arXiv subjects

P. A. Usachev

Publications and source records attributed to P. A. Usachev.

12 recordsLinked to original sources

arXiv2026

Nonlinear Optical Probing of Ferroic-Octupolar Order Parameter in Collinear Altermagnet

Altermagnetism as a new concept in condensed matter physics is currently being thoroughly investigated. Despite the absence of macroscopic magnetization, altermagnets host a hidden spin order whose direct detection remains a challenge. Here we report on the observation of electric and magnetic dipole forbidden optical second-harmonic generation (SHG) in the altermagnet CoF$_2$ with a centrosymmetric lattice and spin order. We demonstrate that below the N\'eel temperature $T_N = 38$ K the SHG signal is sensitive to the ferrotype magnetic octupole $\mathbf{\mathcal{O}}^M$ which is the order parameter in the antiferromagnetic phase. By combining polarization-resolved SHG experimental data and a phenomenological symmetry analysis, we show that the altermagnetic spin structure of CoF$_2$ enables a ferroic-octupole-induced electric-quadrupolar nonlinear polarization $\mathbf{P}^{2\omega} = \mathrm i\varepsilon_{0}{ }^c\mathbf{\chi}^{(3)}(\mathbf{\mathcal{O}}^M) :\mathbf{E}^{\omega} \nabla \mathbf{E}^{\omega}$. The temperature dependence of SHG reveals a phase transition at $T_N$ confirming the spin origin of the observed signal. The SHG response is resonantly enhanced by a coherent three-photon process caused by the electronic $d$-$d$ transitions of the Co$^{2+}$ ion. Model calculations of SHG polarization rotational anisotropies and temperature dependencies give a reasonable agreement with experimental data, proving the disclosed nonlinear contribution. Our results establish SHG as a novel sensitive tool of ferroic-octupolar spin ordering and highlight the potential of CoF$_2$ and other altermagnets for further nonlinear optical investigations and applications.

cond-mat.str-el↗

arXiv2024

Edge-on galaxies in the HST COSMOS field: the evolution of stellar discs up to z$\sim$0.5

We present a sample of 950 edge-on spiral galaxies found with the use of an artificial neural network in the Hubble Space Telescope COSMOS field. This is currently the largest sample of distant edge-on galaxies. For all galaxies we analyzed the 2D brightness distributions in the F814W filter and measured the radial and vertical exponential scales ($h$ and $h_z$ correspondingly) of the brightness distribution. By comparing the characteristics of distant galaxies with those of nearby objects, we conclude that thin stellar discs with $h/h_z \geq 10$ at $z \approx 0.5$ should be rarer than today. Both exponential scales of the stellar disc show evidence of luminosity-dependent evolution: in faint galaxies the $h$ and $h_z$ values do not change with $z$, in bright (and massive) spiral galaxies both scales, on average, grow towards our epoch.

astro-ph.GA↗

arXiv2023

The possible evolution of pitch angles of spiral galaxies

The origin and maintenance of spiral structure in galaxies, the correlation between different types of spiral structure and several proposed mechanisms for their generation, and the evolution of spiral arms of galaxies with time are questions that are still controversial. In this note we study the spiral structure in a sample of distant galaxies in order to infer the evolution of spiral arm characteristics with redshift. We considered a sample of 171 face-on spiral galaxies in the Hubble Space Telescope COSMOS (The Cosmic Evolution Survey) field. The galaxies are distributed up to $z \approx 1$ with a mean value of 0.44. For all galaxies, we determined the pitch angles of the spiral arms and analysed their dependence on redshift; a total of 359 arms were measured. Analyses of our measurements combined with the literature data suggest a possible evolution of the pitch angles of spiral galaxies: by the modern epoch the spiral pattern, on average, becomes more tightly wound. This may be a consequence of the general evolution of the structure of galaxies as galaxies become more massive over time and their bulges grow. In addition, the distribution of the cotangent of pitch angle of galaxies indicates the possibility that the dominant mechanism of spiral pattern generation changes over time.

astro-ph.GA↗

arXiv2023

A Three-Dimensional Analytical Model of the Interstellar Extinction within the Nearest Kiloparsec

We present a new version of our analytical model of the spatial interstellar extinction variations within the nearest kiloparsec. This model treats the 3D dust distribution as a superposition of three overlapping layers: (1) the layer along the Galactic midplane, (2) the layer in the Gould Belt, and (3) the layer passing through the Cepheus and Chamaeleon dust cloud complexes. In each layer the dust density decreases exponentially with increasing distance from the midplane of the layer. In addition, there are sinusoidal longitudinal extinction variations along the midplane of each layer. We have found the most probable values of 29 parameters of our model using four data sets: the 3D stellar reddening maps by Gontcharov (2017), Lallement et al. (2019), and Green et al. (2019) and the extinctions inferred by Anders et al. (2022) for 993291 giants from the Gaia EDR3. All of the data give similar estimates of the model parameters. The extinction for a star or a point in space is predicted by our model with an accuracy from $σ(A_\mathrm{V})=0.07$ to 0.37 for high and low Galactic latitudes, respectively. The natural fluctuations of the dust medium dominate in these values. When ignoring the fluctuations of the medium, the average extinction for an extended object (a galaxy, a star cluster, a dust cloud) or a small region of space is predicted by our model with an accuracy from $σ(A_\mathrm{V})=0.04$ to 0.15 for high and low Galactic latitudes, respectively. Green et al. (2019) and Anders et al. (2022) give in unison an estimate of $A_\mathrm{V}=0.12^m$ for the extinction at high latitudes across the whole Galactic dust half-layer above or below the Sun with the natural fluctuations of the medium $σ(A_\mathrm{V})=0.06^m$. Our model is a step to explain how a substantial amount of dust ended up far from the Galactic midplane.

astro-ph.GA↗

arXiv2023

Evolution of the spiral structure of galaxies from the HST COSMOS field

We have investigated the pitch angle ($ψ$) of the spiral arms of galaxies in the Hubble Space Telescope COSMOS field. The sample consists of 102 face-on galaxies with a two-armed pattern at a mean redshift $\langle z \rangle \approx 0.5$. The typical values of $ψ$ in the spiral arms of distant galaxies are shown to be close to those for nearby spiral galaxies. Within one galaxy the scatter of $ψ$ for different arms is, on average, half the mean pitch angle. In the $z$ range from 1 to 0 we have found a tendency for $ψ$ to decrease. Our analysis of the $ψ$ distributions in galaxies at different redshifts is consistent with the assumption that in most of the galaxies at $z \leq 0.5$ the spiral arms are tidal in origin or they arose from transient recurrent instabilities in their disks.

astro-ph.GA↗

arXiv2023

Interstellar Extinction in Galactic Cirri in SDSS Stripe 82

We have applied the method of star counts with Wolf diagrams to determine the interstellar extinction in five Galactic cirri in Sloan Digital Sky Survey (SDSS) Stripe 82. For this purpose, we have used the photometry of stars in the GALEX NUV filter and the photometry of red dwarfs in five SDSS bands and four SkyMapper Southern Sky Survey DR2 bands. We have identified the cirri as sky regions with an enhanced infrared emission from the Schlegel+1998 map. The extinction in them has been calculated relative to the nearby comparison regions with a reduced emission. The results for different filters agree well, giving the range of distances and the extinction law for each cirrus. The distances in the range 140--415 pc found are consistent with the 3D reddening maps. In the range between the $B$ and $V$ filters the extinctions found are consistent with the estimates from Schlegel+1998 for the Cardelli+1989 extinction law with $R_\mathrm{V}=3.1$. However, the extinctions found for all of the filters are best described not by the Cardelli+1989 extinction law with some $R_\mathrm{V}=3.1$, but by the inverse proportionality of the extinction and wavelength with its own coefficient for each cirrus. In one of the cirri our results suggest a very slight decrease in extinction with wavelength, i.e., a large contribution of gray extinction. In the remaining cirri a manifestation of gray extinction is not ruled out either. This is consistent with the previous measurements of the extinction law far from the Galactic midplane.

astro-ph.GA↗

arXiv2019

Edge-on galaxies in the Hubble Ultra Deep Field

We studied a sample of 58 edge-on spiral galaxies at redshifts $z \sim 1$ selected in the Hubble Ultra Deep Field. For all galaxies we analyzed the 2D brightness distributions in the $V_{606}$ and $i_{775}$ filters and measured the radial ($h_r$) and vertical ($h_z$) exponential scales of the brightness distribution. We obtained evidence that the relative thickness of the disks of distant galaxies, i.e., the ratio of the vertical scale height and radial scale length ($h_z/h_r$), on average, exceeds the relative thickness of the disks of nearby spiral galaxies. The vertical scale height $h_z$ of the stellar disks of galaxies shows no big changes at $z \leq 1$. The possibility of the evolution of the radial scale length $h_r$ for the brightness distribution with redshift is discussed.

astro-ph.GA↗

arXiv2019

Converting Faraday rotation into magnetization in europium chalcogenides

We present a simple semiclassical model to sustain that in europium chalcogenides (EuX), Faraday rotation (FR) in the transparency gap is proportional to the magnetization of the sample, irrespective of the material's magnetic phase, temperature, or applied magnetic field. The model is validated by FR and magnetization measurements in EuSe in the temperature interval 1.7-300K, covering all EuSe magnetic phases (paramagnetic, antiferromagnetic type I or type II, ferrimagnetic and ferromagnetic). Furthermore, by combining the semiclassical model with the explicit electronic energy structure of EuX, the proportionality coefficient between magnetization and FR is shown to be dependent only on the wavelength and the band gap. Due to its simplicity, the model has didactic value, moreover, it provides a working tool for converting FR into magnetization in EuX. Possible extension of the model to other intrinsic magnetic semiconductors is discussed.

cond-mat.mtrl-sci↗

arXiv2018

Ultrafast light switching of ferromagnetism in EuSe

We demonstrate that light resonant with the bandgap forces the antiferromagnetic semiconducor EuSe to enter ferromagnetic alignment in the picosecond time scale. A photon generates an electron-hole pair, whose electron forms a supergiant spin polaron of magnetic moment of nearly 6,000 Bohr magnetons. By increasing the light intensity, the whole of the sample can be fully magnetized. The key to the novel large photoinduced magnetization mechanism is the huge enhancement of the magnetic susceptibility when both antiferromagnetic and ferromagnetic interactions are present in the material, and are of nearly equal magnitude, as is the case in EuSe.

cond-mat.mtrl-sci↗

arXiv2017

Ultrafast laser-induced changes of the magnetic anisotropy in a low-symmetry iron garnet film

We explore a thermal mechanism of changing the anisotropy by femtosecond laser pulses in dielectric ferrimagnetic garnets by taking a low symmetry (YBiPrLu)3(FeGa)5O12 film grown on the (210)-oriented Gd3Ga5O12 substrate as a model media. We demonstrate by means of spectral magneto-optical pump-probe technique and phenomenological analysis, that the magnetization precession in such a film is triggered by laser-induced changes of the growth-induced magnetic anisotropy along with the well-known ultrafast inverse Faraday effect. The change of magnetic anisotropy is mediated by the lattice heating induced by laser pulses of arbitrary polarization on a picosecond time scale. We show that the orientation of the external magnetic field with respect to the magnetization easy plane noticeably affects the precession excited via the anisotropy change. Importantly, the relative contributions from the ultrafast inverse Faraday effect and the change of different growth-induced anisotropy parameters can be controlled by varying the applied magnetic field strength and direction. As a result, the amplitude and the initial phase of the excited magnetization precession can be gradually tuned.

cond-mat.mes-hall↗

arXiv2012

Amplification of the photocurrent in SiO2(Co)/GaAs heterostructure induced by magnetic field in the avalanche regime

Amplification of the photocurrent in heterostructures of silicon dioxide films containing cobalt nanoparticles grown on gallium arsenide SiO2(Co)/GaAs has been observed in magnetic field in the avalanche regime. While the avalanche process is suppressed by the magnetic field and the current decreases, for photon energy E greater than the GaAs bandgap energy the photocurrent significantly increases. The amplification reaches 9.5 for E = 1.50 eV. The effect of the photocurrent amplification is explained by the spin-dependent recombination process at deep impurity centers in GaAs.

cond-mat.mes-hall↗

arXiv2012

Negative photoconductance in SiO2(Co)/GaAs heterostructure in the avalanche regime

In the avalanche regime we observed the negative photoconductance of heterostructures of silicon dioxide films containing cobalt nanoparticles grown on gallium arsenide, SiO2(Co)/GaAs. Light irradiation with the photon energy less than the bandgap energy of the GaAs creates holes trapped on defects within the GaAs bandgap, suppresses the avalanche feedback and causes a reduction of the current flowing in the SiO2(Co)/GaAs.

cond-mat.mes-hall↗