Department of Dielectrics, Institute of Physics, Czech Academy of Sciences

Time 2021-10-13 15:07:54

Web Name: Department of Dielectrics, Institute of Physics, Czech Academy of Sciences

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keywords:Deparment of Dielectrics
description:Deparment of Dielectrics

Department of Dielectrics

Institute of Physics of the Czech Academy of Sciences

HomeResearch groupsDielectric and IR spectroscopyTHz science and technologyLight and neutron scatteringTheory and simulationsSolid-state materials scienceLiquid crystalsNews domain supercrystal and its corresponding X-ray diffraction pattern [1].

Writing in the journal Nature Materials [1], acollaboration led by Dr. Pavlo Zubko from University College Londoninvolving 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 aperiodic 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 avariety 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).

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03.12.2020MediaResearchers' Night in the THz lab

Petr Kužel and his colleagues from the THz science and technology group introduced their research themes in ageneral public video during the European Researchers' Night.

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During the European Researchers' Night, numbers of scientific institutions and infrastructures are animated by popular science activities. The visitors have aunique 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'sResearchers' Night: “Undergoing epidemy circumstances did not allow us to organize popular experiments in the THz lab for visitors. Therefore, in acollaboration with our PR division, we created ageneral public video which introduces the little-known spectral window in the terahertz (THz) range and which gives aflavor 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).

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02.12.2020AwardOnline 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 2^nd place for his image of abeautiful liquid crystal texture resembling the Sun’scorona.

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Avideo 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 aliquid crystal texture seen by polarized optical microscope, taken by Alexey Bubnov. This picture resembling the Sun’scorona was awarded by the 2^nd place.

Video promoting the best photos of the FZU Photo Competition 2020 (European Researchers' Night).

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Latest Highlights New material for 5G mobile networks

Epitaxial strained thin films of (SrTiO₃)_n-1(BaTiO₃)₁SrOwere found to be a promising new material for mobile network of the 5^th generation[Nature Mater. 19, 176 (2020)].

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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.

Reference:
[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).

(show less) 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)].

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Water molecules localized in structural crystal nano-channels give apossibility to eliminate hydrogen bonds, dominating among water molecules at short distances up to ~2Åwhich 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 9.9Åand the water molecules in the channels are by 4.7Åfrom each other. Their dipole moment is perpendicular to the channel axis and our dielectric spectroscopy study have revealed their ordering at ~3K.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 ~3nC/cm² at 0.3K.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 acritical slowing down of dielectric relaxation.ν_p denotes frequency of the maxima in the dielectric loss spectra and τdenotes the relaxation time of the relaxation.

Reference:
[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. GorshunovTargeted chemical pressure yields tuneable millimetre-wave dielectric,
Nat. Commun. 11, 3927 (2020)

(show less) 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)].

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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 E_u ferroelectric soft mode frequency and of the spontaneous polarization P_s in (EuO)₂/(BaO)₂ superlattice obtained from the DFT calculations. Insets show schematic eigenvectors of the E_u and A_2u 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.

Reference:
[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).

(show less) Seemingly anisotropic magnetodielectric effect in isotropic EuTiO₃ ceramics

Anisotropic magnetodielectric effect was observed at 0.3Kin the low-frequency dielectric spectra of antiferromagnetic EuTiO₃ ceramics [Phys. Rev. B 102, 144402 (2020)].

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The effect was theoretically explained by ademagnetizing field and confirmed by magnetization measurements. THz refractive index revealed also alarge 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 2and 15 K.

Reference:
[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).

(show less) 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.

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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.

Reference:
[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 derivativesNanoscale Adv. 1, 2835(2019).

(show less) Mesoscopic polarization dynamics and two ferroelectric sublattices in the uniaxial tetragonal tungsten bronze (Sr_xBa_1−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].

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The archetypical uniaxial tungsten-bronze (Sr_xBa_1−x)Nb₂O₆ (SBN) has been found to show two different ferroelectric subsystems within its structure [Fig 1a]. The uneven shifts of the two Nb atoms create two different polarization regions of dissimilar lengths and volumes: Nb(2) shifts along the polar axis are smaller and less correlated, as a result of the Nb(2)O₆ tilts in the perovskite units. On the contrary the linking octahedra Nb(1)O₆ are isolated and more rigid, and the Nb atom inside is more akin to shift along the c-axis contributing more to the polarization [Fig. 1b]. Because of this, there are at least two correlation lengths associated with the polarization mechanisms.

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 _GHz and _GHz.

Reference:
[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_xBa_1−x)Nb₂O₆,Phys. Rev. B 100, 184113 (2019).

(show less) 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.

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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.

Reference:
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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Last update: 8 October 2021 (admin)

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