Showing papers by "Roman V. Pisarev published in 2021"
TL;DR: In this article, the results of lattice dynamics studies of honeycomb antiferromagnetic ilmenite CoTiO3 single crystal by the complementary polarized infrared, Raman, and dielectric spectroscopic techniques were supplemented by the DFT calculations.
18 citations
TL;DR: In this article, the lattice dynamics of cubic fluoroperovskites were studied and it was shown that these materials exhibit affinity to incipient ferroelectrics, which manifests itself in the softening of polar phonons.
Abstract: Metal halide perovskites are promising materials for modern technologies with unique physical properties. In particular, search and investigation for ferroelectricity and multiferroicity in them are very active research topics. Here, the authors study the lattice dynamics of cubic fluoroperovskites $A\phantom{\rule{0}{0ex}}M$F${}_{3}$, and show that these materials exhibit affinity to incipient ferroelectrics, which manifests itself in the softening of polar phonons. The value of this softening correlates with the perovskite tolerance factor $t$, which speaks for the geometric origin of the observed effect.
10 citations
TL;DR: In this article, the authors demonstrate mechanisms of reciprocity breaking in nonlinear optics driven by the toroidal dipole moment which characterizes nontrivial spatial distributions of spins in solids.
Abstract: We demonstrate mechanisms of reciprocity breaking in nonlinear optics driven by the toroidal dipole moment which characterizes nontrivial spatial distributions of spins in solids. Using high-resolution femtosecond spectroscopy at electronic resonances in the magnetoelectric antiferromagnet $\mathrm{Cu}{\mathrm{B}}_{2}{\mathrm{O}}_{4}$, we show that nonreciprocity reaches 100% for opposite magnetic fields due to the interference of nonlinear coherent sources of second harmonic generation originating from the toroidal dipole moment, applied magnetic field, and noncentrosymmetric crystal structure. The experimental results are corroborated by theoretical analysis based on the crystal and magnetic symmetry of $\mathrm{Cu}{\mathrm{B}}_{2}{\mathrm{O}}_{4}$. Our findings open degrees of freedom in nonlinear optics and pave the way for future nonreciprocal spin-optronic devices operating on the femtosecond timescale.
10 citations
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TL;DR: In this article, the authors studied the optical spectra of antiferromagnetic copper metaborate CuB$_2$O$_4$ with a high spectral resolution in the range of 1.4055-1.4065$ eV in magnetic fields up to 9.5 T and temperatures from 1.6 to 20$ K.
Abstract: The optical spectra of antiferromagnetic copper metaborate CuB$_2$O$_4$ are characterized by an exceptionally rich structure of narrow absorption lines due to electronic transitions within the magnetic Cu$^{2+}$ ions, but their unambiguous identification and behavior in magnetic field remain far from being fully understood. We studied the polarized magneto-absorption spectra of this tetragonal antiferromagnet with a high spectral resolution in the range of $1.4055-1.4065$ eV in magnetic fields up to 9.5 T and temperatures from 1.6 up to $T_N = 20$ K. We observed a set of eight absorption lines at $T=1.6$ K in magnetic fields exceeding 1.4 T which we identified as arising from Frenkel excitons related to the ground and the first excited state of Cu$^{2+}$ ions. The number of these excitons is defined by the presence of the four Cu$^{2+}$ ions with the doubly-degenerate spin state $S = 1/2$ at the 4$b$ positions in the crystallographic unit cell. The energies of these excitons are determined the exchange interaction of 0.5 meV of Cu$^{2+}$ ions in the excited state with surrounding ions and by the Davydov splitting of 0.12 meV. In large magnetic field the observed Zeeman splitting is controlled by the anisotropic $g$-factors of both the ground and excited states. We developed a theoretical model of Frenkel excitons in magnetic field that accounts for specific features of the spin structure and exchange interactions in CuB$_2$O$_4$. The model was used for fitting the experimental data and evaluation of Frenkel exciton parameters, such as the Davydov splitting, the molecular exchange energy, and the $g$-factors of the ground and excited states of the Cu$^{2+}$ ions.
3 citations
TL;DR: In this paper, the authors reported results of the detailed infrared and Raman spectroscopic studies of a Co3(BO3)2 single crystal and found that the large phonon shift between cobalt and nickel compounds are associated with changes of force constants induced by the changes of M-O bond lengths within the two types of octahedral MO6 groups.
2 citations
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TL;DR: In this article, the same pulse launches coherent oscillations of the magnetic linear birefringence at the frequency of 3.4 THz corresponding to the two-magnon mode.
Abstract: Excitation, detection and control of coherent THz magnetic excitation in antiferromagnets are challenging problems that can be addressed using ever shorter laser pulses. We study experimentally excitation of magnetic dynamics at THz frequencies in an antiferromagnetic insulator CoF$_2$ by sub-10 fs laser pulses. Time-resolved pump-probe polarimetric measurements at different temperatures and probe polarizations reveal laser-induced transient circular birefringence oscillating at the frequency of 7.45 THz and present below the Neel temperature. The THz oscillations of circular birefringence are ascribed to oscillations of the magnetic moments of Co$^{2+}$ ions induced by the laser-driven coherent Eg phonon mode via the THz analogue of the transverse piezomagnetic effect. It is also shown that the same pulse launches coherent oscillations of the magnetic linear birefringence at the frequency of 3.4 THz corresponding to the two-magnon mode. Analysis of the probe polarization dependence of the transient magnetic linear birefringence at the frequency of the two-magnon mode enables identifying its symmetry.
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TL;DR: In this article, cubic fluoroperovskites exhibit proximity to incipient ferroelectrics, which manifested in the softening of the low-frequency polar phonons in the Brillouin zone center at cooling.
Abstract: Inorganic metal halide perovskites are promising materials for next-generation technologies due to a plethora of unique physical properties, many of which cannot be observed in the oxide perovskites. On the other hand, the search for ferroelectricity and multiferroicity in lead-free inorganic halide perovskites remains a challenging research topic. Here, we experimentally show that cubic fluoroperovskites exhibit proximity to incipient ferroelectrics, which manifested in the softening of the low-frequency polar phonons in the Brillouin zone center at cooling. Furthermore, we reveal the coupling between harmonic and anharmonic force constants of the softening phonons and their correlation with the perovskite tolerance factor. Next, using first-principles calculations, we examine the lattice dynamics of the cubic fluoroperovskites and disclose the incipient lattice instability at which the harmonic force constants of low-lying phonons tend to decrease with a reduction of tolerance factor at all high-symmetry points of the Brillouin zone. The correlations with the tolerance factor indicate the geometric origin of observed incipient lattice instability in the cubic fluoroperovskites caused by the steric effect due to the volume filling of the unit cell by different radius ions. These results provide insights into the lattice dynamics and potential ferroelectric properties of inorganic lead-free metal halide perovskites, relevant to further design and synthesis of new multifunctional materials.