Moscow University Physics Bulletin最新文献

The structural and electronic properties of perfect and defective MoS({}_{2}) have been calculated with first-principles density functional theory. The defect stability has been evaluated using the defect formation energy. The calculated results show that the formation of Mo, S, and Mo–S defects requires extra energy. Moreover, the S vacancy is energetically more favourable than the Mo vacancy. After doping, the defect level is introduced into the forbidden band. The defect level in the Mo defect model tends to be the acceptor level, while the defect level in the S defect model and Mo–S defect model tends to be the donor level. These three different defect models still maintain the direct band gap characteristics as perfect models. Then, the type of conduction after doping is also analyzed. The projected state density and the charge density of these four models have been analyzed, showing the positions of defect energy levels and change of chemical bonds.

MRI is widely used in radiation therapy planning, particularly in stereotactic radiosurgery for brain metastases. The article addresses the geometric distortion of MRI images, which can lead to errors in radiation therapy planning. A series of experiments with a simple phantom were conducted on two MRI scanners with 0.5 and 1.5 T magnetic fields. Deviations in the positions of the phantom objects from their actual locations were observed. The detected deviations can reach up to 5 mm. A theoretical dependence of the magnetic field gradient on the distance to the center of homogeneity was calculated, which agrees well with the approximation of experimental data. An assessment was made of the dependence of the image area distortion of objects located at the center of the field homogeneity on their actual sizes. It was found that the area deviation can reach up to 20(%).

Using the conservation law for the angular momentum of electromagnetic field in the form of a balance equation, which links the angular momentum density, the angular momentum flux density, and the torque density caused by the anisotropy of the medium in a nonabsorbing medium, formulas for the densities of the orbital and spin parts of the angular momentum and the flux densities of these quantities are derived in the case of the interaction of monochromatic waves in a nonabsorbing medium with spatial dispersion that demonstrates an (n)th order nonlinear optical response to an external light field. In media without spatial and frequency dispersion, the obtained expressions coincide with the canonical expressions for the densities of the orbital and spin parts of the angular momentum, as well as their flux densities. Related to the nonlinearity of the medium, additional terms to the greatest components of the spin parts of the angular momentum and its flux densities can reach ten percent of the corresponding linear parts during the self-focusing of an elliptically polarized Gaussian laser beam in an isotropic gyrotropic medium near the area of its collapse.

In this work, a method for evaluating the cross sections of electroproduction of (K^{+}Lambda^{0}) and (K^{+}Sigma^{0}) off protons in the region of invariant masses of final hadrons (M_{K}+M_{Y}<W<2.65) GeV ((M_{K}) and (M_{Y}) being the masses of the kaon and hyperon, respectively) and squares of four-momentum transfers of virtual photons, i.e., photon virtualities (0<Q^{2}<5) GeV({}^{2}), is developed based on experimental data of these exclusive channels’ cross sections measured by the CLAS detector in Hall B at Jefferson Lab. A set of algorithms has been implemented to evaluate the differential cross sections of these channels, along with their statistical and systematic uncertainties. A program was developed for the evaluation of differential cross sections and structure functions using C++ and Python libraries. An interactive website was created for working with the program, enabling the analysis of one-dimensional and two-dimensional dependences of structure functions and differential cross sections. The evaluation of differential cross sections for the (K^{+}Lambda) and (K^{+}Sigma^{0}) electroproduction channels is necessary for extracting the structure function (sigma_{LT^{prime}}) from the data on the polarization asymmetry of electroproduction reactions of these final states with longitudinally polarized electrons. The obtained results are also important for the development of realistic Monte Carlo event generators in planning future experiments and for evaluating the efficiency of detecting final particles when extracting reaction cross sections from experimental data.

A universal method is proposed for performing calculations within the framework of generalized statistics generated by the parametric Tsallis, Rényi, and Sharma–Mittal entropies. The essence of the approach lies in the use of an auxiliary gamma distribution whose parameters correspond to a particular variant of the statistics. Equations are derived that allow the generalised partition function and the mean energy to be expressed in terms of canonical quantities. The effectiveness of the proposed method is demonstrated using the example of Rényi statistics. The Maxwell–Rényi distribution is obtained and its properties are calculated, based on which assumptions about the possible nature of the generalised parameter are formulated.

Results of experiments in the field of biophysics are often presented as time series obtained with low resolution and not always of great length. In particular, in studies of the effects of various physico-chemical factors on bilayer lipid membranes, transmembrane ion currents and their fluctuations are usually measured. In this case, the mean values and variances of the currents may not differ significantly, making it difficult to determine the nature and degree of impact based on them. Therefore, the development of approaches to time series analysis has never ceased. Attempts to use the entropy of random variable distributions in such analysis have been made for a long time, but in practical work, these approaches have been difficult to implement, especially due to the requirements for the length of the series and the absence of noise. In recent decades, there have been significant changes in this area, and many new methods of time series analysis using various modifications of entropy have been proposed. In this regard, there is a need for a summary of methods based on entropy calculation, indicating their advantages and disadvantages. This is the goal of the proposed brief review of entropy-based methods for analyzing scalar time series, which can be useful in analyzing experimental data. The review considers only some of the basic approaches on which further algorithmic improvements are based. The concept of entropy sometimes causes difficulties for students, so the review can also be useful for educational purposes.

The development of proton therapy techniques imposes strict requirements for the accuracy of proton beam delivery. Existing methods for determining the planning target volume allow one to take into account only the influence of patient positioning errors before the start of irradiation. In this paper, the results of work on taking into account proton therapy system beam delivery errors for determining the planning target volume are presented. The technique for measuring the proton beam parameters and analyzing the measurement results is described. The calculation of the minimum required margin from the clinical target volume is carried out to guarantee its irradiation, taking into account the errors in beam delivery by the proton therapy system.

Solutions in acetonitrile of three mixed complexes of rare-earth elements (terbium and samarium) with organic ligands with various pyridine substituents were studied in this work. The ratios of ligands and metals in the resulting complexes were determined, and the stability constants of samarium complexes were calculated using the spectrophotometric titration method. Measurements of absorption-, emission-, and excitation spectra of luminescence, luminescence kinetics of solutions of mixed complexes of rare-earth elements with an excess of metal relative to the ligand were carried out at various temperatures in the range 298–328 K. For the first time, an increase of luminescence intensity of a samarium ion in complex upon heating was observed. The dependences of the luminescence quantum yield and lifetime of mixed complexes on temperature were obtained. A thermometric parameter—the ratio of the integral luminescence intensities of samarium and terbium ions—was proposed, and the temperature sensitivity coefficient of this parameter was determined for different complexes.

The vacuum polarization effect of a massive scalar field (phi(x)) near the point (delta)-like source is considered. The corresponding interaction is introduced within a technique of self-adjoint extension of the Laplace operator. This method has been widely discussed within the framework of quantum mechanics. We propose to use it to investigate vacuum field effects. This approach allows computing the renormalized Hadamard function and the renormalized vacuum energy density (langle T_{00}(x)rangle_{textrm{ren}}) for massive real scalar field with minimal curvature coupling. The dependence of the vacuum polarization effect upon the fields’s mass is analyzed.

The number of unstable combinations of elastic and Stokes optical modes of the LIGO Voyager gravitational wave detector, frequencies and spatial distributions of displacement vectors of elastic modes of the mirrors were calculated. The values of overlap factors for unstable modes up to ninth-order optical modes were calculated, taking into account the azimuthal condition of parametric instability. An analysis of the influence of the temperature dependence of the Young’s modulus of the mirror material on the number of unstable modes in the Fabry–Perot cavity was performed.