competition not only for students but also for their teachers.
It exceeds common knowledge competitions – the aim is not only solving problems,
but especially the ability of critical thinking, presentation, and discussion of the results.
These are exactly the skills in the knowledge-based economics of the 21th century.
For this reason, it is strongly recommended in the outcomes of the project DIBALI
(Development of Inquiry Based Learning via IYPT) to support the competitions according to
their benefits rather than by the number of participants, and
to acknowledge the work of teachers preparing students for such demanding competitions with high added value.
(website of the competition: https://tmf.fzu.cz/)
text: H. Němec
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Dalibor Repček won the Student Poster Competition at ISAF 2022
Dalibor was awarded for his excellent poster presentation named Multiferroic Quantum Criticality in (Eu,Ba,Sr)TiO3 System in the Student Poster Competition at the International Symposium on Applications of Ferroelectrics (ISAF) within the joint ISAF-PFM-ECAPD conference held in Tours, France from June 27 to July 1, 2022.
The ISAF meeting is actually an important annual international conference gathering scientists all over the world
working on both fundamental science and applications of ferroelectrics.
The first ISAF meeting was held in 1968; since that time, annual meetings have been hosted all around the world,
including Prague in the year 2013 within the joint UFFC, EFTF and ISAF-PFM Symposium,
co-organized by the Department of Dielectrics.
Having this in mind, Dalibor got a prestigious award from the community engaged in ferroelectrics and related materials
for his experimental work on quantum critical phenomena in multiferroic materials.
More specifically, he looks for an experimental proof of the theoretically predicted multiferroic quantum
criticality in (Eu,Ba,Sr)TiO3 solid solution [1].
His PhD research is done under supervision of Dr. Stanislav Kamba and
in collaboration with colleagues from Faculty of Mathematics and Physics, Charles University (Prague, Czech Republic) and
from CEITEC (Brno, Czech Republic).
References
[1] A. Narayan, A. Cano, A.V. Balatsky, and N. A. Spaldin, Multiferroic quantum criticality, Nature Mater 18 , 223–228 (2019).
PhD students succeeded in the Young Researcher Competition of the XXIV Czech-Polish seminar
Vladimir Pushkarev, Dalibor Repček, and André Maia shined in the Young Researcher Competition, consisting of a two-minute talk and a poster presentation, at the XXIV Czech-Polish seminar.
The XXIV Czech-Polish seminar took place in the Giant Mountains (Krkonoše in Czech, Karkonosze in Polish, Riesengebirge in German) in Harrachov (May 23 – 27, 2022) in a warm atmosphere created by 95 participants from 10 countries.
This year, the program of the well-established international conference on Structural and ferroelectric phase transitions (see the highlighted text below)
included not only 8 tutorial lectures to stimulate the scientific development of our younger colleagues, but also a Young Researcher Competition.
All students had an opportunity to take part in a Clip Session to demonstrate their ability
to introduce their research orally in two minutes to a broad audience and in the follow-up poster session.
Finally, the best 5 competitors were awarded:
-
Vladimir Pushkarev (FZU - Institute of Physics of the Czech Academy of Sciences):
Charge confinement and band bending in single-crystalline GaAs nanostructures -
Dalibor Repček (FZU - Institute of Physics of the Czech Academy of Sciences):
Quantum bicriticality tuning in (Eu,Ba,Sr)TiO3 system -
André Maia (FZU - Institute of Physics of the Czech Academy of Sciences):
Lattice dynamics and soft-mode friven gerroelectricity in multiferroic BiMn3Cr4O12 -
Dawid Drozdowski (Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Wroclaw, Poland):
Effect of halide mixing on structural and optoelectrical properties of the 3D and 2D methylhydrazinium lead halide perovskites -
Barbara Loska (Institute of Materials Engineering, University of Silesia, Chorzów, Poland):
Molecular interactions in the twist-bend nematic phase.
Dalibor Repček, Vladimir Pushkarev, André Maia, Dawid Drozdowski, and Barbara Loska.
History of the Czech-Polish seminar:
The idea of regular scientific meetings of physicists involved in studies of ferroelectrics and phase transitions in Poland and Czechoslovakia
followed inevitably from the success of first such an event in Błażejewko in 1979. The Seminar was organized in collaborating of
the Department of Dielectrics of the Institute of Physics of Czechoslovak Academy of Sciences and
the Ferroelectric Lab of the Institute of Molecular Physics of Polish Academy of Sciences.
The Seminars are an international forum for presentation of recent results,
unconstrained discussions and initiating of joint studies.
This conference series results not only in scientific integration but also in close cooperation and friendship.
The bianual event usually brings together about 100–120 participants in a proportion:
approximately one third from the Czech Republic, one third from Poland and the rest from other countries.
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).
Chiral, magnetic, and photosensitive liquid crystalline nanocomposites based on multifunctional nanoparticles and achiral liquid crystals.
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