arXiv subjects

K. Vasin

Publications and source records attributed to K. Vasin.

3 recordsLinked to original sources

arXiv2026

Magnetic circular dichroism of THz modes and selection rules of Raman-active optical phonons in the polar altermagnet candidate \ce{Mn2Mo3O8}

We investigated the magnetic and vibrational excitations in the collinear altermagnet candidate \ce{Mn2Mo3O8} by temperature dependent Raman scattering and magneto-optical THz time-domain transmission spectroscopy. By comparison to \textit{ab initio} calculations accurately capturing the eigenfrequencies of the vibrational eigenmodes, we identify all optical phonons, including the lowest-lying Raman modes of $A_1$ and $E_2$ type, which had remained elusive in a previous Raman study. Moreover, we compare the selection rules for optically active phonons in the paramagnetic and the magnetically ordered phases of \ce{Mn2Mo3O8} and analyze the Raman selection rules with respect to pseudo-angular momentum conservation. No evidence of the expected splitting of the degenerate paramagnetic $E_2$ optical phonons into modes with circular polarization upon magnetic ordering could be resolved, likely due to weak spin-orbit coupling typical for Mn$^{2+}$. In contrast, we observe strong magnetic circular dichroism at a broad THz excitation band, emerging in the magnetically ordered state. This band, potentially originating from two-magnon excitations, is only electric-dipole active and features a field-dependent two-component fine structure. Its magnetic circular dichroism vanishes above the spin-flop transition at 4~T.

cond-mat.str-el↗

arXiv2025

Optical phonons as a testing ground for spin group symmetries

Lattice vibrations are highly sensitive to crystal symmetries and their changes across phase transitions. The latter can modify irreducible (co)representations and corresponding infrared and Raman selection rules of phonons. This concept is established for relativistic magnetic point groups, simultaneously transforming spatial and spin coordinates. However, in altermagnets described by non-relativistic spin groups with disjunct symmetry operations for both vector spaces, the phonon selection rules have remained unexplored. Here, we present a detailed study of the infrared- and Raman-active modes in the collinear antiferromagnet and altermagnet candidate Co$_2$Mo$_3$O$_8$. Comparing to ab initio calculations accurately capturing the eigenfrequencies, we identify all expected phonon modes at room temperature and deduce their selection rules using both symmetry approaches. Importantly, we observe the change of selection rules upon antiferromagnetic ordering, agreeing with the relativistic symmetry approach, while the spin group formalism predicts no changes. Therefore, optical phonons can reveal the appropriate symmetry treatment.

cond-mat.mtrl-sci↗

arXiv2024

Terahertz resonant emission by optically excited infrared-active shear phonons in KY(MoO4)2

Generation of the monochromatic electromagnetic radiation in the terahertz (THz) range of frequencies for many decades remanes a chellenging task. Here we demonstrate the emission of monochromatic sub-THz radiation by coherent optical phonons in dielectric material KY(MoO4)2. The layered crystal structure of KY(MoO4)2 leads to infrared-active shear lattice vibrations with energies below 3.7 meV, which corresponds to the frequencies low than 900 GHz where solid-state compounds based monochromatic radiation sources are rare. Coherent infrared-active optical phonons are excited by broadband THz pulses lasting for tens of picoseconds and re-emit narrow-band subTHz radiation pulses with a decay time of 33 picoseconds, which is exceptionally long for the oscillators with frequencies below 1 THz. Such a long coherent emission allows for the detection of more than 50 periods of radiation with frequencies of 568 and 860 GHz. The remarkably long decay time together with the chemical stability of the employed materials suggest a variety of possible applications in THz laser technology.

cond-mat.mtrl-sci↗