arXiv subjects

F. Borodavka

Publications and source records attributed to F. Borodavka.

5 recordsLinked to original sources

arXiv2025

Suppression of ferromagnetism in van der Waals insulator due to pressure-induced layer stacking variation

Ferromagnetism in van der Waals insulators like CrBr3 is highly sensitive to structural modifications. We explore the pressure-driven structural and magnetic transformations of the van der Waals magnet CrBr3, establishing it as a model platform for phenomena emerging in layered vdW systems. Single-crystal X-ray diffraction revealed intrinsic trimorphism with two known phases (monoclinic and rhombohedral) and a so-far unreported trigonal phase. The paracrystal model well captures the coexistence of the rhombohedral and trigonal phases at ambient conditions. Increasing pressure drives the growth of the AA-stacked trigonal phase at the expense of the rhombohedral phase, which becomes undetectable above 8.4 GPa. Magnetization measurements provided the first direct evidence of a collapse of ferromagnetism above 5.8 GPa. This behavior is attributed to the increasing number of AFM-coupled Cr moments in AA stackings. Ab initio DFT calculations of electronic structure and atomistic simulations of finite-temperature magnetism corroborate the scenario.

cond-mat.mtrl-sci↗

arXiv2024

Orbital angular momentum impact on light scattering by phonons tested experimentally

Polarized Raman spectra of piezoelectric bismuth germanate single crystal were recorded using vortex light in order to verify the predicted symmetry selection rules for Raman scattering of photons with nonzero orbital angular momentum. At first, the phonon spectrum is fully characterized by the infrared reflectivity and Raman scattering with ordinary light. Then, vortex light Raman spectra were compared with the ordinary ones, but no influence of the orbital momentum on the Raman spectra of Bi$_4$Ge$_3$O$_{12}$ was observed. In particular, we experimentally negate the theoretical prediction that Raman scattering using vortex light can yield frequencies of the silent optic modes of Bi$_4$Ge$_3$O$_{12}$. We also have not seen evidence for the predicted symmetry-lowering of the Raman scattering tensors of $E$ modes. We propose that the orbital momentum effects are inaccessible to the ordinary Raman spectroscopy experiments because of the tiny magnitudes of typical phonon coherence lengths.

cond-mat.mtrl-sci↗

arXiv2014

Strong spin-phonon coupling in infrared and Raman spectra of SrMnO3

Infrared reflectivity spectra of cubic SrMnO$_{3}$ ceramics reveal 18 % stiffening of the lowest-frequency phonon below the antiferromagnetic phase transition occurring at T$_{N}$ = 233 K. Such a large temperature change of the polar phonon frequency is extraordinary and we attribute it to an exceptionally strong spin-phonon coupling in this material. This is consistent with our prediction from first principles calculations. Moreover, polar phonons become Raman active below T$_{N}$, although their activation is forbidden by symmetry in $Pm\bar{3}m$ space group. This gives evidence that the cubic $Pm\bar{3}m$ symmetry is locally broken below T$_{N}$ due to a strong magnetoelectric coupling. Multiphonon and multimagnon scattering is also observed in Raman spectra. Microwave and THz permittivity is strongly influenced by hopping electronic conductivity, which is caused by small non-stoichiometry of the sample. Thermoelectric measurements show room-temperature concentration of free carriers $n_{e}=$3.6 10$^{20}$ cm$^{-3}$ and the sample composition Sr$^{2+}$Mn$_{0.98}^{4+}$Mn$_{0.02}^{3+}$O$_{2.99}^{2-}$. The conductivity exhibits very unusual temperature behavior: THz conductivity increases on cooling, while the static conductivity markedly decreases on cooling. We attribute this to different conductivity of the ceramic grains and grain boundaries.

cond-mat.mtrl-sci↗

arXiv2013

Ferroelectric nanodomains in epitaxial PbTiO3 films grown on SmScO3 and TbScO3 substrates

Domain structures of 320 nm thin epitaxial films of ferroelectric PbTiO3 grown by MOCVD technique in identical conditions on SmScO3 and TbScO3 perovskite sub- strates have been investigated by Raman spectroscopy and piezoresponse force microscopy techniques. Phonon frequency shifts and typical domain structure motifs are discussed. The results reveal strikingly different domain structure architecture: domain structures of the PbTiO3 film grown on SmScO3 have dominantly a-domain orientation while strongly preferential c-domain orientation was found in the PbTiO3 film grown on the TbScO3 substrate. Differences between the two cases are traced back to the film-substrate lattice mismatch at the deposition temperature.

cond-mat.mtrl-sci↗

arXiv2012

Absence of ferroelectricity in BiMnO3 ceramics

We performed factor-group analysis of all phonons in possible monoclinic C2/c and C2 structures of BiMnO3 and compared it with our experimental infrared and Raman spectra. We conclude that the crystal structure is centrosymmetric C2/c in the whole investigated temperature range from 10 to 550 K, therefore BiMnO3 cannot be ferroelectric. We revealed a dielectric relaxation in THz spectra above the structural phase transition taking place at T_C1=475 K giving evidence in strong lattice anharmonicity and a large dynamical disorder of Bi cations above T_C1. Step-like dielectric anomaly observed at T_C1 in THz permittivity reminds antiferroelectric phase transition. Nevertheless, the low-temperature dielectric studies did not reveal any antiferroelectric or ferroelectric hysteresis loop. Our experimental results support theoretical paper of P. Baettig et al. (J. Am. Chem. Soc. 129, 9854 (2007)) claiming that BiMnO3 is not multiferroic, but only antipolar ferromagnet.

cond-mat.mtrl-sci↗