R e v i e w s

Condensed matter physics

Moscow University Physics Bulletin 2021. N 3.

Scanning tunneling microscopy, scanning tunneling spectroscopy (STM/STS), and X-ray photoelectron spectroscopy (XPS) were used in this study. The electronic density of states for fluorofullerene molecules on the surface of semiconductors and metals has been calculated ab initio. The behavior of fluorofullerene molecules with different stoichiometric compositions is considered. It has been found that a dipole molecule C$_{60}$F$_{18}$ prefers to be oriented by fluorine atoms towards the semiconductor surface. For a metallic surface, the dipole moment can deviate from the direction of the normal to the surface, which is caused by the balance between the intermolecular interaction and the interaction between the molecule and the surface. Fluorofullerene molecules tend to lose fluorine atoms over time, which allows them to be used as a source of fluorine in the processes of fluorination of metallic surfaces and a local chemical reaction on the semiconducting surface.

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Physics of nuclei and elementary particles

Moscow University Physics Bulletin 2021. N 3.

We study the sensitivity of polarization observables in $\gamma d\to\pi^0d$ reaction near threshold to the choice of elementary $\gamma N\to\pi N$ amplitude and $NN$ potential model adapted for the deuteron wave function (DWF). Numerical results for various beam, target, and beam-target polarization observables are presented and systematic uncertainties caused by the use of different elementary operators and DWFs are evaluated. The calculations are based on a $\gamma d\to\pi^0d$ approach in which realistic models for the elementary pion production amplitude and the DWF are used. We find considerable dependencies of the estimated results for all possible polarization observables on the elementary amplitude. The spin asymmetries $\Sigma$, $T_{21}^c$, $T_{10}^l$, and $T_{20}^l$ show large sensitivities to the DWF. In contrast, the asymmetries $T_{11}$, $T_{2M}$ ($M$=0,1,2), $T_{10}^c$, and $E$ as well as the helicity difference $d(\sigma^P-\sigma^A)/d\Omega$ have slight dependence on the DWF only at photon energies very close to $\pi$-threshold. The unpolarized differential cross section is also predicted and compared with the available experimental data, and a satisfactory agreement is obtained only at forward pion angles. We expect that the results presented here may be useful to interpret the recent measurements from Jefferson Lab, TAPS@ELSA, A2 and GDH@MAMI Collaborations.

Show AbstractTheoretical and mathematical physics

Moscow University Physics Bulletin 2021. N 3.

TTwo types of tapered anechoic chambers have been analyzed by the method of mathematical modeling: conical and pyramidal types. The radar-absorbing coating of the inner surface of both chambers is modeled by a dielectric layer. The electromagnetic field inside the tapered chambers has been calculated by the method of integral equations using the FEKO software. The analysis and comparison of the electromagnetic field distribution in the quiet zone of each chamber have been carried out and the influence of the radiation source position on the distribution of the amplitude and phase of the field has been investigated at different frequencies.

Show AbstractRadiophysics, electronics, acoustics

Moscow University Physics Bulletin 2021. N 3.

This article considers an analytical approach to the description of the electrodynamic interaction of Josephson junctions in a parallel array. Such an analysis is necessary for the design of differential cells of superconducting quantum arrays and active electrically small antennas based on them. Estimates of the required number of elements in a parallel array of Josephson junctions are presented.

Show AbstractMoscow University Physics Bulletin 2021. N 3.

During waveguide sound propagation in the marine environment, one of the main characteristics is the speed of sound. In hydrophysical waveguides, the speed of sound is mainly a function of depth; the radial coordinate introduces second-order corrections of smallness. Nevertheless, in some cases, significant changes in the characteristics of the waveguide along the radial coordinate are possible, for example, as a result of the influence of anthropogenic factors, sea currents, etc. In the present paper, an analytical representation is constructed for the sound field of a point source in a plane-layered sea waveguide that has an inhomogeneous speed of sound in depth and waveguide path. The waveguide model has radial symmetry, while the sound source is located at an arbitrary point on the waveguide, which leads to a solution that substantially depends on all three spatial coordinates. This solution is based on the decomposition of the waveguide region into cylindrical regions, allowing the construction of an analytical solution of the Helmholtz equation. It is shown that in the case of a constant profile of sound velocity and density in each of the waveguide decomposition regions, the solution of the problem can be obtained explicitly. The results of numerical modeling are presented, the influence of heterogeneity on the characteristics of the sound field is analyzed

Show AbstractOptics and spectroscopy. Laser physics

Moscow University Physics Bulletin 2021. N 3.

An optimal arrangement of point reflectors inscribed by femtosecond laser pulses in a standard telecommunication fiber is found, which provides the maximum operating range of distributed sensors based on phase-sensitive optical time-domain reflectometers (φOTDRs). It is shown that the operating distance of an φOTDR utilizing probe pulses with a duration of 200 ns can be increased by 53 km using ultra-low-loss fiber with an attenuation of 0.16 dB/km, thereby extending it to 173 km without using distributed, remotely pumped, or in-line amplifiers. When using the most widespread standard telecom fiber with an attenuation of 0.185 dB/km, the operating distance can be increased by 48 km (up to 152 km total).

Show AbstractCondensed matter physics

Moscow University Physics Bulletin 2021. N 3.

In this study, the effect of hydrogenation on the magnetocaloric properties of extra-pure nanocrystalline terbium was investigated. The investigated compositions were TbHx, where $x = 0 – 1.2$ at H/arb.u. In addition to the major phase $\alpha$-TbH$_x$, all the specimens contained a certain amount of a second phase, terbium dihydride ($\beta$-TbH$_2$). For TbH$_x$ specimens, the effect of hydrogenation on magnetocaloric properties was studied by both direct and indirect methods. The magnetocaloric effect was shown to decrease in a nearly linear fashion with increasing hydrogen content in the specimens.

Show AbstractAstronomy, astrophysics, and cosmology

Moscow University Physics Bulletin 2021. N 3.

Nonstandard techniques to deliver a payload from the Moon to Earth are explored. Two approaches are compared, which are based on using a space elevator and an acceleration device, the Artsimovich railgun. The energy needed to launch a payload to a low lunar orbit and the $L_1$ libration point in the Earth–Moon system is estimated. We conclude that the railgun is economically advantageous compared to the space elevator and standard jet technologies.

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