Vol 193, No 7 (2023)
Reviews of topical problems
Features of spin crossovers in magnetic materials
Abstract
We present experimental and theoretical results of spin crossover studies in magnetically ordered materials. The effect of spin crossovers on the electronic structure of transition metal oxides and on the Bose condensation of spin excitons in the vicinity of the spin crossover is considered. A new method for calculating the interatomic superexchange interaction in transition metal oxides is discussed that allows considering selective contributions of excited magnetic cation terms. Changes in the exchange interaction sign are predicted for spin crossovers for $ d^5-d^7$ ions. In the $R \rm {CoO}_3$ family of rare-earth cobaltites, the ground state is nonmagnetic, but, as the temperature increases, thermal excitations of high-spin states give rise to a number of experimentally detectable features. In defective $R\rm {CoO}_3$ samples, stabilization of the high-spin term and ferromagnetic ordering are possible. Dynamical crossovers under external pumping and the dynamics of multiplicity, magnetization, and local lattice distortions are discussed. Geophysical implications of spin crossovers are considered, and metallic properties of Earth's mantle at a depth of 1400–1800 km are predicted.
Uspehi fizičeskih nauk. 2023;193(7):689-716
689-716
A novel view of the nature of formation of metallic glasses, their structural relaxation, and crystallization
Abstract
An alternative concept of the formation of the defect structure and properties of metallic glasses—the interstitialcy theory (IT), which has been actively developed recently—is systematically laid out. The premises and basic hypotheses of the IT are presented, and the experimental data pertaining to the assessment of its adequacy are considered. The multifaceted relation between the relaxation of the shear elasticity and heat phenomena upon various types of thermal processing of metallic glasses is analyzed in detail. A simple mathematical IT formalism is shown to provide a good description of experimental data. The most important result of the IT is an adequate description of the excess thermodynamic potentials of metallic glasses. Problems surrounding the IT and approaches to its further development are considered.
Uspehi fizičeskih nauk. 2023;193(7):717-736
717-736
Subsecond pulses in microwave emission from the Sun
Abstract
The current state of research on microwave subsecond pulses (SSPs) based on observations of radio-frequency radiation from solar flares with high temporal, spatial, and spectral resolutions is reviewed. It is shown that knowledge about the localization of SSP sources in the flare region, available with the help of large radio interferometers, radically affects the understanding of the nature of SSPs. Based on the unique observational material accumulated in radio observations over the past 30 years, the characteristics of SSPs have been studied, they have been classified according to their spectral characteristics, and the mechanisms of their generation have been determined.
Uspehi fizičeskih nauk. 2023;193(7):737-750
737-750
Instruments and methods of investigation
Explosive emission processes in thermonuclear facilities with magnetic plasma confinement and in linear electron–positron colliders
Abstract
A model of the phenomenon of explosive electron emission based on its similarity to the electrical explosion of conductors is presented. With this model, the microexplosive processes occurring on a cathode surface due to the action of the explosive emission current have been simulated. The simulation results have been used to analyze explosive emission processes caused by the operation of unipolar arcs in thermonuclear reactors with magnetic plasma confinement and by the initiation of radiofrequency vacuum breakdowns in the accelerating structures of linear electron–positron colliders. The structure of the arc discharge cathode spot and the erosion characteristics have been investigated for nanostructured tungsten (W-fuzz) surfaces formed in thermonuclear reactors with magnetic plasma confinement. For radiofrequency vacuum breakdowns, the initiating parameters have been estimated, and prebreakdown and microexplosive processes have been simulated.
Uspehi fizičeskih nauk. 2023;193(7):751-769
751-769
Electromagnetic levitation method as a containerless experimental technique
Abstract
Electromagnetic levitation is a method for containerless high-temperature treatment of metal, semiconductor, and alloy samples. This method is widely used to investigate the thermophysical and thermochemical properties of liquid melts, as well as their crystallization kinetics. An alternating electromagnetic field induces an induction current inside a sample, resulting in a Lorentz force opposing the gravitational force. The Lorentz force lifts the sample, which is heated and melts in a levitation chamber due to the current flowing through it. In this paper, we present an analytical model of the sample levitation process, considering the structure of the electromagnetic levitator coil and options for its optimization for experiments. The kinetics of high-speed solidification of undercooled droplets in the chamber of the electromagnetic levitator is analyzed.
Uspehi fizičeskih nauk. 2023;193(7):770-782
770-782
Methodological notes
783-790
On the problem of boundary conditions for mixed type equations arising in the description of astrophysical transonic flows
Abstract
Using the example of an exactly solvable problem, it is shown that the number of boundary conditions which is necessary to determine the transonic hydrodynamic flow near the so-called ‘nonstandard’ singular point does not depend on whether the separatrix characteristic passes or does not pass through this point. Thus, a quite popular statement is called into question, according to which the critical surface, on which the regularity condition determines the flow structure, generally coincides with the surface of separatrix characteristics, and not with the sonic surface.
Uspehi fizičeskih nauk. 2023;193(7):791-797
791-797
Physics news on the Internet: July 2023
Uspehi fizičeskih nauk. 2023;193(7):798-798
798-798
Personalia
Mikhail Yakovlevich Marov (on his 90th birthday)
Uspehi fizičeskih nauk. 2023;193(7):799-800
799-800