Faculty of Physics
M.V.Lomonosov Moscow State University

# Выпуски журнала

## On the effect of CH$_4$, C$_2$H$_6$, CO$_2$ Xe, and H$_2$S guest molecules on the stability of sI-hydrate

### M. B. Yunusov$^1$, R. M. Khusnutdinoff$^{1,2}$

Memoirs of the Faculty of Physics 2022. N 4.

Currently, hydrates are of considerable interest among researchers, which is due, firstly, to the use of gas hydrates as promising sources of hydrocarbon energy, and secondly, the possibility of using such molecular systems as reservoirs for hydrogen storage. The thermodynamic properties, formation kinetics, and phase diagrams of gas hydrates are well studied, as evidenced by a large number of experimental and theoretical studies. Despite this, questions remain open related to the lack of understanding of the mechanisms of structure formation and stability of molecular complexes of the «guest-host» type. In this work, within the framework of the density functional theory, some aspects of the structural stability of gas hydrates were studied. It is shown that the inclusion of a gas molecule in the hydrate cavity leads to repulsion of the cavity nodes, which indicates the structural stabilization of the hydrate. The binding energies were calculated for gases such as CH$_4$, C$_2$H$_6$, CO$_2$, Xe and H$_2$S placed in small (D-) and large (T-) cavities of clathrate hydrates. It was found that small CH$_4$, Xe, H$_2$S molecules stabilize D-cavities better, while large C$_2$H$_6$ and CO$_2$ molecules stabilize T-cavities of hydrates better. The densities of electronic states for the hydrate with/without the inclusion of a gas molecule are obtained. When a gas molecule is switched on, a shift in the energy of the electronic subsystem of the hydrate was found, which indicates a decrease in the total energy of the system and an increase in its stability.

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## Magnetooptic properties of Fe/polydiphenylenephthalide/Fe thin-film systems

### A. V. Makarov, S. E. Elena, E. A. Gan'shina

Memoirs of the Faculty of Physics 2022. N 4.

Currently, electrically conductive polymers created at the end of the 20th century attract the attention of researchers due to the variety of their mechanical and optical properties, as well as their high conductivity. The most noteworthy polymer is polydiphenylenephthalide - PDP. An analysis of the existing data has shown that by now quite a lot of information has been obtained on the physical properties of PDP samples, but there are practically no data on the effect of the polymer, which is part of composite multilayer magnetic systems, on their magneto-optical properties. In addition, of particular interest is the study of the previously unexplored spectral dependences of the equatorial Kerr effect (TKE) for three-layer Fe/PDP/Fe systems, which can be used to obtain information about the electronic and magnetic structure of the samples. It should be noted that the magnetic characteristics and features of the magnetic field behavior of Fe/PDP/Fe systems were studied and published by the authors of this work earlier in the journal Solid State Physics, volume 60, issue № 9., P. 1693. At the same time, the magnetooptical properties of these systems have practically not been studied. Taking this fact into account, the present work is devoted to the magneto-optical study of three-layer Fe/PDP/Fe low-dimensional thin-film magnetic systems. This paper presents for the first time the results of a study of the magneto-optical properties of three-layer Fe/PDP/Fe low-dimensional thin-film magnetic systems. The measurements were made on a magneto-optical magnetometer using the equatorial Kerr effect. It is found that the values of the saturation field observed for the studied thin-film systems depend on the thickness of both the iron layer and the PDP layer. This fact is explained by the interaction of ferromagnetic layers with PDP layers. It is found that the shape of the spectral dependences of the equatorial Kerr effect observed for Fe/PDP/Fe thin-film systems with an iron thickness of 30 nm is similar to the magneto-optical spectra found for ferromagnetic films. At the same time, the magnitude of the magneto-optical effect depends on the thickness of the PDP layer; in particular, it decreases with an increase in the thickness of the PDP layer. This fact is explained by the decrease in the influence of the ferromagnetic metal layer on the magneto-optical signal with an increase in the thickness of the non-ferromagnetic layer.

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## Neural network for local atomic structure analysis

### E. F. Khametova$^2$, O. R. Bakieva$^1$

Memoirs of the Faculty of Physics 2022. N 4.

In this paper, we consider the possibility of studying the local atomic structure of a substance by analyzing extended fine structures of electron energy loss spectra (EXELFS - Extended Electron Energy Loss Fine Structure) using neural network technologies to determine the structure parameters: partial coordination numbers, chemical bond lengths and thermal dispersion parameters. The experimental data of EXELFS spectra of one-component systems obtained in the transmission mode were chosen as model objects. At the first stage of the work, the interatomic distance was chosen as the parameter to be determined, since minimal processing is required to obtain this characteristic from the initial experimental data. Various learning methods and types of neural networks are considered: supervised learning, unsupervised learning and reinforcement learning. The analysis of the work of researchers in the field of spectroscopy, solving various problems using machine learning methods, is carried out. The results of the operation of an additional module capable of analyzing the spectra of one-component systems are shown.

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## Development of the phenomenological model of metamagnetic phase transition in La(Fe,Si)13 compounds

### R. A. Makarin$^1$, M. V. Zheleznyi$^{2,3}$, D. Yu. Karpenkov$^{1,2}$

Memoirs of the Faculty of Physics 2022. N 4.

The phenomenological model that predicts the behavior of a magnetic material near a first-order phase transition under the simultaneous action of several generalized forces: magnetic field, temperature, and external pressure, is presented in this article. Today, there is a variety of such models, based on the classical thermodynamic approach developed by Bean and Rodbell in 1962. One of their main disadvantages is the use of a linear approximation of the Curie temperature dependence on the crystal lattice volume the change. As consequence, this limits applicability of the previous models in a narrow range of temperatures and external pressures. According to the emerging paradigm, when creating a basically new generation of magnetic refrigerators, it is supposed to adjust the phase transition temperature of the working bodies in a wide temperature range down to cryogenic. The latter will require the application of high pressures up to 1 GPa. The prediction of material behavior under such extreme conditions required refinement of the constructed models: taking into account the phonon contribution to the Gibbs potential of the system, and processing the influence of external pressure on the behavior of the material. The constructed model made it possible to predict changes in the Curie temperature under the simultaneous applying of external pressure and magnetic field, to estimate the value of internal hysteresis within phase transition, and to determine the required value of external pressure to minimize the field hysteresis during the isothermal process of magnetization - demagnetization.

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## Effect of sublattice symmetry breaking on the phase diagram of the extended Hubbard model

### S. D. Mostovoy$^{1,2}$, O. V. Pavlovsky$^{1,2}$

Memoirs of the Faculty of Physics 2022. N 4.

The effect of sublattice symmetry breaking in the extended Hubbard model on the position of semimetal-antiferromagnet phase transition is considered. It is shown that setting different values of on-site interaction intensity on the sublattices leads to a shift of the phase transition point. The formation of spin condensate on sublattices is discussed. Changes in kinetic and potential energies of electrons are demonstrated.

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## Using the Monte Carlo method to calculate several isomers of some neurotransmitters

### I. S. Gusev

Memoirs of the Faculty of Physics 2022. N 4.

The paper presents and applies a modern, computationally efficient and economical numerical method for solving the Schrodinger equation in terms of constructing a several energy isomers of the neurotransmitter molecules glycine and dopamine in order to research their internal structure. A number of energy isomers of these molecules has been constructed, including both already known configurations and fundamentally new ones.

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## Nonlocal extension of relativistic causal thermodynamics and general relativistic Burgers equation

### A. S. Ilin, A. B. Balakin

Memoirs of the Faculty of Physics 2022. N 4.

A phenomenological approach to the construction of a relativistic model of thermo-visco-elasticity is elaborated. The main element of presented approach is the generalized Burgers equation. As a key step, we have constructed a non-local generalization of the Israel-Stewart model for relativistic causal thermodynamics of a homogeneous, isotropic cosmic fluid, in which the coefficient at the integral operator is responsible for the elastic properties of the medium. Based on the second low of thermodynamics we derive the integro-differential equation for the evolution of non-equilibrium pressure scalar and we show that the differential version of this equation is a relativistic analog of the Burgers equation describing visco-elastic processes in classical media. Based on the obtained equations for a medium model with bulk visco-elasticity, we present a hypothesis that the equations for a model with shear visco-elasticity can also be represented using the corresponding relativistic generalization of the Burgers equation; in other words, the equation for the traceless shear part of the non-equilibrium pressure tensor was obtained phenomenologically using an analogy with the exact equation for the bulk part of this tensor.

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## Research of the influence of agglomerates with a high Pu content in MOX fuel on the neutron-physical characteristics of a nuclear reactor pin

### E. P. Averchenkova, K. А. Silicheva, R. A. Vnukov$^1$, V. V. Kolesov$^1$, E. A. Ivanov$^2$

Memoirs of the Faculty of Physics 2022. N 4.

The main topik of the reserch is to find out is it possible to load fully PWR's and BN-800's cores without consideretion of disperse structure of MOX-fuel. Two types of lattice were considered for PWR and BN-800: homogeneous and heterogeneous. At first, the calculation was for pins. Each fuel pin was divided axially or radially into zones. Different number of plutonium agglomerates were simulated in each zone, the volume fraction was 7%, 10% and 22% for each model. The typical diameter of Pu-rich agglomerate was selected: 30 μm as an average value and 50 μm as a probable maximum. The focus of the study was on the following characteristics: energy distribution in plutonium agglomerates and in uranium matrix; Doppler effect; reproduction coefficient. In conclusion, the more volume fraction of Pu-rich agglomerates - the more energy release in pins, but agglomerates' diameters have no influence and also there is no dependence of axial partition.

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## Experimental study of deformation and filtration processes in low-permeability reservoir rocks of Achimov deposits when implementing the method of directional unloading of the reservoir

### V. I. Karev, Yu. F. Kovalenko, V. V. Khimulia

Memoirs of the Faculty of Physics 2022. N 4.

The article describes the studies of rheological and filtration characteristics of low-permeability rocks of the Achimov deposits of the Urengoy gas-condensate field in the course of modeling the implementation of the method of directional unloading formation. The experiments were carried out on the Triaxial Independent Load Test System (TILTS) at the Institute for Problems in Mechanics of the Russian Academy of Sciences. The results of physical modeling of actually occurring stresses in the vicinity of the wells during application of the method of directional unloading formation are presented. The statistical analysis of the influence of the creep process on the permeability of the rocks under study is carried out. The patterns of changes in the filtration characteristics of the studied rocks of the Achimov deposits during the simulation of the implementation of the method of directional unloading are established. Based on the experimental studies, conclusions were made about ways of preventing negative processes in reservoirs and ways of increasing well productivity were outlined.

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## Using a birdcage resonator for wireless power transfer at higher-order mode in MRI

### O. I. Burmistrov, P. S. Seregin

Memoirs of the Faculty of Physics 2022. N 4.

This work is devoted to a wireless power transfer in magnetic resonance imaging (MRI) at a higher-order mode of a birdcage resonator. A numerical model of the resonator with a phantom and the Siemens Avanto 1.5 T clinical MRI scanner have equal parameters. We use two orthogonally located loop antennas as a system of receiving antennas. In the course of numerical simulations of the voltage on the receiving antenna system, electric and magnetic field distribution, and the specific absorption rate (with a detailed voxel model of a human), we found that the optimal mode is at a frequency of 44.2 MHz. Ways of the practical application of the wireless power transfer system presented.

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