Vol 127, No 6 (2018)

A model is presented and the interaction of accelerated electrons with atoms in a thin film “shooting through” Be target is theoretically described. The algorithm is based on the Monte Carlo simulation of the electron motion in the target accompanied by energy losses in elastic and inelastic interactions. The conversion of the electron energy to the radiation energy in the 11.4-nm K_α line of Be and the emission spectrum are calculated. The maximum conversion efficiency to the solid angle 4π CE = 3.0 × 10^–4 is achieved for the electron energy E_e = 2.0 keV and a 40-nm-thick freely suspended beryllium film. 200-nm and 400-nm-thick films were experimentally investigated. The maximum conversion efficiency to the solid angle 4π for a 200‑nm-thick film and the electron energy E_e = 2.75 keV was CE_exp = 9.2 × 10^–5, whereas the calculated value was CE_calc = 2.5 × 10^–4. The observed discrepancy between the theory and experiment is explained in the paper.

The absorption of ultrashort laser pulses by diatomic CO and HF molecules is studied theoretically. The probability of absorption in the total pulse action time and its dependence on the pulse duration and carrier frequency are calculated. In contrast to atomic systems, where the finite pulse duration effects are noticeable on time scales of the order of femtoseconds, these effects in molecular structures manifest themselves on time scales of the order of picoseconds. In this case, the vibrational–rotational structure of molecular transitions plays a significant role. The results of our calculations of the vibrational–rotational spectra for the absorption of ultrashort laser pulses by diatomic CO and HF molecules are presented. The transition from the nonlinear dependence of the absorption probability for short pulse durations to the standard dependence at long pulse durations corresponding to the transition probabilities per unit time is investigated.

Recently, the concept of supersonic N-crowdions was offered. In molecular dynamics simulations, they can be excited by initial kick of N neighboring atoms located in one close-packed atomic row along this row. In the present study, in 2D Morse crystal, we apply initial kick to M neighboring atoms located in neighboring close-packed atomic rows along these rows. This way, we initiate crowdion clusters called subsonic M-crowdions. It is well known that static 1-crowdion in 2D Morse lattice is unstable; as a result, the interstitial atom leaves the close-packed atomic row and becomes immobile. However, we show that 1-crowdion moving with sufficiently large subsonic velocity remains in the close-packed atomic row. Crowdion clusters with M equal to or greater than 2 appear to be stable even at rest, with growing M transforming into prismatic dislocation loops. It is important to note that stable subsonic M-crowdions (M > 1) remain mobile and they can carry interstitial atoms over long distances.

The dynamics of the motion of the magnetic moment averaged over an ensemble of nonequilibrium spin-injected electrons in a ferromagnetic junction is considered with allowance for the exchange interaction, as well as the interaction with an external electromagnetic field and a thermostat. The solution of this problem is important for the experimental development of compact terahertz-band radiation sources. The rate of quantum transitions of electrons with opposite spins, which determine the spin relaxation under the interaction with a thermostat, is calculated within the density matrix formalism. It is shown that two spin-relaxation modes can be implemented that correspond to low- and high-Q precession of spin-nonequilibrium injected electrons. The effect of the characteristic features of spin-flip transitions under the relaxation of the magnetic moment on the emission and absorption of photons with-energy corresponding to the energy of effective exchange splitting of spin subbands is discussed.

Monte Carlo methods are used to study the anisotropic Ising model with competing interactions in the region of the phase transition from the modulated to the paramagnetic phase. Using histogram analysis of Monte Carlo data and the theory of finite-size scaling, it is shown that the transition from the modulated state to the paramagnetic one is a second-order phase transition. The critical parameters and temperatures of phase transitions in this region are calculated. It is shown that the modulated–paramagnetic phase transition cannot be described within the framework of the known universality classes of critical behavior.

Based on a microscopic approach, we have derived equations for the local magnetization dynamics of a spin system coupled by a dipole–dipole interaction in a uniform magnetic field in the continuum approximation. Using the generalized method of multiple scales, we have found the corrections to the Larmor precession frequency of the magnetic moments due to the interparticle interaction, which lead to a broadening of spectral lines and the formation of satellites far from the Larmor frequency. We have derived nonlinear equations for the magnetization amplitude in higher expansion orders, which admit wave and soliton solutions. We have analyzed the influence of the nonsecular part of the dipole–dipole interaction on the stability of solitons and determined the conditions for their existence.

A dynamic method is used to measure the thermodynamic functions and the electrical resistivity of fluid lead for a wide range of specific volume and pressure. The measurements errors have been reduced to a level comparable to those of stationary methods. It is found that the transition of the fluid from the metallic state to a nonmetallic state occurs when its specific volume increases 2.7 times relative to the normal value. In each individual experiment the transition occurred at almost a constant pressure, and was detected by the change in sign of the isochoric temperature coefficient of resistance determined for the entire series of the experiments. In the pressure range of 0.6–5 GPa, the specific volume at which the metal–nonmetal transition occurs is found to be independent of pressure, and therefore, the transition line is close to an isochore. For the whole range of states of fluid lead investigated here, the speed of sound and the Grüneisen coefficient have been determined.

We consider some general aspects of dependence of magneto-conductivity on a magnetic field in metals having complicated Fermi surfaces. As it is well known, a nontrivial behavior of conductivity in metals in strong magnetic fields is connected usually with appearance of non-closed quasiclassical electron trajectories on the Fermi surface in a magnetic field. The structure of the electron trajectories depends strongly on the direction of the magnetic field and usually the greatest interest is caused by open trajectories that are stable to small rotations of the direction of B. The geometry of the corresponding Stability Zones on the angular diagram in the space of directions of B represents a very important characteristic of the electron spectrum in a metal linking the parameters of the spectrum to the experimental data. Here we will consider some very general features inherent in the angular diagrams of metals with Fermi surfaces of the most arbitrary form. In particular, we will show here that any Stability Zone actually has a second boundary, restricting a larger region with a certain behavior of conductivity. Besides that, we shall discuss here general questions of complexity of the angular diagrams for the conductivity and propose a theoretical scheme for dividing the angular diagrams into “simple” and “complex” diagrams. The proposed scheme will in fact also be closely related to behavior of the Hall conductivity in a metal in strong magnetic fields. In conclusion, we will also discuss the relationship of the questions under consideration to the general features of an (abstract) angular diagram describing the behavior of quasiclassical electron trajectories at all energy levels in the conduction band.

The volume and the electrical resistivity of glassy selenium (g-Se) are precisely measured at a high hydrostatic pressure up to 9 GPa. The bulk modulus at normal pressure (B = 9.05 ± 0.15 GPa) and its baric derivative (\(B_{P}^{'}\) = 6.4 ± 0.2) fall on the general concentration dependence of the properties of Se–Ge glasses. The bulk modulus is found to behave substantially nonlinearly in the pressure range P < 3 GPa, and this behavior is not related to glass density relaxation (which is absent in this pressure range). The electrical resistivity of g-Se decreases almost exponentially with increasing pressure and reaches 20 Ω cm at 8.75 GPa. The inelastic behavior and weak volume relaxation in g-Se begin at a pressure above 3.5 GPa. Both the volume and the logarithm of electrical resistivity exhibit noticeable (logarithmic in time) relaxation at above 8 GPa. The detected volume hysteresis (1%) and significant (two orders of magnitude) electrical resistivity hysteresis are associated with the pressure-induced structural changes in the glass. They cannot be explained by partial glass crystallization, since no crystalline phase impurity was detected right after experiments. A noticeable (about 1.5%) crystalline phase impurity appears in a sample only after long-term (1 day) storage at a pressure above 8 GPa. Moreover, the crystallization kinetics of g-Se is studied under normal conditions after the action of a high pressure.

It is shown that two electrons situated in a quantum well near a metallic electrode are attracted to each other due to the Bychkov–Rashba spin–orbit interaction (SOI) and electrostatic image forces. For quite accessible values of the characteristic parameters of the system, the effective attraction due to the SOI dominates over the Coulomb repulsion, and the formation of a bielectron becomes possible. The theory of the effect is especially simple and clear in the case of a quantum wire. The binding energy of the electron pair significantly increases under the application of a gate voltage of appropriate polarity.

Two systems of magnetohydrodynamic equations in the shallow water approximation are proposed as a basis for studies in the field of plasma astrophysics: the system of equations with a full allowance for the Coriolis force and the system of equations on the β-plane in which the changes of the Coriolis parameter are linear in coordinate. Both systems of equations take into account such fundamentally important phenomena in plasma astrophysics as the compressibility and external magnetic field effects, increasing significantly the potential for applying these equations to study astrophysical objects. Compressibility in plasma astrophysics is shown to change significantly the dispersion laws for magneto-Poincare, magnetostrophic, and magneto-Rossby waves. The same nonlinear interactions as those in the absence of compressibility have been found to be realized in the case of a compressible rotating plasma. Three-wave equations in the weak nonlinearity approximation, in which the interaction coefficients depend on plasma large-scale compressibility and thermodynamic characteristics, have been derived by the method of multiscale expansions. Expressions for the growth rates of the parametric instabilities of three-wave interactions with large-scale compressibility have been derived.

A unified mechanism explaining the rotation of a suspension and the azimuthal rotation of a dust structure in the plasma of a vertically oriented gas discharge placed in a longitudinal magnetic field is proposed. Basically, it consists in the action of a magnetic field on the electric current produced by the directed motion of ions in the layer of space charge around the solid bodies placed in a plasma. The derived expressions, which define the dependences of the torque of the magnetomechanical effect acting on the plate and the angular velocity of azimuthal rotation of the dust structure on external plasma parameters, qualitatively correspond to the experimental results.