Journal of Experimental and Theoretical Physics最新文献

The structure of nuclear matter at short internucleon distances is one of the poorly studied aspects of nuclear physics. At distances of the order of the nucleon radius, nuclear matter is represented by the pairs of correlated nucleons with relative momenta exceeding the Fermi momenta that emerge for a short time. Such structures, whose local density is comparable to the density of neutron stars, arise from fluctuations in the average nuclear density. One of the important characteristics of nucleon–nucleon correlations is their universality implying the independence of their properties from the nuclear mass number. Therefore, the peculiarities of these objects of nuclear structure reflect the properties of nuclear matter rather than specific nuclei. Information about the short-distance physics is extracted from the analysis of processes with high energy–momentum transfers. Until now, the property of universality has been observed in electron–nucleus collisions only for the breakup of nucleon pairs. In this paper we analyze the data on the cumulative production of pions by protons on a set of nuclear targets and for the first time have established the existence of universality of two-nucleon correlations in the production of π⁺ and π^– mesons. We have obtained evidence for the involvement of three-nucleon correlations in the production of pions beyond the kinematics of their production in interactions with two-nucleon objects.

The gauge SU(2)_L × U(1) model with the extension of the lepton sector by three right-handed Majorana sterile neutrinos is considered. The complete mass matrix of 6 × 6 active and sterile neutrinos is diagonalized. The cosmological bounds following from the lifetime of sterile neutrinos and the fraction of energy carried by sterile neutrino dark matter are obtained. The deviations from the “fine-tuned” mixing scenario sensitive to the mass of the lightest standard (active) neutrino are considered, and the regions of the model parameter space corresponding to these deviations are distinguished.

Exact analytic expressions are derived for the spectrum and wavefunctions of electrons in a monolayer of transition metal dichalcogenides in crossed electric and magnetic fields. The electric field dependence of interband transition intensity is calculated. The valley selectivity of interband absorption considerably depends on the electric field and can reverse its sign multiply upon an increase in this field.

The growth kinetics of ⁴He crystals nucleated in a metastable liquid at a supersaturation of up to 10mbar is studied in the temperature range 90–180 mK. An optical dilution cryostat is used to observe and film crystal growth. The boundary supersaturation separating the normal (slow) and abnormal (fast) growth regions is determined. The growth rate of crystal facets in the normal growth mode is measured. The kinetics of fast crystal growth is investigated as a function of supersaturation and temperature.

Coupled nonlinear Schrödinger equations for paraxial optics with two circular polarizations of light in a defocusing Kerr medium with anomalous dispersion coincide in form with the Gross–Pitaevskii equations for a binary Bose—Einstein condensate (BEC) of cold atoms in the phase separation regime. In this case, the helical symmetry of an optical waveguide corresponds to rotation of the transverse potential confining the BEC. The “centrifugal force” considerably affects the propagation of a light wave in such a system. Numerical experiments for a waveguide with an elliptical cross section have revealed characteristic structures consisting of quantized vortices and domain walls between two polarizations, which have not been observed earlier in optics.

The Zeeman splitting in the GaAs/AlGaAs heterostructure is investigated experimentally. Numerical analysis performed for the wavefunctions of exciton states, which takes into account the bands of heavy holes, light holes, and the band split by the spin–orbit interaction, is the quantitative agreement with experimental data both for an exciton with a heavy hole and for that with a light hole. It is shown that for explaining the experimental values of the Zeeman splitting in the quantum well under investigation, it is necessary to take into account both the Coulomb interaction and the contribution from the three bands in the valence band. The effect of screening of exciton states by a 2D gas of electrons with concentration n ≈ 10⁹ cm^–2 is described. Numerical calculations are performed for a large range of quantum well widths and aluminum concentrations in barriers; the chart of the dependence of the effective g factor on these parameters is plotted for magnetic field B = 5 T.

We present the experimental results of our study of the formation and dynamics of chain structures by active Brownian particles in a DC glow discharge. The mechanism of active particle motion is associated with the conversion of laser radiation by particles into the energy of their own (nonthermal) motion. Through our analysis of the motion parameters (the trajectories, the root-mean-square displacement, the kinetic energy) as a function of the intensity of laser radiation incident on the particles, we have shown that the particles are active Brownian ones. It is possible to control their motion without changing the discharge parameters. It has been experimentally found that the formation of chain structures and their stable state are not violated under kinetic heating of the particles as their kinetic energy increases by more than an order of magnitude. This suggests the realization of a mechanism for the formation of chains with a strong coupling between the particles that is not explained by the simple (ion) wake behind the upstream particle.

This paper presents an overview of Monte Carlo (MC) event generators for simulation of proton-proton collisions along with the results on hadron production at nuclotron-based ion collider facility (NICA) energies. Namely, mean multiplicities, mean transverse momenta, and rapidity distributions of p((bar {p})), π^±, K^± at different collision energies are presented. We also study two-particle angular correlations for stable charged particles. Results of simulations with PYTHIA, EPOS, SMASH, and UrQMD event generators are compared to available data. Connections of studied quantities with physics mechanisms in MC generators are discussed. We suggest a tuned set of parameters to address observed discrepancies between data and PYTHIA.

The spin states of Co atoms in PrBaCo₂O_{5 + δ} with regard to the paramagnetic contribution of Pr³⁺ ions have been determined from magnetic property studies for δ = 0.52 and 0.74. Results obtained without considering the paramagnetic contribution of Pr³⁺ ions are inconsistent with available experimental data. With a decrease in temperature, the metal–insulator transition in PrBaCo₂O_5.52 becomes sharper according to a sharp change in spin states (from HS/LS to LS/IS) of Co³⁺ ions. In this case, Co³⁺ ions occupying octahedra pass from the high-spin state (HS, S = 2) to the low-spin one (LS, S = 0) and those occupying pyramids pass from the LS-state to the intermediate spin state (IS, S = 1), as follows from the available structural data. In PrBaCo₂O_5.74, the metal–semiconductor transition occurs smoothly from HS/LS to the HS/IS state upon the smooth transition of the Co³⁺ ion state from (LS, S = 0) to the (IS, S = 1) state in pyramides without change in the spin state of ions Co³⁺ (HS, S = 2) and Co⁴⁺ (LS, S = 1/2) in octahedrons.

A theory of conversion of an intense initial wave pulse into solitons for asymptotically long evolution times has been developed using the approach based on the fact that such a transformation occurs via an intermediate stage of formation and evolution of dispersion shock waves. The number of nonlinear oscillations in such waves turns out to be equal to the number of solitons in the asymptotic state. Using the Poincaré–Cartan integral invariant theory, it is shown that the number of oscillations equal to the classical action of a particle associated with the wave packet in the vicinity of the small-amplitude edge of a dispersion shock wave remains unchanged upon a transfer by a flow described by a nondispersive limit of the nonlinear wave equations considered here. This makes it possible to formulate a generalized Bohr–Sommerfeld quantization rule that determines the set of “eigenvalues” associated with soliton physical parameters in the asymptotic state (in particular, with their velocities). In the theory, the properties of full integrability of nonlinear wave equations are not used, but the corresponding results are reproduced in this case also. The analytical results are confirmed by numerical solutions to nonlinear wave equations.