Vol 165, No 5 (2024)

A theory of Ramsey resonance excitation has been developed, taking into account the complete magnetic structure of levels D₁-line of ⁸⁷Rb atoms, as well as the finite temperature of the ensemble. The dependences of the shape and shifts of resonances on parameters such as external magnetic field magnitude, degree of laser field ellipticity, and medium temperature have been analyzed. The possibility of interference between different channels of Ramsey resonance excitation, observed when varying the magnetic field magnitude, is shown. The existence of optimal field ellipticity at certain polarization, leading to the highest resonance amplitude, has also been discovered.

This paper focuses on describing energy transfer by coherent thermal excitations in dielectrics, metamaterials, and nanoscale systems. Using the second quantization technique, a general formalism of thermal conductivity is proposed, considering both the model of free phonons in heat transfer and the formation of coherent Schrödinger states of the oscillator system. A general form of the time-dependent problem solution with arbitrary initial conditions is obtained. An exact solution is analytically derived for the heat flux carried by coherent phonons created by an electronic wave packet produced by a laser pulse effecting a nanomaterial. The obtained exact form of solution in quadratures provides a basis for quantitative description of coherent phonons with various initial conditions, as well as taking into account thermal distributions, which allows for evaluation of thermal properties of nanocrystals. It is shown that under certain ratios of constants characterizing the interaction of phonons with the electronic subsystem, a time-independent heat flux can be established in the crystal.

High pressure affects melt solidification and its glass-forming ability. Ab initio molecular dynamics calculations show how the local structure of the melt changes with increasing pressure. High pressure promotes the formation of icosahedral clusters in the melt. Rare earth elements: gadolinium, terbium facilitate the formation of icosahedra. At a pressure of 10 GPa and melt temperature of 1800 K, icosahedra atoms form a "percolation cluster". As pressure decreases, the concentration of icosahedra decreases, and at atmospheric pressure, icosahedra are practically absent. Thus, the glass-forming ability of the melt increases with increasing pressure. Using deep machine learning techniques, the dependence of glass transition temperature on high pressure was evaluated: pressure increase from 0 to 10 GPa increases by 1.3 times. The structure of solid alloy samples obtained by cooling its melt from 1800 K at a rate of 1000 degrees/s under 10 GPa pressure was studied. X-ray diffraction and electron microscopy methods showed that the samples are dense and homogeneous, with a fine-dispersed structure. New crystalline phases with cubic (cP 4 / 2) and tetragonal (tI 26 / 1) structures, stable for long periods under normal conditions, were synthesized in the alloy. Rare earth elements play a major role in the formation of the phase with cubic structur (cP 4 / 2). Studies showed that the average hardness of samples obtained at 10 GPa is almost 2 times higher than that of the initial sample obtained at atmospheric pressure, and is about 2 GPa.

The manifestations of the flexomagnetoelectric effect in thin ferromagnetic films with uniaxial easy-plane anisotropy and artificially created perforations in the presence of an external electric field normal to the film plane are investigated. It is shown that the influence of inhomogeneous magnetoelectric interaction in this case leads to the transformation of magnetic structures, which is necessarily accompanied by the deviation of the magnetization vector from the sample plane. For cases where the deviation angles are small, explicit expressions describing the magnetization distribution are obtained. It is proven that the impact of an electric field of certain strength can lead to changes in the topology of the ground state of the system. A simplified model is considered, explaining the features of changes in structures of this type, as well as allowing to establish conditions for their implementation.

Strips with a length of 20 mm and width from 0.1 to 2 mm were fabricated by laser lithography from permalloy (Fe₂₀Ni₈₀) films with thicknesses of 50, 100, and 200 nm, obtained by magnetron sputtering on quartz substrates. In the first series of samples, the uniaxial magnetic anisotropy induced by the presence of a constant magnetic field in the film plane during deposition was oriented along the long axes of the strips, and in the second series perpendicular to them. The anisotropic properties of the samples were determined from the angular dependencies of ferromagnetic resonance fields measured on a scanning spectrometer. It was found that in the first series of samples, with decreasing strip width, the anisotropy monotonically increases several times while barely changing its direction. In the samples of the second series, it first decreases almost to zero at a certain strip width, and then rapidly grows while simultaneously rotating by ~ 90◦. The phenomenological calculation of uniaxial anisotropy in uniformly magnetized film strips shows good agreement with the experiment.

Using a simple molecular model of passive, active non-chiral and chiral nematics, molecular dynamics simulations were performed to study the behavior of their binary mixtures in a two- dimensional bounded circular domain. Equilibrium structures in these systems were studied under normal and tangential anchoring of particles at the boundaries. It is shown that in mixtures consisting of passive and active model particles, as well as in mixtures of active particles with different chirality, at sufficiently large self-propelling forces, the bounded domain splits into clusters predominantly consisting of particles of the same type. To characterize the degree of separation of mixtures into these clusters, a segregation parameter is introduced. The values of this parameter are calculated for different magnitudes of selfpropelling forces and chirality of model particles.

The processes of plasma formation from helium bubbles-containing tungsten nanofibers when exposed to energy and particle flux from helium plasma under conditions of near-wall potential increased to hundreds of volts, when spontaneous initiation of explosive electron emission bursts is observed, have been considered. It is shown that the development of initiation models under external influence of energy and particle flux requires consideration of nanofibers heterophase structure. Using molecular dynamics method, atomistic modeling of interaction between an incident high-energy helium atom (100-500 eV) with an ensemble of helium atoms in a nanoscale bubble with solid-state of nanofibers heterophase structure 1029 m–3, retained in the near-surface tungsten layer, was performed. The energy relaxation time in the heterophase system of a nanobubble in tungsten was obtained, amounting to several picoseconds. It is shown that at incident particle energies of hundreds of electronvolts, overheating of near-surface nanobubbles is possible, leading to their explosion within times of about 10 ps. Such energy is comparable to the total energy of nanobubble particles, and at such near-wall potential, spontaneous initiations of explosive electron emission bursts are observed.