Search

It has been previously demonstrated [Panov et al. Physics of Atomic Nuclei 84(2021)1037] that the TAIGA-HiSCORE Cherenkov array, originally built for cosmic ray physics and ultrahigh-energy gamma-ray astronomy studies using the extensive air shower method, can be used in conventional optical astronomy for wide-field searches for rare nanosecond optical transients of astrophysical origin. The FOV of the facility is on the scale of 1~ster, and it is capable of detecting very rare transients in the visible light range with fluxes greater than approximately 3000~quanta/m$^2$/10~ns (10~ns is the apparatus integration time) and pulse durations of 10\,ns. Among the potential sources of distant nanosecond optical transients are the evaporation of primary black holes, magnetic reconnection in the accretion disks of black holes, and signals from distant lasers of extraterrestrial civilizations. The paper describes the methods and results of the search for optical transients using the TAIGA-HiSCORE Cherenkov array from 2018 to 2022 (four winter seasons of data collection). No reliable astrophysical candidates for optical transients were found. We set an upper bound on the flux of the searched events as $\sim 1\times10^{-3}$\,events/ster/h., Comment: 20 figures, 46 pages, submitted to Astronomy Reports

The peculiarities of phase states of the triplet boson model for orthonickelates are investigated analytically and by means of numerical simulations. The conditions of thermodynamic stability of homogeneous phases are found. It is shown that the description of the phase inhomogeneous state in the mean-field approximation qualitatively agrees with the observed state of the system in numerical simulations by the classical Monte Carlo method

Rare-earth orthonickelates RNiO3 are Jahn-Teller magnets, unstable with respect to the anti-Jahn-Teller disproportionation reaction with the formation of a system equivalent to a system of effective spin-triplet composite bosons moving in a non-magnetic lattice. Within the framework of the two-sublattice approximation, we have developed a mean field theory for a model nickelate with competition between phases of charge ordering, an antiferromagnetic insulator and a spin-triplet superconductor, and constructed phase diagrams taking into account phase separation.

We analyze frustrated states of the one-dimensional dilute Ising chain with charged interacting impurities of two types with mapping of the system to some Markov chain. We perform classification and reveal two types of Markov chains: periodic with period 2 and aperiodic. Frustrated phases with various types of chains have different properties. In phases with periodic Markov chains, long-range order is observed in the sublattice while another sublattice remains disordered. This results in a conjunction of the non-zero residual entropy and the infinite correlation length. In frustrated phases with aperiodic chains, there is no long-range order, and the correlation length remains finite. It is shown that under the magnetic field the most significant change in the spin chain structure corresponds to the change of the Markov chain type., Comment: 19 pages, 4 figures, for associated pdf file, see https://journals.ioffe.ru/articles/viewPDF/58978

We consider a system of Ising spins s=1/2 with nonmagnetic impurities with charge associated with pseudospin S=1. The charge density is fixed pursuant to the concentration n. Analysis of the thermodynamic properties in the one-dimensional case showed the presence of so-called pseudotransitions at the boundaries between the staggered charge ordering and (anti)ferromagnetic ordering. In the case of n=0, a "1st order" pseudotransition was discovered. This type of pseudotransition is inherent for a series of other one-dimensional frustrated models. However, for n != 0 we discovered a new type of "2nd order" pseudotransition, which had not previously been observed in other systems., Comment: 6 pages, 4 figures

We argue that the unusual properties of a wide class of materials based on Jahn-Teller 3d and 4d ions with different crystal and electronic structures, from quasi-two-dimensional unconventional superconductors (cuprates, nickelates, ferropnictides/chalcogenides, ruthenate SrRuO4), manganites with local superconductivity to 3D ferrates (CaSr)FeO3, nickelates RNiO3 and silver oxide AgO with unusual charge and magnetic order can be explained within a single scenario. The properties of these materials are related to the instability of their highly symmetric Jahn-Teller "progenitors" with the ground orbital E-state to charge transfer with anti-Jahn-Teller disproportionation and the formation of a system of effective local composite spin-singlet or spin-triplet, electronic or hole bosons moving in a non-magnetic or magnetic lattice. These unusual systems are characterized by an extremely rich variety of phase states from non-magnetic and magnetic insulators to unusual metallic and superconducting states., Comment: 6 pages, 1 table. arXiv admin note: text overlap with arXiv:2306.06612

In this paper the dependence of the spectra of cosmic ray nuclei on the charges of nuclei was studied, according to the data of the NUCLEON space experiment. First, we studied the dependence of the spectral index of magnetic rigidity spectra on the charge for abundant nuclei. Secondly, for the charge range $Z=9\div20$, the differences in the total spectra of rare odd and abundant even nuclei were studied. Using the GALPROP package, the inverse problem of CR propagation from a source (near supernova) to an observer was solved, a component-by-component spectrum in the source was reconstructed, and it was shown that a systematic change in the spectral index in the source exist. It is supposed that this change may be interpreted as incomplete ionization of cosmic rays at the stage of acceleration in the supernova remnant shock. The ratio of the total spectra of magnetic rigidity for low-abundance odd and abundant even nuclei from the charge range $Z=9\div20$ is obtained, and it was shown that the spectra of odd rare nuclei are harder than the stpectra of abundat even nuclei in the rigidity range 300--10000~GV., Comment: 7 pages 4 figures, accepted in Bulletin of the Russian Academy of Sciences: Physics V.87(7), 2023

We introduce a minimal model to describe the charge degree of freedom in cuprates with the on-site Hilbert space reduced to only the three valence states CuO$_4^{7-,6-,5-}$ (nominally Cu$^{1+,2+,3+}$) and make use of the S=1 pseudospin formalism. The formalism constitutes a powerful method to study complex phenomena in interacting quantum systems characterized by the coexistence and competition of various ordered states. Overall, such a framework provides a simple and systematic methodology to predict and discover new kinds of orders. In particular, the pseudospin formalism provides the most effective way to describe different topological structures, in particular, due to a possibility of a geometrical two-vector description of the on-site states. We introduce and analyze effective pseudospin Hamiltonian with on-site and inter-site charge correlations, two types of a correlated one-particle transfer and two-particle, or the composite boson transfer. The 2D S=1 pseudospin system is prone to a creation of different topological structures, which form topologically protected inhomogeneous distributions of the eight local S=1 pseudospin order parameters. We present a short overview of localized topological structures, typical for S=1 (pseudo)spin systems, focusing on unexpected antiphase domain walls in parent cuprates and so-called quadrupole skyrmion, which are believed to be candidates for a topological charge excitation in parent or underdoped cuprates. Puzzlingly, these unconventional structures can be characterized by an uniform distribution of the mean on-site charge, that makes these invisible for X-rays. Quasiclassical approximation and computer simulation are applied to analyze localized topological defects and evolution of the domain structures in "negative-$U$" model under charge order-superfluid phase transition., Comment: 25 pages, 7 figures. arXiv admin note: text overlap with arXiv:1409.5064

It is demonstrated in the (pseudo)spin S=1 formalism that the structure of antiphase domain boundaries in the phase of charge ordering of a mixed-valence system of the Cu1+, 2+, 3+ "triplet" type in cuprates on a two-dimensional square lattice depends to a considerable extent on on-site correlation parameter U. The results of computer modeling on large square lattices illustrate the change in the boundary structure (from a homogeneous monovalent nonconducting structure of the Cu2+ type to a filamentary superconducting one) induced by a relatively small variation of positive U values., Comment: 3 pages, 2 figures

The competition between the magnetic and charge orderings in a model cuprate is considered in terms of a simplified static 2D spin-pseudospin model. This model is equivalent to the 2D dilute antiferromagnetic (AFM) Ising model with charged impurities. The mean-field approximation results are presented for the system under study and briefly compared to the classical Monte Carlo (MC) calculations. The numerical simulation shows that the cases of the strong exchange and the strong charge correlation differ qualitatively. In the case of a strong exchange, the AMF phase is instable with respect to the phase separation (PS) into the pseudospin (charge) and magnetic (spin) subsystems that behave as immiscible quantum liquids. The analytical expression has been obtained for the PS temperature., Comment: 7 pages, 3 figures

It is shown that the structure of antiphase domain boundaries in the antiferromagnetic (AFM) phase of a highly anisotropic magnet with S = 1 on a two-dimensional square lattice depends greatly on single-ion anisotropy parameter D. Computer modeling on large square lattices illustrates the changes in the boundary structure from the quantum paramagnet (QP) to the XY phase, including the intermediate QP-XY phase at fairly small variations in positive D., Comment: 3 pages, 2 figures

We consider the effects of the magnetic field on the frustrated phase states of the dilute Ising chain, especially, the behavior of the magnetic entropy change and the isentropic dependence of the temperature on the magnetic field, which are the key parameters of the magnetocaloric effect. The found temperature dependences of entropy demonstrate the nonequivalence of frustrated phases in the antiferromagnetic and ferromagnetic cases. In the antiferromagnetic case, the nonzero magnetic field at certain parameters causes a charge ordering for nonmagnetic impurities at a half-filling, while in the ferromagnetic case, the magnetic field reduces the frustration of the ground state only partially. It is also shown, that impurities radically change the magnetic Gr\"uneisen parameter in comparison with the case of a pure Ising chain., Comment: 15 pages, 8 figures

This paper is devoted to computational algorithms designed to describe the classical Ising magnet in some specific cases when an additional macroscopic restriction in form of constant charge density exists in the system. We developed and implemented a parallel algorithm for modeling such a systems on GPU with CUDA technology. This work focuses on technical aspects of implementing the algorithm.

A classical Monte Carlo algorithm based on the quasi-classical approximation is applied to the pseudospin Hamiltonian of the model cuprate. The model takes into account both local and non-local correlations, Heisenberg spin-exchange interaction, single-particle and correlated two-particle transfer. We define the state selection rule that gives both the uniform distribution of states in the phase space and the doped charge conservation. The simulation results show a qualitative agreement of a phase diagrams with the experimental ones.

The corrected dependence of the mean depth of the EAS maximum $X_{max}$ on the energy was obtained from the data of the Tunka-133 array for 7 years and the TAIGA-HiSCORE array for 2 years. The parameter $\langle\ln A\rangle$, characterizing the mean mass compositon was derived from these results. The differential energy spectrum of primary cosmic rays in the energy range of $2\cdot 10^{14}$ - $2\cdot 10^{16}$\,eV was reconstructed using the new parameter $Q_{100}$ the Cherenkov light flux at the core distance 100 m.}, Comment: 6 pages, 3 figures, Submitted to SciPost Phys.Proc

We develop a minimal non-BCS model for the CuO$_2$ planes with the on-site Hilbert space reduced to only three effective valence centers CuO$_4$ with different charge, conventional spin, and orbital symmetry, combined in a charge triplet. Using the S=1 pseudospin algebra we introduce an effective spin-pseudospin Hamiltonian. To illustrate the possibilities of the molecular field approximation we start with the analysis of the atomic and the "large negative-$U$" limits of the model in comparison with the Bethe cluster approximation, classical and quantum Monte Carlo methods. Both limiting systems exhibit the phase separation effect typical of systems with competing order parameters. The $T$\,-\,$n$ phase diagrams of the complete spin-pseudospin model were reproduced by means of a site-dependent variational approach within effective field approximation typical for spin-magnetic systems. Limiting ourselves to two-sublattice approximation and $nn$-couplings we arrived at several N\'{e}el-like phases in CuO$_2$ planes for parent and doped systems with a single nonzero local order parameter: antiferromagnetic insulator, charge order, glueless $d$-wave Bose superfluid phase, and unusual metallic phase. However, the global minimum of free energy is realized for phase separated states which are bounded by the third-order phase transition line $T^{\star}(n)$, which is believed to be responsible for the onset of the pseudogap phenomenon. With a certain choice of the Hamiltonian parameters the model phase diagrams can quite reasonably reproduce the main features of experimental phase diagrams for T- and T$^{\prime}$-cuprates and novel nickelates. The superconducting phase of cuprates/nickelates is determined by the on-site composite boson transport, it is not a consequence of pairing of doped holes/electrons, but represents one of the possible phase states of parent systems., Comment: 7 pages, oral presentation at SCES 2020. arXiv admin note: text overlap with arXiv:2009.12790

Starting with a minimal model for the CuO$_2$ planes with the on-site Hilbert space reduced to a charge triplet of the three effective valence centers [CuO$_4$]$^{7-,6-,5-}$ (nominally Cu$^{1+,2+,3+}$) with different conventional spin, different orbital symmetry, and different local lattice configuration, we develop a unified non-BCS spin-pseudospin model to describe the main phase states of doped cuprates. We argue that antiferromagnetic insulating, charge ordered, superconducting, and Fermi-liquid phases are possible phase states of a model parent cuprate, while typical phase state of a doped cuprate, in particular mysterious pseudogap phase, is a result of a phase separation. Superconductivity of cuprates is not a consequence of pairing of doped holes, but the result of quantum transport of on-site composite hole bosons, whereas main peculiarities of normal state can be related to an electron-hole interplay for unusual Fermi-liquid phase and features of the phase separation. Puzzlingly, but it is the electron-lattice interaction, which in the BCS model determines $s$-wave pairing, in the model of local composite bosons gives $d_{x^2-y^2}$-symmetry of the superconducting order parameter, thus showing once again a substantial involvement of the lattice in the cuprate's HTSC., Comment: 21 pages, oral presentation at the European Conference "Physics of Magnetism 2021"

We develop a classical Monte Carlo algorithm based on a quasi-classical approximation for a pseudospin S=1 Hamiltonian in real space to construct a phase diagram of a model cuprate with a high Tc. A model description takes into account both local and nonlocal correlations, Heisenberg spin-exchange interaction, correlated single-particle, and two-particle transport. We formulate a state selection algorithm for a given parameterization of the wave function in order to ensure a uniform distribution of states in the phase space. The simulation results show a qualitative agreement with the experimental phase diagrams., Comment: 9 pages, 3 figures

Phase diagrams of charged semi-hard-core bosons are studied in the mean field approximation. An increase in the parameter of local correlations is shown to lead to the transformation of the phase diagram of the system from the form characteristic of hard-core bosons to the limiting form with a parabolic dependence of the critical temperature of charge ordering on the boson concentration. The evolution between these limiting cases is dependent on the ratio between the model parameters and is accompanied by various effects, such as the change in the phase transition type, the appearance of new order-order transition, and the appearance of new critical points., Comment: 10 pages, 3 pdf figures

The problem of competing orderings in the high-temperature cuprate materials is widely discussed for the last years. We present the mean-field approximation results for the spin-pseudospin model accounting for the on-site and inter-site correlations, the antiferromagnetic exchange coupling, the one- and two-particle transport. The explicit form of the equations for the critical temperatures of the most significant order parameters of the model are given., Comment: 6 pages, 2 figures

The short-range correlations are considered for a two-dimensional hard-core boson model on square lattice within Bethe approximation for the clusters consisting of two and four sites. Explicit equations are derived for the critical temperatures of charge and superfluid ordering and their solutions are considered for various ratios of the charge-charge correlation parameter to the transfer integral. It is shown that taking into account short-range correlations for the temperatures of charge ordering results in the appearance of the critical concentration of bosons, which restricts the existence domain of the solutions of charge ordering type. In the case of superfluid ordering with the assumption of short-range correlations, the critical temperature is reduced up to zero values at half-filling. A phase diagram of the hard-core boson model is constructed with the assumption of phase separation within Maxwell's construction and it is shown that taking into account short-range correlations within Bethe approximation quantitatively approximates the form of phase diagram to the results of the quantum Monte Carlo method., Comment: 6 pages, 5 figures

The classical Monte Carlo method is used to study the properties of the ground state and phase transitions of the spin-pseudospin model, which describes a two-dimensional Ising magnet with competing charge and spin interactions. This competition leads to the ground state degeneracy and the frustration of the system. It is shown that the ground state degeneracy is observed in the frustration area with nonzero probabilities of the formation of two different ordered states. Based on the histogram analysis of the Monte Carlo data, the type of phase transitions is analyzed. It is found that, near the frustration point, first-order phase transitions are observed in the dependence on the ratio between the spin ($s = 1/2$) and pseudospin ($S = 1$) interactions., Comment: 13 pages, 4 figures

The properties of a dilute Ising magnet are studied using a two-dimensional spin-pseudospin model with charged impurities and a frustration caused by the competition of the charge and magnetic orderings. Based on the classical Monte Carlo method, the ground state phase diagram has been obtained and also unusual phase states appeared at finite temperatures have been studied. The regions in which order-order phase transitions and also reentrant phase transition are observed have been found., Comment: 11 pages, 6 figures

We report on classical Monte Carlo study of phase transitions and critical behavior of a 2D spin-pseudospin model describing a dilute magnet with competing charge and spin interactions. The static critical exponents of the specific heat and correlation length are calculated using the finite-size scaling theory in a wide range of model parameters. The order of phase transitions is analyzed within the energy histogram method. It is found that approaching the frustration point and increasing the density of non-magnetic impurities leads to non-universal critical behavior and first-order phase transitions. Features of non-universal critical behavior are shown to depend on the relationship between the parameters of the spin and pseudospin interactions., Comment: 11 pages, 4 figures. IEEE Trans. on Magn. (2021)

We study the 2D static spin-pseudospin model equivalent to the dilute frustrated antiferromagnetic Ising model with charge impurities. We present the results of classical Monte Carlo simulation on a square lattice with periodic boundary conditions in a strong exchange limit. In the framework of the finite-size scaling theory we obtained the static critical exponents for the specific heat $\alpha$ and the correlation length $\nu$ for a wide range of the local density-density interaction parameter $\Delta$ and charge density $n$. It was shown that the system exhibits non-universal critical behavior depending on these parameters., Comment: 7 pages, 2 figures

The magnetocaloric effect in a two-dimensional Ising model is considered for different ratios between parameters of inter-site repulsion of nonmagnetic impurities and exchange coupling. Classical Monte Carlo simulations on a square lattice show that in case of weak coupling and at sufficiently high concentrations of nonmagnetic impurities the long-range ferromagnetic ordering breaks down to give isolated spin clusters in the ground state of the system. This leads to appearance of a paramagnetic response in the system at the zero temperature and nonzero entropy of the ground state. The feasibility to detect frustration of ground state using data on the magnetic entropy variation is discussed

A wide-angle Cerenkov array TAIGA-HiSCORE (FOV $\sim$0.6 sr), was originally created as a part of TAIGA installation for high-energy gamma-ray astronomy and cosmic ray physics. Array now consist on nearly 100 optical stations on the area of 1 km$^2$. Due to high accuracy and stability ($\sim$1 ns) of time synchronization of the optical stations the accuracy of EAS arrival direction reconstruction is reached 0.1$^\mathrm{o}$. It was proven that the array can also be used to search for nanosecond events of the optical range. The report discusses the method of searching for optical transients using the HiSCORE array and demonstrates its performance on a real example of detecting signals from an artificial Earth satellite. The search for this short flares in the HiSCORE data of the winter season 2018--2019 is carried out. One candidate for double repeater has been detected, but the estimated probability of random simulation of such a transient by background EAS events is not less than 10%, which does not allow us to say that the detected candidate corresponds to a real astrophysical transient. An upper bound on the frequency of optical spikes with flux density of more than $10^{-4} \mathrm{erg/s/cm}^2$ and a duration of more than 5\,ns is established as $\sim 2 \times 10^{-3}$ events/sr/hour., Comment: 15 pages, 6 figures, reported at the conference ISCRA-2021, Accepted for publication in Physics of Atomic Nuclei

Starting with a minimal model for the CuO$_2$ planes with the on-site Hilbert space reduced to only three effective valence centers [CuO$_4$]$^{7-,6-,5-}$ (nominally Cu$^{1+,2+,3+}$) with different conventional spin and different orbital symmetry we propose a unified non-BCS model that allows one to describe the main features of the phase diagrams of doped cuprates within the framework of a simple effective field theory., Comment: 7 pages, 1 figure

The local distributions of the one-dimensional dilute annealed Ising model with charged impurities are studied. Explicit expressions are obtained for the pair distribution functions and correlation lengths, and their low-temperature asymptotic behavior is explored depending on the concentration of impurities. For a more detailed consideration of the ordering processes, we study local distributions. Based on the Markov property of the dilute Ising chain, we obtain an explicit expression for the probability of any finite sequence and find a geometric probability distribution for the lengths of sequences consisting of repeating blocks. An analysis of distributions shows that the critical behavior of the spin correlation length is defined by ferromagnetic or antiferromagnetic sequences, while the critical behavior of the impurity correlation length is defined by the sequences of impurities or by the charge-ordered sequences. For the dilute Ising chain, there are no other repeating sequences whose mean length diverges at zero temperature. While both the spin correlation and the impurity correlation lengths can diverge only at zero temperature, the ordering processes result in a maximum of the specific heat at finite temperature defined by the maximum rate of change of the impurity-spin pairs concentration. A simple approximate equation is found for this temperature. We show that the non-ordered dilute Ising chains correspond to the regular Markov chains, while various orderings generate the irregular Markov chains of different types., Comment: 10 pages, 4 figures; submitted to the Journal of Magnetism and Magnetic Materials

We address the intensively studied extended bosonic Hubbard model (EBHM) with truncation of the on-site Hilbert space to the three lowest occupation states n=0,1,2 in frames of the S=1 pseudospin formalism. Similar model was recently proposed to describe the charge degree of freedom in a model high-Tc cuprate with the on-site Hilbert space reduced to the three effective valence centers, nominally Cu^{1+;2+;3+} . With small corrections the model becomes equivalent to a strongly anisotropic S=1 quantum magnet in an external magnetic field. We have applied a generalized mean-field approach and quantum Monte-Carlo technique for the model 2D S=1 system with a two-particle transport to find the ground state phase with its evolution under deviation from half-filling., Comment: 9 pages, 3 figures

The continuous quasi-classical two-sublattice approximation is constructed for the 2D system of charged hard-core bosons to explore metastable inhomogeneous states analogous to inhomogeneous localized excitations in magnetic systems. The types of localized excitations are determined by asymptotic analysis and compared with numerical results. Depending on the homogeneous ground state, the excitations are the ferro and antiferro type vortices, the skyrmion-like topological excitations or linear domain walls., Comment: 5 pages, 2 figures

We consider the competition of magnetic and charge ordering in high-Tc cuprates within the framework of the simplified static 2D spin-pseudospin model. This model is equivalent to the 2D dilute antiferromagnetic (AFM) Ising model with charged impurities. We present the mean-field results for the system under study and make a brief comparison with classical Monte Carlo (MC) calculations. Numerical simulations show that the cases of strong exchange and strong charge correlation differ qualitatively. For a strong exchange, the AFM phase is unstable with respect to the phase separation (PS) into the charge and spin subsystems, which behave like immiscible quantum liquids. An analytical expression was obtained for the PS temperature., Comment: 14 pages, 3 figures

We make use of the S=1 pseudospin formalism to describe the charge degree of freedom in a model high-$T_c$ cuprate with the on-site Hilbert space reduced to the three effective valence centers, nominally Cu$^{1+,\,2+,\,3+}$. Starting with a parent cuprate as an analogue of the quantum paramagnet ground state and using the Schwinger boson technique we found the pseudospin spectrum and conditions for the pseudomagnon condensation with phase transition to a superconducting state., Comment: Version to be published in JLTP

The Drake equation pertains to the essentially equilibrium situation in a population of communicative civilizations (CCs) of the Galaxy, but it does not describe dynamical processes which can occur in it. Both linear and non-linear dynamical population analysis is build out and discussed instead of the Drake equation., Comment: 10 pages, 5 figures

One of the main results of the ATIC (Advanced Thin Ionization Calorimeter) experiment is a collection of energy spectra of abundant cosmic-ray nuclei: protons, He, C, O, Ne, Mg, Si, Fe measured in terms of energy per particle in the energy range from 50 GeV to tens of teraelectronvolts. In this paper, the ATIC energy spectra of abundant primary nuclei are back-propagated to the spectra in sources in terms of magnetic rigidity using a leaky-box approximation of three different GALPROP-based diffusion models of propagation that fit the latest B/C data of the AMS-02 experiment. It is shown that the results of a comparison of the slopes of the spectra in sources are weakly model dependent; therefore the differences of spectral indices are reliable data. A regular growth of the steepness of spectra in sources in the range of magnetic rigidity of 50--1350~GV is found for a charge range from helium to iron. This conclusion is statistically reliable with significance better than 3.2 standard deviations. The results are discussed and compared to the data of other modern experiments., Comment: 8 pages, 11 figures