Moscow University Physics Bulletin最新文献

There is a version of the Landau–Lifshitz equation that takes into account the Coulomb exchange interactions between atoms, expressed by the term ({sim}mathbf{s}timestrianglemathbf{s}). On the other hand, ions in magnetic materials have several valence electrons on the (d)-shell, and, therefore, the Hamiltonian of many-electron atoms with spins (S>1) should include a biquadratic exchange interaction. We propose a novel fundamental microscopic derivation of the spin density evolution equation with an explicit form of biquadratic exchange interaction using the method of many-particle quantum hydrodynamics. The equation for the spin density evolution is obtained from the many-particle Schrödinger–Pauli equation and contains the contributions of the usual Coulomb exchange interaction and the biquadratic exchange. Furthermore, the derived biquadratic exchange torque in the spin density evolution equation is proportional to the nematic tensor for the medium of atoms with spin (S=1). Our method may be very attractive for further studies of the magnetoelectric effect in multiferroics.

Based on the band structure of TlGaTe({}_{2}) compound, the extrema of the valence and conduction bands were determined and the tensor of the components of the inverse effective mass of electrons and holes was calculated.

The paper presents the results of the analysis of the sloar flare SOL2012-04-27 emission in hydrogen, helium, and calcium spectral lines observed on the horizontal solar facility HSFA-2 of the Ondřejov Observatory (Astronomical Institute of the Czech Academy of Sciences). After processing the spectra, the integrated emission fluxes in the lines were determined. The data collected are treated using the approach of rapid energy release inside the chromosphere, accompanied by gas evaporation. Within the framework of the heated gas model, a theoretical calculation of the plasma parameters was performed, taking into account the physical conditions in the chromosphere, including spectral line self-absorption. Comparison across six lines allowed, with a high degree of validity, the reconstruction of the temperature (T), density, and spatial structure of the emitting gas. The best agreement of theory with observations is obtained in the model of the spread of inhomogeneous clouds, each of which has hot ((Tapprox 18,000{-}19,500) K) and less heated ((Tapprox 8000{-}9000) K) regions. On average, one cloud contributes about ten percent to the total emission flux.

The paper, like the seminar where it was presented, is dedicated to the memory of the outstanding physicist and nuclear scientist, Professor of Moscow State University B.S. Ishkhanov, more precisely, to his contribution to the research and development of LEXG—laser–electron X-ray generators. The relevance and prospects of this direction are associated with the ‘‘gap’’ between two existing types of X-ray generators: on the one hand, X-ray tubes, and on the other, electron accelerators, storage rings, and free electron lasers. This involves a vast difference in beam characteristics, cost, scale, energy consumption, etc. The history of LEXG, modern projects abroad, as well as the Lomonosov Moscow State University–Jilin University project, carried out under the supervision of B.S. Ishkhanov, are briefly presented. From the point of view of their applications, LEXG designs can be divided into two types and, accordingly, devices of different scales. In the first case, it is the application area of common X-ray tubes, and in the second, it is radiation medicine, nuclear technologies, and nuclear physics research. International conferences on LEXG have been held since 2008. Their list is provided in the Appendix to the article.

In the study, the characteristics of the phase transition from a face-centered cubic crystal to a homogeneous phase in helium were calculated based on the correlation cell–cluster extension. A comparison of theoretical data with experimental data was conducted, and good agreement was obtained. The use of data on phase transitions in helium for estimating the thermodynamic characteristics of the phase transition in a quantum system of hard spheres is discussed.

The degree of DNA compaction in various regions of the cell nucleus determines whether the corresponding genes should be expressed and other crucial cellular processes, such as DNA replication and repair, should occur. Therefore, the development of approaches to the experimental assessment of DNA compactness in cell nuclei, as well as its indicator, dynamic disorder which determines the degree of thermal fluctuation in the position and mutual orientation of molecular fragments, is highly relevant. In this paper, using single-stranded guanine and cytosine oligonucleotides, as well as their double-stranded combination as an example, it is shown that dynamic disorder in DNA can be assessed based on low-frequency Raman scattering (RS) spectroscopy data. For the first time, Raman spectra of oligonucleotides have been measured over a wide frequency range, including the low-frequency (10–200 cm({}^{-1})) and high-frequency (200–2000 cm({}^{-1})) regions. It was found that the low-frequency Raman intensity is maximal in single-stranded oligocytosine and minimal in the double-stranded oligonucleotide, which is in full agreement with the magnitude of dynamic disorder estimated from molecular dynamics simulations. The obtained results indicate the promising application of low-frequency Raman spectroscopy for assessing dynamic disorder and DNA compactness. The use of such a technique is expected to contribute to the understanding of key cellular processes and the physical mechanisms that underlie them, which is necessary for the development of advanced methods in molecular biophysics and cell biology.

The features of methods for obtaining information on photonuclear reaction cross sections using beams of bremsstrahlung and quasimonoenergetic photons formed during the in-flight annihilation of relativistic positrons are discussed. These methods have provided the vast majority of data in the nuclear excitation energy up to ({sim}40) MeV. It is shown that significant disagreements between the results of such experiments, both in absolute value and shape, are due to certain shortcomings of both methods. These shortcomings can be to a certain extent eliminated by using beams of photons formed in the processes of backward Compton scattering.

In this paper, the magnetic structure of the storage ring for an X-ray source based on the effect of Compton backscattering and designed for use in the electron beam energy range from 35 to 120 MeV is investigated. For this storage ring, the results of calculating the dynamics of the electron beam are presented, and the effects of radiation damping, quantum excitation, laser damping, and intrabeam scattering are considered. The dynamic aperture and energy acceptance of this ring are discussed. The results of calculating the magnitude of the spectral brightness of X-ray radiation and its change over time are provided.

The results of the study on the impact of continuous visible laser irradiation on the field emission properties of point cathodes which are diamond needle-like microsized crystallites with a nanometer tip radius, are presented. The measurements were conducted in a vacuum diode configuration with a flat metal anode using DC voltage source. An increase in the field emission current was observed when the diamond needle was illuminated by laser radiation. The magnitude of the current changes directly proportional to the power of the radiation. At a maximum power density value of about 400 W/cm({}^{2}), the relative increase in current under the action of laser irradiation was 13(%). The relative increase in current is determined by the parameters of the dark current–voltage characteristic and reaches its maximum value in the region corresponding to the minimal increase in dark current with voltage. It is shown that the most probable mechanism for the current increase is the change in the electrical resistance of the diamond microneedle as a result of the absorption of laser radiation with the involvement of electronic levels located in the bandgap of diamond, associated with impurities or structural defects in the near surface layer of the diamond microneedle.

To create a compact beam scanning system along the conveyor, this paper considers the horizontal placement of the accelerator with a magnetic system for rotating the beam by 90({}^{circ}). Three-dimensional modelling of the magnetic system and electron dynamics within it was carried out. Dose distributions in the near-surface area of the irradiated object were also obtained, based on which the optimal laws of changing the current and voltage of the scanning magnet were determined, ensuring the required conditions for product irradiation.