Vol 127, No 4 (2018)

The phenomenon of superradiance of electron bunches provides a promising method for generating high-power ultrashort electromagnetic pulses. In the present paper, it is shown that the equations describing the interaction between an electron beam and an electromagnetic wave in a BWO-type oscillator admit a self-similar solution that describes an amplifying and compressing pulse. It is demonstrated by numerical simulation that the generation of short superradiance pulses is of self-similar character at the initial stage of the transient process. The specific features of the main characteristics of such pulses (the amplitude and width of a pulse and the coordinates of its maximum) are investigated as a function of time and control parameters. It is shown that, after a transient process, the solution reaches the self-similar stage, where the pulse amplitude and the inverse of its squared width grow linearly with time.

The frequency dependence of the effective complex permittivity and effective complex permeability of a heterostructure based on a dielectric medium containing metallic nanoparticles of spherical shape is calculated by an original method. In contrast to the Bruggeman [21] and the Maxwell Garnett [17] approaches, which use the quasi-static approximation in calculations, a nonuniform distribution of electromagnetic fields inside metallic particles is calculated, which allows the analysis of the electromagnetic parameters of the heterostructure not only as a function of frequency but also as a function of the nanoparticle size. It is shown that the plasmon resonant frequency decreases with increasing both the size and the concentration of particles in the heterostructure. It is also shown that a dielectric medium containing nonmagnetic metallic nanoparticles exhibits diamagnetic properties. In this case, the position of the maximum on the frequency dependence of the imaginary part of the magnetic susceptibility coincides with the relaxation frequency of charge carriers. The calculated spectra of the real and imaginary components of the permittivity of the heterostructure with a size of metallic particles less than 10 nm are in good agreement with Bruggeman calculations; however, the agreement with Maxwell Garnett calculations is observed only at nanoparticle concentrations lower than 10^–6.

Quantum mechanics admits collective measurements that are related to the projection onto entangled states and allow one to retrieve more classical information from an ensemble of quantum states compared with individual measurements. In this relation, a fundamental question arises for key secrecy in quantum cryptography. Should the secrecy criterion be formulated with regard to all keys distributed both on previous and future quantum key distribution (QKD) sessions, or it suffices to guarantee key secrecy only in an individual QKD session? The study of this question is the subject of the present paper.

Schwinger’s idea about the magnetic world of the early Universe, in which magnetic charges (monopoles) and magnetic atoms (g⁺g^–) could be formed, is developed. In the present-day Universe magnetic charges with energies in the GeV range can be formed in the magnetospheres of young pulsars in superstrong magnetic fields. Spectroscopic features of magnetic atoms and possibilities for their observations are discussed. Relic magnetic atoms can contribute up to 18% to the dark matter density. The gamma-ray excess at our Galactic center could arise under two-photon annihilation of magnetic charges as a cooperative effect from neutron stars. A sharp physical difference of Schwinger’s magnetic world from Dirac’s present-day electric world is pointed out. Artificial magnetic monopoles are also mentioned briefly.

Results of investigations of the angular and energy distributions of prompt neutrons from ²³⁹Pu fission observed on WWR-M type research reactor (Gatchina) are presented. Peculiarities observed in both angular and energy distributions of prompt neutrons can be explained by assuming the existence of neutrons emitted near the instant of fissioning nucleus rupture. Anisotropy of the angular distribution of prompt neutrons in the frame of the center of mass of fission fragments (ψ(0°)/ψ(90°) ≈ 1.07–1.09) should also be taken into consideration.

The existence of degenerate stationary bound states with square-integrable radial wavefunctions was proven when second-order equations are used with the effective potential of the Reissner–Nordström (RN) field with two event horizons for charged and uncharged fermions. The fermions in such states are localized near event horizons within the ranges from zero to several fractions of Compton wavelength of fermions versus the values of gravitational and electromagnetic coupling constants and the values of angular and orbital momenta j, l. In case of extreme RN fields, the absence of stationary bound states of fermions with the energies of E < mc² is shown for solutions of the second-order equation for any value of gravitational and electromagnetic coupling constants. The existence of a discrete energy spectrum is shown for the naked RN singularity, due to the solution of the second-order equation at definite values of physical parameters. The discrete spectrum exists for both charged and uncharged fermions. The naked RN singularity in quantum mechanics with the second-order equation for half-spin particles poses no threat to cosmic censorship, since it is covered with an infinitely large potential barrier. Electrically neutral systems of atomic type (RN collapsars with the definite number of fermions in degenerate bound states) are proposed to consider as particles of dark matter.

The temperature dependences of the specific heat and the transport characteristics of thermal-frequency phonons in single crystals of the solid solutions of rare-earth aluminum garnets are studied at liquid-helium temperatures in the presence of Schottky-type low-energy excitations. The kinetic characteristics of phonons as functions of the solid solution composition are measured. The relation between the kinetic and thermophysical characteristics of the material in the solid solutions of rare-earth aluminum garnets is analyzed under conditions of a nonstationary process and the spatial inhomogeneity caused by the coordinate dependence of the state of low-energy excitations. The thermalization conditions in the nonequilibrium phonon–low-energy excitation system are estimated.

We present the derivation of a microscopic superexchange Hamiltonian for undoped magnetic insulators with an arbitrary spin. It is established that the sign of the (ferromagnetic or antiferromagnetic) superexchange between magnetic ions in the dⁿ configuration depends on the spin nature of virtual multielectron states d^{n± 1}, viz., low-spin or high-spin partners with S ± 1/2 relative to ground state of the dⁿ configuration with spin S. A macroscopic substantiation is given for the Goodenough–Kanamori rules and simple mean-field estimates connecting the magnetic ordering temperature with the exchange constant. The conventional Anderson superexchange for magnetic materials with spin S = 1/2 and the P/T magnetic phase diagram for ferroborate FeBO₃ with spin crossover S = (5/2 ↔ 1/2) at the Fe³⁺ ion under a high pressure are also reproduced as a test.

The nonequilibrium behavior of the magnetic superstructures consisting of alternating magnetic and nonmagnetic nanolayers is numerically simulated by Monte Carlo methods. An analysis of the calculated two-time dependence of an autocorrelation function during the evolution from a high-temperature initial state has revealed aging effects, which are characterized by slowing down of the correlation effects in the system when the waiting time increases. In contrast to bulk magnetic systems, the aging effects are shown to appear in magnetic superstructures both near the critical ferromagnetic ordering temperature T_c in films and in a low-temperature phase at T ≤ T_c. The aging effects in the correlation processes in a magnetic multilayer structure weaken when ferromagnetic film thickness N increases at T = T_c(N), and these effects increase with film thickness N at temperatures T = T_c(N)/2. When simulating the transport properties of the Co/Cu(001)/Co structure, we calculated the temperature dependence of equilibrium magnetoresistance and were the first to reveal the influence of nonequilibrium behavior of the structure on the magnetoresistance and the manifestation of the aging effects in it.

Within the generalized DMFT+Σ approach, we study disorder effects in the temperature dependence of paramagnetic critical magnetic field H_cp(T) for Hubbard model with attractive interaction. We consider the wide range of attraction potentials U—from the weak coupling limit, when superconductivity is described by BCS model, up to the limit of very strong coupling, when superconducting transition is related to Bose–Einstein condensation (BEC) of compact Cooper pairs. The growth of the coupling strength leads to the rapid growth of H_cp(T) at all temperatures. However, at low temperatures, paramagnetic critical magnetic field H_cp grows with U much more slowly, than the orbital critical field, and in BCS limit, the main contribution to the upper critical magnetic filed is of paramagnetic origin. The growth of the coupling strength also leads to the disappearance of the low temperature region of instability towards type I phase transition and Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) phase, characteristic of BCS weak coupling limit. Disordering leads to the rapid drop of H_cp(T) in BCS weak coupling limit, while in BCS–BEC crossover region and BEC limit H_cp(T → 0) dependence on disorder is rather weak. Within DMFT+Σ approach, disorder influence on H_cp(T) is of universal nature at any coupling strength and is related only to disorder widening of the conduction band. In particular, this leads to the drop of the effective coupling strength with disorder, so that disordering restores the region of type I transition in the intermediate coupling region.

Wave properties of damped solitons in a collisional unmagnetized four-components dusty fluid plasma system contains superthermal distributed electrons, mobile ions, and negative-positive dusty grains have been examined. To study dissipative DIA mode properties, a reductive perturbation (RP) analysis is used under convenient geometrical coordinate transformation, three-dimensional damped Kadomtsev–Petviashvili (3D-CDKP) equation in cylindrical coordinates is obtained. Effects of collisional parameters on damped soliton pulse structures are studied. More specifically, the impact of axial, radial, and polar coordinates with the time on solitary propagation are examined. This investigation may be viable in plasma of the Earth’s mesosphere.

The Riemann compression wave describes a 2D flow behind an infinite wave front, but is used for describing its propagation in channels. The effect of walls on the process of its propagation is disregarded in this case. However, friction against the walls and diffraction divergence of elementary plane waves compensating the friction change the conditions of wave propagation. These effects, which are inevitable in a channel, violate the constancy of entropy and the jet nature of the flow, which contradicts the definition of a “simple wave.” Analysis of diffraction divergence makes it possible to solve the problem of formation of a finite-aperture simple wave (a wave beam with a large Rayleigh length). The evolution of friction processes explains the emergence of a turbulent flow and makes it possible to describe the mechanism of a spontaneous transition of deflagration into detonation more accurately.

A new method is developed to measure the phase-transition temperatures in artificial phospholipid membranes. This method is based on studying the temperature dependence of dark conductivity and photoconductivity in a symmetric cell with current-conducting indium–tin oxide (ITO) electrodes. Internal electron photoemission into a thin liposome layer is induced by visible and near IR light from the ITO electrodes. This method is applied to study the lyotropic phases in 1,2-dipalmitoyl-rac-glycero-3-phosphocholine (DPPC) and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) with ethylene glycol (EG) and glycerol (G). The results of time-of-flight measurements are used to calculate the carrier mobilities in liposome vesicles. The measurement results are compared with the results obtained by dc conductometry. We are the first to detect the effect of a positive temperature coefficient of resistivity in a liquid-crystal phase. The proposed method makes it possible to detect the phase transitions in lyotropic liquid-crystal systems and, hence, can be used to create biocompatible drug carriers based on thermosensitive liposomes.