Vol 515, No 1 (2024)

Flow contraction within cylindrical samples causes instability in motion, leading to the formation of circumferential compression stress. This then results in perturbations in the form of Mach three-shock configurations (protrusions) at the shock front. The area at the front of the perturbed shock wave increases due to these protrusions. Furthermore, the protrusions are amplified as a consequence of the absorption of smaller perturbations, which are continuously generated at the shock front. The shock front undergoes sharp growth at the final stage, giving rise to several large protrusions that divide it into separate sectors. These sectors undergo oscillatory movements. As the counter configurations collide, a high-pressure zone is generated, which propels some of the compressed material forward. The protrusions reach their maximum height when they become equal to the distance of the front to the axis. The near-axis region is taken up by the frontal protrusions, and the consequent rarefaction shock wave decelerates the incoming flow.

Andreev bound states are formed in multiterminal structures based on normal metals and superconductors. Their spectrum is determined by the system parameters, in particular the scattering phases and transmission coefficients at the nodes. The article found conditions under which Andreev bound states are universal: they don’t change with any change in the reflection phases. As a consequence, the spectrum is completely determined by the transport characteristics of the system. The result was obtained for a structure in the form of a normal metal M-finite star, each of the rays (terminals) N_k of which is in contact with its superconductor S_k, 1 ≤ k ≤ M. Together they form a multiterminal Josephson junction. At the center of the structure there is a non-magnetic impurity with its some scattering matrix.

In this paper, we consider the effect of the emergence of transparency of a resonantly absorbing medium for electromagnetic radiation due to the excitation of piston-like acoustic oscillations of the medium along the direction of radiation propagation – acoustically induced transparency. The physical mechanism and basic conditions for the implementation of the effect are discussed, as well as its application to reduce the propagation velocity of gamma-ray photons and on-demand recovery of an arbitrary part of a single-photon wave packet resonantly absorbed in the medium.

Exact solutions for the thermal conductivity coefficient of a two-phase dispersed medium are obtained using the most general physical principles of locality and symmetry. Two solutions define the well-known Hashin–Shtrikman bounds. The third solution, invariant under the phase inversion transformation, significantly narrows the Hashin–Shtrikman boundaries; this is confirmed by comparison with numerous experiments by other authors. It has been shown that taking into account the remote interaction of dispersed particles at their increased concentration only slightly (less than 3%) affects the result.

In this work, we study the nonlinear dynamics of parametrically excited bending vibrations of two weakly coupled beam microresonators under electrothermal excitation. A steady-state harmonic temperature distribution in the volume of the resonators in the frequency domain was obtained. A system of equations for mechanically coupled beam resonators is derived, considering the deposited particle on one of them. Using asymptotic methods of nonlinear dynamics, equations in slow variables were obtained, which were studied by methods of the theory of bifurcations. It is shown that in a perfectly symmetrical system in a certain frequency range, the effect of symmetry breaking is observed – the emergence of a mode with different amplitudes of oscillations of two beam resonators, which can be the basis for a new principle of high-precision measurements of weak disturbances of various physical natures, in particular – measurements of ultra-low masses of deposited particles.

It has been shown that the strength of copper-coated optical fiber in air at elevated temperatures decreases over time, primarily due to degradation of the copper coating due to oxidation, which leads to the appearance and growth of point defects on the surface of silica glass. In this case, the optical fibers remain operational at 600°C for ~1.5 hours, and at 500°C for ~16 hours. The activation energy of the process is in the range of 120–123 kJ/mol. The results obtained make it possible to predict the stability of the optical fibers in air at 300°C for ~1.5 years, and at 250°C for ~17 years.

Holographic signal processing of noise signals by using of the vector-scalar receivers is described. The results of an experiment on the detection and direction estimation of an underwater noise source are presented. The experiment was conducted in Black Sea. The source noise emission was received by vector-scalar receivers. Holographic signal processing made it possible to detect, localize and resolve an underwater source with a low input signal/interference ratio against the background of intensive surface navigation.