Vol 165, No 6 (2024)

For the electronic subsystem of transition metal ions embedded in a crystal lattice or formed a complex with ligands, an effective decrease in interelectron repulsion is observed compared to free ions, which in modern literature is referred to as the nephelauxetic effect. In this work, we study the role of the nephelauxetic effect in the Fe2+ ions electronic spectrum formation in CdTe and ZnSe matrices. Experimental assessment of the corresponding corrections was carried out based on the analysis of two transitions – the well-known ⁵T₂(⁵D) → ⁵E(⁵D), enabling  us  to  record  the  magnitude  of  the  crystal  field,  and  the  less  studied ³T₁(³H) → ⁵E(⁵D). The discovery of the zero-phonon line of this transition in CdTe:Fe enabled us to compare the two luminescent systems properties and demonstrate that for the Fe²⁺ ion in CdTe the nephelauxetic effect role increases noticeably. Based on the experimental data obtained in combination with calculations within crystal field theory, we have refined the values of the Racah parameters for Fe²⁺ ions in CdTe and ZnSe matrices. The role of the nephelauxetic effect for Fe²⁺ ions in two matrices similar in structure is important both for practical problems related to IR laser systems improvement, and for resolving some fundamental questions of quantum chemistry.

The paper presents the results of an experimental and theoretical study of electromechanical self- oscillations in systems consisting of a vacuum diode with a flexible field emission cathode, depending on its elastic properties and ability to deform. Self-oscillation regime experimentally demonstrated for field electron emitters based on carbon nanotubes and diamond microneedles. A mathematical model is developed to describe the electromechanical processes in the self-oscillating systems under consideration. Based on the analysis of the experimental data and simulation results, it is shown that the excitation of self-oscillations in a system with a flexible field emission cathode is determined by a combination of system parameters that result in a negative effective damping coefficient. The potential practical applications of self-oscillations of field emission cathodes in various micro- and nano-electromechanical systems are explored.

In this paper, using the example of the Lamy metric, the polarization properties of black holes and wormholes are investigated. Maps of linear polarization and the position of the angle of the electric vector are constructed for the toroidal and radial distribution of the magnetic field in thin disks. Using these results from future RSDB observations on the next-generation Event Horizon telescope and the Millimetron spacemission, it will be possible to determine whether the source is a black hole or a wormhole.

Within the transition state theory and the projector augmented-wave method, the mechanisms of iron diffusion in α-Ti were studied. The formation energies of interstitial and substitution defects, as well as the barriers of iron migration in α-Ti along possible paths through both interstitial and vacancy mechanisms were calculated. It was confirmed that the most preferred position for an iron interstitial atom is a crowdion, which formation energy is only 0.17 eV higher than that of iron defect on titanium site. Analytical expressions for the temperature-dependent diffusion coefficients of iron in two crystallographic directions for the interstitial mechanism were obtained by the Landman method. In general, the coefficients of iron diffusion in α-Ti and its anisotropy are consistent with experimental data, while the corresponding diffusion coefficients for the vacancy mechanism are several orders of magnitude lower. The obtained results allow us to conclude that the anomalously fast diffusion of iron in α-Ti is due to the interstitial mechanism.

Systematic studies of 2G HTS wires irradiated by high-energy 167 MeV Xe ions and fluences up to 1·10¹² cm⁻² have been carried out. The optimal fluence value (the number of particles passing through 1 cm² of the sample surface) for obtaining the maximum critical current at different temperatures and external magnetic fields has been determined. An increase in the external magnetic field leads to a shift of the critical current peak towards higher fluences in the whole temperature range. The results of microstructural investigations by transmission electron microscopy and X-ray diffraction methods are given. It is shown that because of irradiation ion tracks with a diameter of about 5-8 nm are formed, acting as effective pinning centers. X-ray diffraction analysis indicates a decrease in texture sharpness under the influence of irradiation.

The aim of the present work is to calculate the recombination time of Shockley–Read–Hall (SRH) process with the capture of charge carriers on mercury vacancy states in HgCdTe ternary alloys with a bandgap of about 40 meV. In the considered case the capture of both electron and hole is possible due to the emission of a single optical phonon. It is found that at T = 4.2 K and T = 77 K the SRH recombination determines the total lifetime of carriers in the p-type material with recombination centers density more than ~2∙10¹⁵ cm^–3, which makes it possible to control the lifetime of carriers by changing the concentration of mercury vacancies.

Amplifiers with dielectric and plasma-dielectric filling, based on excitation of surface electromagnetic waves at dielectric boundary by relativistic electron beam, are considered. The focus is on the subterahertz operating frequency range. In linear approximation dispersion equation is obtained and areas of amplified frequencies and field structure are determined. Two limiting modes of Cherenkov beam instability depending on electron beam density are identified. The role of plasma near the dielectric boundary was evaluated. Efficiency of conversion of energy of directed motion of electron beam into energy of electromagnetic waves is determined on the basis of solution of shortened system of nonlinear equations. Schemes of radiation output from amplifier working area are proposed.

The thermodynamic functions and electrical resistivity of dense lead plasma were assessed at specific volumes ranging from 5 to 20 times greater than the standard value, under pressures between and 4.0 hPa, and with specific internal energies 3 to 18 times higher than the energy required for The recorded dependencies were later evaluated against those estimated through a classical plasma chemical model. This research aimed to uncover the effects of non-ideality on the thermodynamic characteristics and resistivity behavior of plasma. A significant finding was that the Grüneisen coefficient for this plasma varied between 0.2 and 0.4 across the entire range of states examined. Findings from the research showed that the chemical model did not accurately reflect the energy expenditure for plasma ionization and atom excitation, underestimating it by close to a factor of two, while also overestimating the temperature by a similar proportion. The inquiry additionally disclosed that in the whole spectrum of plasma states being analyzed, “pressure ionization” was a key element, and that resistivity lessened with a decrease in specific volume along isotherms.