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The spin-$\frac{1}{2}$ Heisenberg diamond chain with ferro- and antiferromagnetic exchange interactions is studied. The phase boundary in the parametric space of these interactions is determined, where the transition between the ferromagnetic and other (singlet or ferrimagnetic) ground state phases occurs. On this phase boundary there is a dispersionless (flat) energy band in the one-magnon spectrum and these states can be represented as localized magnons in the trapping cells between neighboring diamonds. The ground state consists of the localized magnons and special magnon complexes and it is macroscopically degenerate. A remarkable feature of the model is the existence, on a certain part of the phase boundary, of two- and three-magnon localized states, forming flat bands in two- and three-magnon spectrum. All these localized states also belong to the ground state manifold, which turns out to be exactly the same as for the system of independent spins-$\frac{3}{2}$. This implies a large macroscopic degeneracy of the ground state $4^n$ ($n$ is number of diamonds in the chain) and a high residual entropy per spin $s_{0}=\frac{2}{3}\ln 2$., Comment: 20 pages, 5 figures

We investigate a nonlinear response of the physical vacuum by the example of the third harmonic in radiation of the electron-positron-photon plasma excited by a strong time dependent electric field at the focus spot of counterpropagating laser pulses. The investigation was developed within the framework of the Bogoliubov-Born-Green-Kirkwood-Yvon kinetic theory nonperturbatively describing vacuum creation of the quasiparticle electron-positron plasma (EPP) and different channels of its interaction with the photon subsystem in the single-photon approximation that are opened in the presence of a strong semiclassical field. We investigate the radiation of EPP in the annihilation channel on the basis of the well known approximations and compare the obtained result with the semiclassical radiation of the plasma oscillations. In the high-frequency range for large adiabaticity parameter the quantum radiation dominates in comparison with the semiclassical one.

Identification of particles generated by ion collisions in the NICA collider is one of the basic functions of the Multipurpose Detector (MPD). The main means of identification in MPD are the time-of-flight system (TOF) and the time-projection chamber (TPC). The article considers the optimization of the algorithms of particles identification by these systems. Under certain conditions, the use of the statistical Bayesian approach has made it possible to achieve an optimal ratio of the efficiency of particle identification and contamination by incorrectly defined particles.

We study the frustrated spin-$\frac{1}{2}$ model consisting of a linear chain of triangles with ferro (F)- and antiferromagnetic (AF) interactions connected by ferromagnetic interactions (triangles chain). The ground state phase diagram as a function of the frustration parameter (a ratio of F and AF interactions) and the interaction between triangles consists of the ferromagnetic, two ferrimagnetic and the singlet phases. We study the magnetic properties in these phases and analyze the magnetization curves. We show that there are the magnetization plateau in the singlet phase and the magnetization jumps in some region of this phase. We study the low-thermodynamics and its relation to the specific structure of the excitation spectrum of the triangle chain., Comment: 20 pages, 17 figures

We study the delta-chain with spin-$1$ on basal sites and spin-$\frac{1}{2}$ on apical sites. The Heisenberg interaction between neighbor basal spins is antiferromagnetic (AF) and the interaction between basal and apical spins is ferromagnetic (F). We show that the magnetization curve of this model is the same as that of the spin-$\frac{1}{2}$ kagome-like chain with competing Heisenberg interactions. The ground state phase diagram of the latter as a function of the ratio between the AF and F interaction, $\alpha $, consists of the ferromagnetic, ferrimagnetic and singlet phases. We study the magnetic properties in each ground state phase and analyze the magnetization curves. We show that there are magnetization plateaus and jumps in definite regions of value $\alpha$. We compare the magnetic properties of considered models with those of the spin-$\frac{1}{2}$ delta chain., Comment: 20 pages, 9 figures

It has been shown that during the outburst of accretion activity observed in UX Ori type star RZ Psc in 2013, the accretion rate increased approximately by an order of magnitude. This means that the accretion process at the late stages of the Pre-Main Sequence evolution is very unstable. Using the spectra obtained during this episode we have studied the magnetospheric emission in the H$\alpha$ line. Models of magnetospheric accretion are calculated to obtain the parameters of the magnetosphere from this observation. In present work we have taken into account the influence of the recombination delay effect during gas motion in the stellar magnetosphere. The accounting for this effect and the presence of the magnetospheric absorption in the IR CaII triplet lines and its absence in D Na I resonance lines allowed us to place a lower limit on the temperature in the magnetosphere at $\approx$ 10000 K, which significantly improved precision of our estimate of accretion rate. According to the best fit model the logarithm of accretion rate is $\log\dot{M} = -10.1\pm0.3$ ($\dot{M} \approx 7\times10^{-11}$ M$_\odot$yr$^{-1}$) and the inclination angle of RZ Psc is $43\pm 3^\circ$. It is less than the inclination, typical for the UX Ori stars (about 70$^\circ$), that explains the weak photometric variability of this star. Using the obtained accretion rate and magnetosphere radius we estimate the strength of the dipole component of the magnetic field of RZ Psc $\approx$ 0.1 kGs., Comment: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society Published by Oxford University Press on behalf of the Royal Astronomical Society

The nonlinear dynamics of a one-dimensional molecular crystal in the form of a chain of planar coronene molecules is analyzed. Using molecular dynamics, it is shown that a chain of coronene molecules supports acoustic solitons, rotobreathers, and discrete breathers. An increase in the size of planar molecules in a chain leads to an increase in the number of internal degrees of freedom. This results in an increase in the rate of emission of phonons from spatially localized nonlinear excitations and a decrease in their lifetime. Presented results contribute to the understanding of the effect of the rotational and internal vibrational modes of molecules on the nonlinear dynamics of molecular crystals., Comment: 12 pages, 13 figures

We present results of high magnetic field experiments in pure $^3$He (in the absence of $^4$He coverage) in nematic aerogel. In this case the aerogel strands are covered with few atomic layers of solid paramagnetic $^3$He, which enables the spin-exchange mechanism for $^3$He quasiparticles scattering. Our earlier NMR experiments showed that in low fields, instead of the polar phase, the A phase is expected to emerge in nematic aerogel. We use a vibrating wire resonator with the sample of aerogel attached to it and measure temperature dependencies of resonance properties of the resonator at different magnetic fields. A superfluid transition temperature of $^3$He in aerogel, obtained from the experiments, increases nonlinearly in applied magnetic field. And this increase is suppressed compared with that for bulk A$_1$ phase, which we attribute to an influence of the magnetic scattering channel, previously considered theoretically for the case of $^3$He confined in isotropic silica aerogel. However, we observe the essential quantitative mismatch with theoretical expectations., Comment: 5 pages, 3 figures

We investigate spin-$\frac{1}{2}$ anisotropic model of a linear chain of triangles with competing ferro- and antiferromagnetic interactions and ferromagneic Heisenberg interactions between triangles. For a certain ratio between interactions the one-magnon excitation band is dispersionless leading to an existence of localized-magnon states which form macroscopically degenerated ground states. The spectrum of excitations has a specific structure depending on the value of the triangle-triangle interaction. Such specific structure determines the low-temperature thermodynamics and, in particular, the temperature dependence of a specific heat. In the limit of strong anisotropy of interactions the spectrum has a multi-scale structure which consists of subsets rank-ordered on small parameter. Each subset is responsible for the appearance of the peak in the temperature dependence of the specific heat., Comment: 23 pages, 13 figures

We train a deep learning artificial neural network model, Spatial Attention U-Net to recover useful ionospheric signals from noisy ionogram data measured by Hualien's Vertical Incidence Pulsed Ionospheric Radar. Our results show that the model can well identify F2 layer ordinary and extraordinary modes (F2o, F2x) and the combined signals of the E layer (ordinary and extraordinary modes and sporadic Es). The model is also capable of identifying some signals that were not labeled. The performance of the model can be significantly degraded by insufficient number of samples in the data set. From the recovered signals, we determine the critical frequencies of F2o and F2x and the intersection frequency between the two signals. The difference between the two critical frequencies is peaking at 0.63 MHz, with the uncertainty being 0.18 MHz., Comment: 17 pages, 7 figures, 3 tables