Vol 163, No 3 (2023)

In the paper we theoretically investigate the features of RABBITT (Reconstruction of Attosecond Beating By Interference of Two-photon Transitions) spectroscopy under conditions when transitions through discrete spectrum states play a significant role. Two approaches are applied in the article: the numerical solution of rate equations with continuum discretization and the perturbation theory up to the third order in amplitude. Both approaches use transition matrix elements and photoionization amplitudes obtained by the high-precision R-matrix method. Within the framework of these approaches, photoelectron spectra, the amplitude and phase of RABBITT oscillations were obtained, and the effect of the seed optical field intensity and detuning from a resonance upon excitation of discrete states was studied.

We consider the problem of interaction of three charged particles, the size of one of which can be disregarded. The equations for the expansion coefficients of the electric field potential are derived using the method of expansion in spherical harmonics. Expressions are obtained for the Cartesian components of the interaction force and the torque due to this force. It is shown that in spite of the axial symmetry breaking after the addition of the third particle, if the free charge is distributed uniformly over the surface of a spherical particle, all vector components of the torque acting on this particle are equal to zero. By separating the contributions from image charges in explicit form, we have derived the expressions for the surface charge density and the force of interaction of the particles. The conditions for the emergence of attraction between similarly charged spherical particles depending on the position of the point particle are investigated.

A magnetic state diagram for ferromagnets exhibiting a first-order phase transition in a magnetic field has been calculated. Calculations have been made using the exchange-striction model of a ferromagnet. It has been shown that the curve of first-order magnetic phase transition ends up at a critical point (Hcr, Tcr) that is similar to the critical point of the liquid–gas transition on the (T, P) plane. Analytical expressions for thermodynamic quantities, namely, magnetic susceptibility, specific heat, and compressibility, which anomalously grow near the critical point, have been derived. Calculation results have been compared with available experimental data for La(Fe0.88Si0.12)13 ferromagnet, which experiences the first-order magnetic phase transition.

The dynamics of domain walls in ferrimagnets in which spatial dynamics invariance is violated because of the presence of the chiral Dzyaloshinskii–Moriya interaction with energy linear in sublattice spin density gradients is investigated theoretically. Analysis is performed based on numerical integration of equations in the sigma model generalized to the case of a ferrimagnet near the sublattice spin compensation point. It is shown that in contrast to conventional or chiral ferromagnets, chiral ferrimagnets can exhibit effects of dynamic transformation of the domain wall structure with the formation of more complex walls with a nonmonotonic behavior of the spin density in a wall upon an increase in the wall velocity. These effects are possible in a quite narrow neighborhood of the compensation point, and the width of this region increases upon an increase in the Dzyaloshinskii–Moriya interaction constant.

The nonlinear dynamics of a semi-infinite isotropic ferromagnet with partial spin pinning at the sample edge, as well as in the limiting cases of full spin pinning and in its absence, is investigated based on the Landau–Lifshitz model using the inverse scattering transform. Two types of solitons are predicted. The first of them represents magnetization oscillations with discrete frequencies, which are localized near the sample surface. The second type contains moving particle-like objects with deformable cores, which are elastically reflected from the sample boundary, whereas at large distances from the boundary they are transformed into the typical solitons of an extended ferromagnet. Peculiarities in collisions of solitons with the sample boundary are analyzed for various degrees of spin pinning on the surface. A set of new conservation laws is obtained, which guarantee the fulfillment of the required boundary conditions for solitons and ensure the localization of solitons near the sample surface or their reflection from it.

A mechanism of transverse charge transfer through hexagonal boron nitride (h-BN) in a MIS structure has been studied. Experimental data for charge transfer have been analyzed in terms of different models of charge transfer in insulators. It has been shown that charge transfer in h-BN is described by the model of phonon-assisted tunneling between neutral traps. The thermal and optical energies of phonon-coupled traps in h-BN have been determined. Based on charge transfer measurements, XPS spectra, and the ab initio electronic structure of intrinsic defects in h-BN it has been found that boron–nitrogen divacancies are most probably responsible for charge transfer in h-BN and transfer is provided by electrons.

We present theoretical analysis of Hall effect in doped Mott–Hubbard insulator, considered as a prototype of cuprate superconductor. We consider the standard Hubbard model within DMFT approximation. As a typical case we consider the partially filled (hole doping) lower Hubbard band. We calculate the doping dependence of both the Hall coefficient and Hall number and determine the value of carrier concentration, where Hall effect changes its sign. We obtain a significant dependence of Hall effect parameters on temperature. Disorder effects are taken into account in a qualitative way. We also perform a comparison of our theoretical results with some known experiments on doping dependence of Hall number in the normal state of YBCO and Nd-LSCO, demonstrating rather satisfactory agreement of theory and experiment. Thus the doping dependence of Hall effect parameters obtained within Hubbard model can be considered as an alternative to a popular model of the quantum critical point.

The quasi-isentropic compressibility of a strongly nonideal helium plasma in the pressure range 250–600 GPa is experimentally studied in devices with cylindrical geometry. The temperature at the front of a cylindrical shock wave in helium (T ≈ 10 000 K) and the flight speed of the inner cascade (W ≈ 3.5 km/s), in the cavity of which the maximum compressed plasma density is achieved, are measured. Data on the compression of a nonideal helium plasma to a density ρ ≈ 3 g/cm3 at an approximately constant final temperature of 21000 K are obtained. The trajectories of the metallic shells compressing the plasma are detected using high-power pulsed X-ray sources with a boundary electron energy of up to 60 MeV. The helium plasma density is determined using the radii of the shells measured at the time of their “stop.” The compressed plasma pressure is obtained using gasdynamic calculations. Comparative theoretical calculations of the quasi-isentropic compression parameters have been carried out using the following two theoretical models: the traditional chemical plasma model (SAHA code) and an ab initio quantum molecular dynamics (QMD) approach. No anomaly of the experimental data in the pressure range of the plasma phase transition theoretically assumed in helium is detected.