arXiv subjects

Fedir Borodavka

Publications and source records attributed to Fedir Borodavka.

8 recordsLinked to original sources

arXiv2026

Experimentally constrained modeling of the Pockels response of KNbO3 and KTaNbO3

The soft-mode of electro-optic (EO) metal-oxide perovskites plays a critical role in determining their Pockels responses. This is the case of potassium tantalate niobate (KTN), which exhibits an intrinsic Pockels response 2.5 times larger than that of state-of-the-art barium titanate (BTO), highlighting its potential for high-performance EO applications. By combining ab initio calculations at the density-functional theory level and far-IR measurements, we reveal that the harmonic approximation combined with semi-local exchange-correlation functionals fails to accurately capture the soft transverse optical (TO) Slater mode of both potassium niobate and KTN, which dominates the technologically relevant r51 Pockels coefficient. Replacing the calculated mode frequency with its measured value provides an experimentally constrained approach that substantially improves the predicted Pockels response. For that purpose, the previously unreported TO Slater-mode frequency of KTN is extracted from far-IR reflectivity measurements. The results emphasize the potential of KTN-based EO modulators as an alternative to standard lithium niobate and BTO technologies, with a potentially lower energy consumption and device footprint.

cond-mat.mtrl-sci↗

arXiv2026

Soft and chiral phonons in chiral phase of K3NiO2

Raman scattering measurements confirmed the theoretical prediction that the structural phase transition from the achiral tetragonal to the chiral tetragonal phase, which occurs near 400 K, is induced by a doubly degenerate soft phonon at the Z point of the Brillouin zone. In the low-temperature chiral phase, the soft mode activates in Raman spectra, splits into two components with A1 and B1 symmetries and harden with cooling according to Cochran law. Circularly polarized Raman scattering did not reveal the angular momentum of these singly degenerate phonons at the Gamma point, which is consistent with theory. We also calculated the phonon branches in the whole Brillouin zone for both crystalline phases and compared the results with the phonons observable in the Raman spectra. The calculations revealed that some phonons with nonzero k have angular momentum in the chiral phase. A pronounced circular motion of atoms can be observed, for example, in a Dirac-type topological phonon at the M-point of the Brillouin zone with a frequency of 168 cm-1.

cond-mat.mtrl-sci↗

arXiv2026

Dielectric, magnetic and lattice dynamics properties of double perovskite (Ca0.5Mn1.5)MnWO6

Recent dielectric and magnetic studies of (Ca0.5Mn1.5)MnWO6 ceramics [A.A. Belik, Chem. Mater. 36, 7604 (2024)] have classified this material as a rare hybrid multiferroic, with both antiferromagnetic and (anti)ferroelectric ordering occurring at the same temperature of 22 K. The pronounced dielectric anomaly observed at this temperature indicated that the structural change is primarily induced by a phonon soft mode and not by a spin arrangement, as is usually the case in type II multiferroics. However, our comprehensive investigation involving new ceramic samples as well as the sample from the above-mentioned reference does not support this conclusion. Low-temperature polarization measurements revealed no evidence of either ferroelectric or antiferroelectric order in both sample series. The dielectric permittivity exhibits only a slight change at the antiferromagnetic transition, and phonon modes observed in IR and Raman spectra show no indication of a symmetry change at low temperatures. In the new samples the Neel temperature is shifted to TN = 18 K. XRD, SEM, EDS and WDS analyses confirmed the composition (Ca0.5Mn1.5)MnWO6 of both ceramics, but also indicated a small amount (percentage points) of MnO and CaO impurities in the sample from the previous publication and Mn3O4, CaWO4 secondary phases (<4%) in the new ceramics. The differences in dielectric and magnetic properties of the two samples can therefore be explained by their different chemical purity. The small dielectric anomaly of the new sample at the antiferromagnetic transition temperature is explained by a spin-phonon coupling. We conclude that (Ca0.5Mn1.5)MnWO6 is not a multiferroic, but a paraelectric antiferromagnet.

cond-mat.mtrl-sci↗

arXiv2026

Phonon properties and unconventional heat transfer in quasi-2D $Bi_2O_2Se$ crystal

Bi2O2Se belongs to a group of quasi-2D semiconductors that can replace silicon in future high-speed/low-power electronics. However, the correlation between crystal/band structure and other physical properties still eludes understanding: carrier mobility increases non-intuitively with carrier concentration; the observed $T^2$ temperature dependence of resistivity lacks explanation. Moreover, a very high relative out-of-plane permittivity of about 150 has been reported in the literature. A proper explanation for such a high permittivity is still lacking. We have performed infrared (IR) reflectivity and Raman scattering experiments on a large perfect single crystal with defined mosaicity, carrier concentration and mobility. Five of the eight phonons allowed by factor group theory have been observed and their symmetries determined. The IR spectra show that the permittivity measured in the tetragonal plane is as high as $ε_r{\approx}500$, and this high value is due to a strong polar phonon with a low frequency of ~34 $cm^{-1}$ (~1 THz). Such an unusually high permittivity allows the screening of charge defects, leading to the observation of high electron mobility at low temperatures. It also allows effective modulation doping providing a platform for high performance 2D electronics. DFT calculations suggest the existence of a very low frequency acoustic phonon ~14 $cm^{-1}$ (~0.4 THz). Both the low frequency phonons cause anomalous phonon DOS, which is reflected in the unconventional temperature dependence of the heat capacity, $c_M{\approx}T^{3.5}$. The temperature-dependent, two-component group velocity is proposed to explains the unusual temperature dependence of the thermal conductivity, $κ{\approx}T^{1.5}$

cond-mat.mtrl-sci↗

arXiv2022

Terahertz Magnetic and Lattice Excitations in van der Waals Ferromagnet VI3

We use the synergy of infrared, terahertz, and Raman spectroscopies with DFT calculations to shed light on the magnetic and lattice properties of VI3. The structural transition at TS1 = 79 K is accompanied by a large splitting of polar phonon modes. Below TS1, strong ferromagnetic fluctuations are observed. The variations of phonon frequencies at 55 K induced by magnetoelastic coupling enhanced by spin-orbit interaction indicate the proximity of long-range ferromagnetic order. Below TC = 50 K, two Raman modes simultaneously appear and show dramatic softening in the narrow interval around the temperature TS2 of the second structural transition associated with the order-order magnetic phase transition. Below TS2, a magnon in the THz range appears in Raman spectra. The THz magnon observed in VI3 indicates the application potential of 2D van der Waals ferromagnets in ultrafast THz spintronics, which has previously been considered an exclusive domain of antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2018

Electromagnon in Y-type hexaferrite BaSrCoZnFe$_{11}$AlO$_{22}$

We investigated static and dynamic magnetoelectric properties of single crystalline BaSrCoZnFe$_{11}$AlO$_{22}$ which is a room-temperature multiferroic with Y-type hexaferrite crystal structure. Below $300\,\rm K$, a purely electric-dipole-active electromagnon at $\approx 1.2\,\rm THz$ with the electric polarization oscillating along the hexagonal axis was observed by THz and Raman spectroscopies. We investigated the behavior of the electromagnon with applied DC magnetic field and linked its properties to static measurements of the magnetic structure. Our analytical calculations determined selection rules for electromagnons activated by the magnetostriction mechanism in various magnetic structures of Y-type hexaferrite. Comparison with our experiment supports that the electromagnon is indeed activated by the magnetostriction mechanism involving spin vibrations along the hexagonal axis.

cond-mat.mtrl-sci↗

arXiv2017

Unusual ferroelectric and magnetic phases in multiferroic 2H-BaMnO$_3$ ceramics

The structural phase transition in hexagonal BaMnO$_3$ occurring at $T_c$=130 K was studied in ceramic samples using electron and X-ray diffraction, second harmonic generation as well as by dielectric and lattice dynamic spectroscopies. The low-temperature phase (space group $P6_{3}cm$) is ferroelectric with a triplicated unit cell. The phase transition is driven by an optical soft mode from the Brillouin-zone boundary [$q = (\frac{1}{3},\frac{1}{3},0)$]; this mode activates in infrared and Raman spectra below $T_c$ and it hardens according to the Cochran law. Upon cooling below $T_c$, the permittivity exhibits an unusual linear increase with temperature; below 60 K, in turn, a frequency-dependent decrease is observed, which can be explained by slowing-down of ferroelectric domain wall motions. Based on our data we could not distinguish whether the high-temperature phase is paraelectric or polar (space groups $P6_{3}/mmc$ or $P6_{3}mc$, respectively). Both variants of the phase transition to the ferroelectric phase are discussed based on the Landau theory. Electron paramagnetic resonance and magnetic susceptibility measurements reveal an onset of one-dimensional antiferromagnetic ordering below $\approx220\,\rm K$ which develops fully near 140 K and, below $T_{n} \approx 59\,\rm K$, it transforms into a three-dimensional antiferromagnetic order.

cond-mat.mtrl-sci↗

arXiv2016

Electromagnon in the Z-type hexaferrite $({\rm Ba}_{x}{\rm Sr}_{1-x})_3\rm Co_2Fe_{24}O_{41}$

We studied experimentally the high-temperature magnetoelectric $({\rm Ba}_{x}{\rm Sr}_{1-x})_3\rm Co_2Fe_{24}O_{41}$ prepared as ceramics (x = 0, 0.2) and a single crystal (x = 0.5) using inelastic neutron scattering, THz time-domain, Raman and far-infrared spectroscopies. The spectra, measured with varying temperature and magnetic field, reveal rich information about the collective spin and lattice excitations. In the ceramics, we observed an infrared-active magnon which is absent in $E^ω\perp z$ polarized THz spectra of the crystal, and we assume that it is an electromagnon active in $E^ω \| z$ polarized spectra. On heating from 7 to 250 K, the frequency of this electromagnon drops from 36 to 25 cm$^{-1}$ and its damping gradually increases, so it becomes overdamped at room temperature. Applying external magnetic field has a similar effect on the damping and frequency of the electromagnon, and the mode is no more observable in the THz spectra above 2 T, as the transverse-conical magnetic structure transforms into a collinear one. Raman spectra reveal another spin excitation with a slightly different frequency and much higher damping. Upon applying magnetic field higher than 3 T, in the low-frequency part of the THz spectra, a narrow excitation appears whose frequency linearly increases with magnetic field. We interpret this feature as the ferromagnetic resonance.

cond-mat.mtrl-sci↗