Volume 107, Nº 10 (2018)

Twisted bilayer graphene is an excellent example of highly correlated system demonstrating a nearly flat electron band, the Mott transition and probably a spin liquid state. Besides the one-electron picture, analysis of Dirac points is performed in terms of spinon Fermi surface in the limit of strong correlations. Application of gauge field theory to describe deconfined spin liquid phase is treated. Topological quantum transitions, including those from small to large Fermi surface in the presence of van Hove singularities, are discussed.

It is shown that the preliminary exposure of ZnO, triglycine sulfate, and potassium hydrogen phthalate crystals in ultralow crossed magnetic fields—Earth’s magnetic field and ac pump field—leads to a resonance change in their microhardness. The resonance frequency of microhardness peaks is determined by the classical condition of electron paramagnetic resonance only at certain orientations of the crystals with respect to the Earth’s magnetic field B_Earth. Rotations of all samples with respect to the direction B_Earth by angle θ reduce the resonance frequency in proportion to cosθ. The observed anisotropy has been attributed to the presence of their own local magnetic fields B_loc ≫ B_Earth in the crystals.

The process of homogeneous crystal nucleation has been considered in a model liquid, where the interparticle interaction is described by a short-range spherical oscillatory potential. Mechanisms of initiating structural ordering in the liquid at various supercooling levels, including those corresponding to an amorphous state, have been determined. The sizes and shapes of formed crystal grains have been estimated statistically. The results indicate that the mechanism of nucleation occurs throughout the entire considered temperature range. The crystallization of the system at low supercooling levels occurs through a mononuclear scenario. A high concentration of crystal nuclei formed at high supercooling levels (i.e., at temperatures comparable to and below the glass transition temperature T_g) creates the semblance of the presence of branched structures, which is sometimes erroneously interpreted as a signature of phase separation. The temperature dependence of the maximum concentration of crystal grains demonstrates two regimes the transition between which occurs at a temperature comparable to the glass transition temperature T_g.

The stiffness of spin waves in the Fe_0.75Co_0.25Si helimagnet with the Dzyaloshinskii–Moriya interaction in a state fully magnetized by an external field has been measured by the small-angle neutron scattering method. It has been shown that the dispersion of magnons in this state is anisotropic because the neutron scattering pattern consists of two circles for neutrons with obtaining and losing the magnon energy, respectively. The centers of the circles are shifted by the momentum transfer oriented along the applied magnetic field H and equal to the wave vector of the spiral ±k_s measured in inverse nanometers. The radius of the circles is directly related to the stiffness of spin waves and depends on the magnitude of the magnetic field. It has been shown that the stiffness of spin waves A for the helimagnet is equal to 46.0 meV Ų at T = 0 K and decreases weakly (by 20%) with increasing temperature up to the critical value T_c = 38 K.

Resonance excitation of the ⁸³Kr first nuclear level (E = 9.4 keV) by solar axions formed via the Primakoff mechanism is sought. The γ- and X-ray photons and the conversion and Auger electrons arising from the excited-level relaxation are detected with a gas proportional counter of a low-background detector in the underground Baksan Neutrino Observatory. The following experimental constraint is obtained for the product of the axion–photon coupling constant and the axion mass:|g_Aγ × m_A| ≤ 6.3 × 10 ^-17 In the framework of the hadronic-axion model, this corresponds to a new axion-mass constraint of m_A ≤ 12.7 eV at 95% C.L.

A dispersion law for the system of three-level atoms with an equidistant energy spectrum interacting with resonant laser radiation has been obtained taking into account two successive optically allowed one-photon transitions and an optically allowed two-photon transition between the lower and upper levels. It has been shown that the dispersion law consists of three polariton branches. The effects of repulsion and attraction of branches of the dispersion law and their intersection, as well as the self-consistent variation of the photon–atom coupling constant, have been predicted.

The dynamics of formation of streamers in a “needle–plane” gap filled with air at atmospheric pressure has been studied. It has been shown that the time dependence of the dynamic displacement current measured by either a current shunt or a collector placed behind a grid electrode is determined by the rate of variation of the shape and dimensions of a streamer. The presence of a single peak on oscillograms of the dynamic displacement current means that a ball streamer is formed in the gap and does not cross the gap during the time of voltage application. The presence of two peaks on oscillograms of the dynamic displacement current means that the ball streamer crosses the gap and reaches the opposite electrode. In this case, the ball streamer is usually transformed to a cylindrical one. It has been shown that the measurement of the dynamic displacement current makes it possible to determine the time dependence of the electric field strength near the planar electrode.

A theory of the diffuse X-ray scattering on randomly distributed surface blisters, which constitute a new type of defects, has been developed. An expression for the amplitude of diffuse X-ray scattering from surface blisters has been obtained within the model of a truncated spherical layer. Scattering intensity distribution maps from surface blisters have been numerically simulated. The effect of the elastic bending of reflecting planes of the lattice of surface blisters on the angular distribution of the scattering intensity in the reciprocal space has been revealed.