Moscow University Physics Bulletin最新文献

Analytical expressions have been obtained describing the spatio-temporal dynamics of terahertz pulses in a medium with refractive index dispersion, representing a single-period oscillation of the electric field have been obtained. The calculation results made it possible to approximate the actual radiation from a photoconductive semiconductor antenna based on LT-GaN. Comparison with experimental spectra showed that the proposed formulae successfully describe the single-period emission of terahertz pulses generated by photoconductive antennas under the action of femtosecond laser pulses. It is shown that in the far-field diffraction zone, terahertz pulses undergo diffractive broadening, acquiring the character of a one-and-a-half-period oscillation.

Expressions for the magnetorefractive effect and the equatorial Kerr effect of collectivised ferromagnetic metal electrons in the terahertz spectral range are obtained using Maxwell’s equations and the Boltzmann kinetic equation. It is shown that the magnetorefractive effect (the change in the reflection coefficient upon reflection of unpolarized terahertz radiation from the surface of a magnetoresistive metal) is proportional to the magnetoresistance, and the Kerr effect can reach values of the same order of magnitude as for the visible spectral range. The possibility of using these effects is discussed.

In the present work, based on a combined model of photonucleon reactions, the activities of the prospective medical isotope ({}^{161})Tb, produced via the reactions ({}^{162})Dy((gamma),1p) and ({}^{163})Dy((gamma),1n1p) at electron beam energies ranging from the reaction thresholds up to 55 MeV, were calculated. For photonuclear production of isotopes, information on the formation of by-product nuclides is also necessary, therefore the reactions ({}^{162})Dy((gamma),1n1p)({}^{160})Tb, ({}^{163})Dy((gamma),1p)({}^{162})Tb and ({}^{163})Dy((gamma),2n1p)({}^{160})Tb were studied. The calculation results indicate the feasibility of using a monoisotopic ({}^{162})Dy target for photonuclear production of ({}^{161})Tb at electron beam energies of 21–22 MeV, and a monoisotopic ({}^{163})Dy target at energies of 29–30 MeV.

The new expressions for the form factor components and mean-square radius for the vector current of the composite system of two relativistic particles with arbitrary masses and spin 1/2 are obtained. The pseudoscalar, vector, and pseudovector composite systems were considered. For them, an identity was derived that establishes the dependence between masses and quark spins forming the composite systems. Values of the mean-square radius of the ground-state (s)-wave level of pseudoscalar (pi^{pm})-, (K^{pm})-, and (K_{0})-mesons with Coulomb interaction are calculated. The analysis is performed, and the dependence of the behaviour of the mean-square radius concerning the differences in quark masses forming the mesons is established. The consideration is conducted within the framework of the relativistic quasipotential approach based on the covariant Hamiltonian formulation of quantum field theory by transition to the three-dimensional relativistic configurational representation in the case of two relativistic spinor particles with arbitrary masses.

The effect of intensity modulation of narrow-linewidth laser radiation in the 1590 nm range, transmitted through the waveguide structure of a quantum cascade laser, has been obtained and observed upon application of an electric voltage to the structure. This effect reveals the thermodynamic behaviour of the active region of a quantum cascade laser operating in a pulsed mode, which is relevant to the task of optical communication based on the principle of high-frequency amplitude signal modulation.

The architecture of the ConvFormer S18 neural network has been shown to enable reconstruction of the wavefront shape of a laser beam based on the intensity distribution near the lens focus. Incorporating a standardization method for sparse data into the wavefront reconstruction procedure reduced the root mean square error by a factor of two. Experiments have demonstrated the ability to use a neural network to reconstruct the wavefront of laser radiation based on the intensity profile near the focus of the collecting lens, even in the presence of multiple aberrations. Errors in determining wavefront based on intensity distribution measured before focus, after focus, and based on the data from both positions relative to experimentally measured values were found to be (lambda/13) , (lambda/18), and (lambda/31), respectively. The method’s accuracy is proven by the match between the intensity distribution calculated using the data obtained by the neural network and the measured experimental values.

At present, most measuring systems allow obtaining monostatic radar cross section (RCS), but the ability to obtain bistatic RCS is very important from a practical point of view. Therefore, it is necessary to modernise existing compact ranges with the aim of measuring bistatic RCS. In practice, this is often achieved using a near-field scanner with the subsequent computation of the far-field and the corresponding bistatic RCS. In this paper, a method is proposed, based on the principles of the discrete sources method for computing the RCS from near-field values obtained through scanning on a plane or a portion of a cylindrical surface. The proposed method is validated through numerical experimentation. A comparison is performed between the results obtained using the presented method and those obtained via the traditional near-field to far-field transformation technique. It is demonstrated that the use of discrete sources in the considered configurations allows for more accurate computation of the target bistatic RCS.

The studies carried out at the newly discovered methane seep site, identified during sonar surveys near Cape Ayu-Dag, complement existing data on the distribution of shallow-water gas emissions off the Crimean coast of the Black Sea. Using the passive acoustic method, the nature of the gas emissions was determined, and the bubble gas flow was estimated to range from 0.1 to 0.2 L/h. Preliminary data from isotopic and gas chemical analyses demonstrated the biogenic nature of the released gas. Further monitoring will make it possible to assess the seasonal and interannual dynamics of activity at the detected seep site near Cape Ayu-Dag, as well as the isotopic and chemical characteristics of the gases.

The results of the analysis of time lags between variations in the global near-surface temperature T and the atmospheric CO({}_{2}) content q, obtained using numerical simulations with the IAP RAS–MSU Earth System model for the period 2020–3000 AD, are presented. In these simulations, variations in the energy flux into the climate system associated with natural climate variability, in particular with a period of about 60 years, were taken into account, as well as changes in anthropogenic CO({}_{2}) emissions into the atmosphere in accordance with the SSP1-2.6 scenario, extended up to the year 3000 and supplemented with 50-year periodic variations simulating Kondratiev’s economic cycles. It is found that changes in T may both lead and lag behind changes in q, depending on the time interval over which the time lag between T and q is investigated. The results depend on the ratio of the amplitudes of the variations in anthropogenic CO({}_{2}) emissions and the variations in the energy flux into the climate system associated with natural variability. The effects observed under their comparable contribution to changes in T are explained using analytical solutions of a system of equations that describe in simplified form the dynamics of the Earth system. The results obtained indicate that, in general, it is not possible to infer the nature of the cause-and-effect relationship between the variables based solely on the time lag between changes in them, without involving physical concepts concerning the nature of their interaction.

In the present work, the influence of the surface quantum effect on the field intensity of a nanoparticle pair with a nanometre-scale gap is investigated. Gold and sodium are chosen as the particle materials. The quantum effect is taken into account using mesoscopic boundary conditions involving the Feibelman parameters. The modelling was carried out using the discrete sources method, which allows the near fields to be computed with guaranteed accuracy. As a result of comparative analysis, significant differences in the field intensity within the nanometre-scale gap between gold and sodium particles were identified. It was established that, when the quantum effect is taken into account, the field intensity in the gap between sodium particles turns out to be higher than in the classical case. This markedly distinguishes them from the gold pair, where accounting for the quantum effect leads to a reduction in intensity.