Vol 105, No 4 (2017)

We present a set of formulas to extract the gluon density from the Berger–Block–Tan form of the deep inelastic structure function F₂ at small x in the leading order of perturbation theory.

In the framework of the Nambu–Jona-Lasinio (NJL) model, a mechanism for eliminating the off-diagonal πa₁ transitions from the effective meson Lagrangian is investigated. The problem is considered in the oneloop approximation for the quark loops. We employ the minimal superposition linear in the fields, a_μ^'= a_μ − c∂_μπ, and show that the covariant transformation properties of both the axial-vector and vector physical fields are affected by this substitution. Contrary to some claims found in the literature, we demonstrate that the discussed change in covariant transformation properties of these fields does not result in various violations of chiral symmetry, but rather reduces to a redefinition of the S-matrix elements beyond the mass shell. At the same time, the covariant and noncovariant approaches yield identical on-shell results.

A macroscopic counterpart to the microscopic mechanism of the straightening dimer mesogens conformations, proposed recently by S.M. Saliti, M.G. Tamba, S.N. Sprunt, C. Welch, G.H. Mehl, A. Jakli, and J.T. Gleeson [Phys. Rev. Lett. 116, 217801 (2016)] to explain their experimental observation of the unprecedentedly large shift of the nematic–isotropic transition temperature is discussed. The proposed interpretation is based on singular longitudinal fluctuations of the nematic order parameter. Since these fluctuations are governed by the Goldstone director fluctuations, they exist only in the nematic state. External magnetic field suppresses the singular longitudinal fluctuations of the order parameter (similarly as is the case for the transverse director fluctuations, although with a different scaling over the magnetic field). The reduction of the fluctuations changes the equilibrium value of the magnitude of the order parameter in the nematic state. Therefore, it leads to additional (with respect to the mean field contribution) fluctuation shift of the nematic–isotropic transition temperature. Our mechanism works for any nematic liquid crystals, however the magnitude of the fluctuation shift increases with decrease in the Frank elastic moduli. Since some of these moduli supposed to be anomalously small for so-called bent-core or dimer nematic liquid crystals, just these liquid crystals are promising candidates for the observation of the predicted fluctuation shift of the phase transition temperature.

We perform theoretical investigation of the localized state dynamics in the presence of interaction with the reservoir and Coulomb correlations. We analyze kinetic equations for electron occupation numbers with different spins taking into account high order correlation functions for the localized electrons. We reveal that in the stationary state electron occupation numbers with the opposite spins always have the same value: the stationary state is a “paramagnetic” one. “Magnetic” properties can appear only in the non-stationary characteristics of the single-impurity Anderson model and in the dynamics of the localized electrons second order correlation functions. We found that for deep energy levels and strong Coulomb correlations, relaxation time for initial “magnetic” state can be several orders larger than for “paramagnetic” one. So, long-living “magnetic” moment can exist in the system. We also found non-stationary spin polarized currents flowing in opposite directions for the different spins in the particular time interval.

The coherent spin dynamics of a two-dimensional electron gas in a GaAs/AlGaAs quantum well is experimentally studied near the filling factors ν = 3 and 1. The nonmonotonic character of the dependence of the spin dephasing time of a Goldstone spin exciton on the filling factor is found experimentally. The observed effect can be due to the formation of a new spin relaxation channel, when the main state of the two-dimensional electron system is a spin-textured liquid.

New heterophase superlattices based on MoS₂ are studied in detail by the electron density functional theory. It is shown that the incorporation of the 1Т phase in the 2H-MoS₂ monolayer is responsible for the formation of electronic levels near the Fermi level and quantum wells in the transverse direction of superlattices. The proposed lateral heterophase structures of transition metal dichalcogenides are promising for the construction of new elements of nanoelectronics.

Gor’kov theory of superconductivity [L. P. Gor’kov, Sov. Phys. JETP 7, 505 (1958)] opened the application of the methods of quantum field theory to condensed matter physics. Later the results became relevant to relativistic quantum fields.

The Cherenkov effect is a well-known phenomenon and its properties are widely used in many fields of physics. However, some features of the polarization characteristics of Cherenkov radiation that appears when charged particles pass near azimuthally asymmetric, finite dielectric targets are still poorly studied. This problem is solved in this work. The polarization characteristics of Cherenkov radiation in the case of a rectangular dielectric screen are analyzed using the Stokes approach. Owing to the azimuthal asymmetry of the target, radiation acquires an elliptic polarization whose rotation direction and inclination angle depend both on the direction of radiation propagation and on the dielectric properties of a substance. The results demonstrate that the Cherenkov effect can be used to create sources of elliptically polarized radiation with the controlled direction of polarization rotation.