Journal of Experimental and Theoretical Physics最新文献

The correspondence of the results obtained in the Neutrino-4 experiment with the results of the NEOS, DANSS, STEREO, and PROSPECT experiments at reactors, the MiniBooNE, LSND, and MicroBoone experiments at accelerators, and the IceCube and BEST experiments with a ⁵¹Cr neutrino source is analyzed. The agreement between the results of the Neutrino-4 experiment, the BEST experiment, and the gallium anomaly on the mixing angle is discussed. The discrepancy between the results of the listed direct experiments with the results of the reactor anomaly, as well as with constraints from solar and cosmological data, is discussed. It is shown that the results of these direct experiments on the search for sterile neutrinos and the IceCube experiment do not contradict the Neutrino-4 experiment within the 3 + 1 neutrino model within 3σ contours of experimental errors. The sterile neutrino parameters from the Neutrino-4 and BEST experiments make it possible to estimate the sterile neutrino mass as m₄ = (2.70 ± 0.22) eV and the effective mass of the electron neutrino as ({{m}_{{4{{nu }_{e}}}}}) = (0.86 ± 0.21) eV. The matrix of the absolute values of the 3 + 1 neutrino model mixing parameters and the mixing scheme are presented.

Modern theories of melting of two-dimensional systems are discussed that are mainly based on the concepts of the Berezinskii–Kosterlitz–Thouless (BKT) theory of phase transitions in two-dimensional systems with continuous symmetry. Today there exist three basic scenarios of melting of two-dimensional crystals. First of all, this is the Berezinskii–Kosterlitz–Thouless–Halperin–Nelson–Young (BKTHNY) theory, in which two-dimensional crystals are melted through two BKT-type continuous transitions with an intermediate hexatic phase. In this case a first-order phase transition can also occur. The third scenario has recently been proposed by Bernard and Krauth (BK), in which melting can occur through a BKT-type transition; in this case the hexatic phase–isotropic fluid transition is a first-order transition. The review presents a critical analysis of the approaches used to determine the parameters and the type of transition by computer simulation methods.

Using the successively inverted projection method, we studied the dynamics of ultralean hydrogen–air flames propagating freely in a horizontal cylindrical Hele–Shaw cell. To quantify the two revealed characteristics of the flame dynamics—the dependence of the average flame velocities on time and the dependence of the initial flame velocity on the stoichiometry of the initial hydrogen–air mixture—we proposed time and stoichiometric scaling relations. The first relation approximates the dependence of the path of the flame front in hydrogen–air mixtures with an initial hydrogen concentration exceeding a certain critical value. The second relation approximates the dependencies of the initial flame front velocities on the hydrogen concentration. The general relationships for topologically different types of ultralean hydrogen–air flames can be interpreted as additional evidence of the presence of a general mechanism for the transition from discrete fronts of isolated drifting ball flames to a quasi-continuous deflagration flame front through a cascade of bifurcations.

The multiphoton excitation and high harmonic generation (HHG) processes are considered using the microscopic quantum theory of nonlinear interaction of strong coherent electromagnetic (EM) radiation with rectangular graphene quantum dot (GQD) with zigzag edges and more than 80 atoms. The dynamic Hartree–Fock approximation has been used to consider the quantum dot-laser field nonlinear interaction at the nonadiabatic multiphoton excitation regime. The many-body Coulomb interaction is described in the extended Hubbard approximation. By numerical results, we show the significance of the rectangular GQD lateral size, shape, and EM wavefield orientation in rectangular GQD of the zigzag edge in the HHG process allowing for increasing the cutoff photon energy and the quantum yield of higher harmonics.

The modification of magnetic and elastic properties of YBaCo₄O_{7 + x} (x = 0, 0.1) cobaltites at a slight controlled deviation from stoichiometry (x) has been investigated. The magnetic properties of stoichiometric YBaCo₄O₇ demonstrate nontrivial behavior, which is inconsistent with the generally accepted notion of phase transitions with long-range magnetic order. Only magnetic moment ΔM = M_FC – M_ZFC induced by an external magnetic field (an analog of thermoremanent magnetization) exhibits anomalies at magnetic phase transition temperatures T_N1 and T_N2 that coincide with those of Young’s modulus anomalies, whereas in the magnetic susceptibility curves taken in the FC and ZFC modes, phase transitions are not discerned. At a small off-stoichiometry (x = 0.1), induced moment ΔM rises by an order of magnitude and a residual ferromagnetic moment of about 10^–3 μ_B arises in the magnetization curves. Two scenarios of the cobalt subsystem magnetic behavior with increasing part of cobalt ions Co³⁺ have been discussed. It has been found that when YBaCo₄O_{7 + x} cobaltites deviate from stoichiometry, the evolution of their magnetic properties is similar to that observed at the transition from Y-based to Ca-based cobaltite.

We present nonlinear dynamic equations for nematic and smectic A liquid crystals in the presence of an alternating electric field and explain their derivation in detail. The local electric field acting in any liquid-crystalline system is expressed as a sum of external electric field, the fields originating from feedback of liquid crystal order parameter, and a field, created by charged impurities. The system tends to decrease the total electric field, because it lowers the energy density. This basically nonlinear problem is not a pure academic interest. In the realm of liquid crystals and their applications, utilized nowadays modern experimental techniques have progressed to the point where even small deviations from the linear behavior can be observed and measured with a high accuracy. We follow hydrodynamic approach which is the macroscopic description of condensed matter systems in the low frequency and long wavelength limit. Nonlinear hydrodynamic equations are well established to describe simple fluids. Similar approaches (with degrees of freedom related to the broken orientational or translational symmetry included) have been used also for liquid crystals. However to study behavior of strongly perturbed (well above the thresholds of various electro-hydrodynamic instabilities) liquid crystals, the nonlinear equations should include soft electromagnetic degrees of freedom as well. There are many examples of such instabilities, e.g., classical Carr-Helfrich instability triggered by the competitive electric and viscous torques, flexoelectric instability, and so one. Therefore the self-consistent derivation of the complete set of the nonlinear electro-hydrodynamic equations for liquid crystals became an actual task. The aim of our work is to present these equations, which is a mandatory step to handle any nonlinear phenomenon in liquid crystals.

The isomers of ion–molecular H⁺(H₂O)_n complexes including up to six water molecules have been studied using quantum chemistry methods. The atomic positions in the isomers corresponding to the global and deepest local minima of potential energy are calculated using the basin-hopping algorithm. The activation energies of some configuration transformations are estimated. The thermodynamic characteristics of the clustering and decomposition of complexes are determined in a harmonic approximation, and they are in good agreement with experimental data. The possibility of simplifying the theoretical investigation of reactions by averaging the thermodynamic characteristics over various channels for energetically close isomers is shown. A weak dependence of the entropy of a reaction on the complex size has been found. A simplified model is proposed to explain the calculation results, and its use for estimating the entropies of complex clustering and decomposition reactions gives good agreement with the experimental results.

Motivated by recent experimental data on dichloro-tetrakis thiourea-nickel (DTN) [Soldatov et al., Phys. Rev. B 101, 104410 (2020)], a model of antiferromagnet on a tetragonal lattice with single-ion easy-plane anisotropy in the tilted external magnetic field is considered. Using the smallness of the in-plane field component, we analytically address field dependence of the energy gap in “acoustic” magnon mode and transverse uniform magnetic susceptibility in the ordered phase. It is shown that the former is non-monotonic due to quantum fluctuations, which was indeed observed experimentally. The latter is essentially dependent on the “optical” magnon rate of decay on two magnons. At magnetic fields close to the one which corresponds to the center of the ordered phase, it leads to experimentally observed dynamical diamagnetism phenomenon.

In this research paper, we consider the process of neutral Higgs production from e⁺e^– annihilation in Inert Higgs Doublet Model (IHDM) in the absence and presence of an external field. The latter is assumed to be a plane and monochromatic wave with linear polarization. In the theoretical framework, we present the analytic calculation of the lowest order differential cross section by using the scattering matrix approach and Dirac–Volkov formalism for charged incident particles. The total cross section is computed by performing a numerical integration of the differential cross section over the solid angle. The results obtained are analyzed and discussed for different centre of mass energies and laser parameters. We found that inserting a laser wave with linear polarization is a suitable mechanism to enhance the total cross section of the process. Indeed, the probability of the process to occur increases with the presence of a linearly polarized laser field, especially with low frequency and high strength.

The effect of a dielectric film on the surface of conducting dust particles on their electrostatic interaction is investigated. Special attention is paid to the case when the radius of one of particles is much larger than the radius of the other particle and to a nonuniform distribution of the surface charge with variants of equilibrium free charge distribution on each of the macroparticles over the entire surface and over the left and/or right hemispheres. The technique for calculating of slowly converging series is worked out using the hypergeometric Gauss functions and by introducing new functions for which recurrent relations and numerical calculation technique were determined.