Том 103, № 11 (2016)

An order-of-magnitude enhancement of the pulsed photocurrent in a polycrystalline diamond sample synthesized by chemical vapor deposition is observed under the conditions of formation of an electron–hole liquid. Nonequilibrium charge carriers are excited by laser pulses at a wavelength of 222 nm with FWHM pulse duration of 18 ns and peak intensity above 2.5 MW/cm² upon cooling the sample to 90 K. For peak intensities of laser excitation lower than 1 MW/cm², sample cooling from 300 to 90 K leads to a decrease in pulsed photocurrent by about a factor of 5. The observed increase in pulsed photocurrent is attributed to the formation of the electron–hole liquid.

High-precision measurements of the Hall effect and Raman scattering have been performed for single crystals of ZrB₁₂ superconductor in the wide temperature range of 5–300 K. For ZrB₁₂, the boson peak with ω_max ~ 100 cm^–1 has been observed for the first time within the low-frequency range of the Raman spectrum I(ω). The sizes of vibrational clusters with the correlation length ranging from 25 to 35 Å are estimated. The relation between the renormalization of the low-frequency density of vibrational states accompanying the transition to the cage-glass phase (T* ~ 90 K) and the enhancement of superconductivity in ZrB₁₂ is discussed.

A physical model of the action of radiation from a bolide on Earth’s lower ionosphere has been presented. The calculations within this model applied to the Chelyabinsk bolide on February 15, 2013, have reproduced the luminosity curves in the visible and infrared ranges in agreement with ground and satellite observations. The calculated disturbances of the lower ionosphere induced by the radiation of the bolide at the stage of the flight from an altitude of 60 to 30 km are in quantitative agreement with the results of processing of radar and satellite (GPS) observations in the Ural region.

Singular points of the autocorrelation function on the imaginary time axis that is averaged over the location of spins in the magnetically dilute spin lattice with isotropic spin–spin interaction at a high temperature have been studied. For the autocorrelation function in the approximation of the self-consistent fluctuating local field, nonlinear integral equations have been proposed which reflect the separation of the inhomogeneous spin systems into close spins and other spins. The coordinates of the nearest singular points have been determined in terms of the radius of convergence of the expansion in powers of time, the coefficients of which have been calculated from recurrence equations. It has been shown that the coordinates of singular points and, consequently, the wings of the autocorrelation function spectrum at strong magnetic dilution are determined by the modulation of the local field by the nearest pairs of spins leading to its logarithmic concentration dependence.

Inhomogeneous deformation induced by an external electric field in thin SrTiO₃ single-crystal plates has been studied by the interferometric method.

Radiationless relaxation in hybrid systems quantum dot (QD)–plasmonic nanostructure is considered. For the system QD–2D plasma the relaxation rate extremely steeply depends on the radius of quantum dot while in the pair QD–cylindrical wire contacting each other this dependence is logarithmic weak.

Recently, a new type of Weyl semimetal called type-II Weyl semimetal has been proposed. Unlike the usual (type-I) Weyl semimetal, which has a point-like Fermi surface, this new type of Weyl semimetal has a tilted conical spectrum around the Weyl point. Here we calculate the anomalous Hall conductivity of a Weyl semimetal with a tilted conical spectrum for a pair of Weyl points, using the Kubo formula. We find that the Hall conductivity is not universal and can change sign as a function of the parameters quantifying the tilts. Our results suggest that even for the case where the separation between the Weyl points vanishes, tilting of the conical spectrum could give rise to a finite anomalous Hall effect, if the tilts of the two cones are not identical.

The generation of X rays at the grazing incidence of 18-MeV electrons with a 50-μm-thick Si crystal 4 mm in length along the electron beam has been studied. The crystal has been placed in a goniometer inside the chamber of a B-18 betatron. The results exhibit strong changes in the angular distribution of bremsstrahlung at the variation of the orientation of the crystal. This effect is not observed in the case of the normal incidence of electrons on the surface of a thin crystal where the channeling of electrons, which occurs at certain orientation of the crystal, is absent. Images of a reference microstructure have been obtained with a high resolution of details of the microstructure owing to the smallness of the source of radiation. The dependence of the contrast of an image on the position of the microstructure in the radiation cone has been demonstrated.

In this short review, we first discuss the results, which are mainly devoted to the generalizations of the famous Kohn–Luttinger mechanism of superconductivity in purely repulsive fermion systems at low electron densities. In the context of repulsive-U Hubbard model and Shubin–Vonsovsky model we consider briefly the superconducting phase diagrams and the symmetries of the order parameter in novel strongly correlated electron systems including idealized monolayer and bilayer graphene. We stress that purely repulsive fermion systems are mainly the subject of unconventional low-temperature superconductivity. To get the high temperature superconductivity in cuprates (with T_C of the order of 100 K) we should proceed to the t–J model with the van der Waals interaction potential and the competition between short-range repulsion and long-range attraction. Finally we note that to describe superconductivity in metallic hydrogen alloys under pressure (with T_C of the order of 200 K) it is reasonable to reexamine more conventional mechanisms connected with electron–phonon interaction. These mechanisms arise in the attractive-U Hubbard model with static onsite or intersite attractive potential or in more realistic theories (which include retardation effects) such as Migdal–Eliashberg strong coupling theory or even Fermi–Bose mixture theory of Ranninger et al. and its generalizations.