卷 108, 编号 11 (2018)

We report the results of theoretical and experimental investigation of spin diffusion in the normal phase of liquid ³He confined in planar aerogel, which is a material consisting of nanostrands almost parallel to a specific plane and randomly oriented in this plane. Using spin echo technique, we measure the spin diffusion coefficients in the directions perpendicular and parallel to the plane. We see good agreement between the experiment and the theory.

The dynamics of the formation of different-scale vortices and antivortices in bulk and film YBa₂Cu₃O_7–x samples in the process of their “decomposition” into twins with an increase in the field has been demonstrated. The spatial sizes and energies of the revealed twin boundary vortices and antivortices have been determined. It has been shown that the transverse dimensions of twin boundary antivortices coincide with the spatial dimensions of twins. These antivortices keep their shape and decrease with increasing field, becoming smaller than Abrikosov vortices. At any stage of the decomposition of a sample, the transverse dimensions of twin boundary vortices are smaller than the dimensions of twin boundary antivortices and the energy of twin boundary vortices is higher than the energy of twin boundary antivortices.

It has been shown that the presence of narrow-gap quantum jumpers in “dirty” (low concentrations of identical nonmagnetic impurities in the insulator layer) superconductor–insulator–superconductor (S–I–S) junctions results in a significant deviation of the critical supercurrent (Josephson current) in the temperature range 0 ≤ T ≤ T_c (T_c is the superconducting transition temperature in the S shores of the junction) from the value given by the known Ambegaokar–Baratoff relation. It has been found that the character of this deviation changes at a certain nonzero temperature T_b < T_c: this deviation is negative at 0 ≤ T< T_b (deficiency of the supercurrent), whereas it is positive at T_b< T ≤ T_c (excess of the supercurrent). Estimates indicate that experimental manifestation of this effect can be detected.

Majorana zero modes can be simulated in structures based on spin or quasi-spin degrees of freedom, e.g., Josephson-qubit chains. Braiding of Majorana degrees of freedom is realized using T-junctions supplied with an auxiliary spin (ancilla). Motivated by prospective experiments, we analyze the braiding in the spin representation, which provides the basis for the analysis of imperfections characteristic to the spin and qubit designs. The result of the braiding operation is straightforwardly found for the initial basis states of the two qubits and the ancilla, up to phase factors. Here, we fix these phase factors and thus describe the complete two-qubit operation. This result is relevant for physical simulation of the Majorana qubits in Josephson-qubit chains and other spin or qubit structures.

We perform quantum calculations of fluctuations of the electromagnetic fields in AA collisions at RHIC and LHC energies. Calculations are performed with the help of the fluctuation–dissipation theorem taking into account the giant dipole and quadrupole resonances. We find that in the quantum picture the field fluctuations are much weaker than that predicted by the classical Monte Carlo simulation with the Woods–Saxon nuclear density used in previous analyses.

It has been shown that effective fermion vertices appear in lattice gravity associated with fermions. These vertices are generated by gravitational instantons, similar to the appearance of effective fermion vertices in the Yang–Mills theory because of the existence of zeroth fermion modes associated with instantons.

Dependences of experimental K, L_I, L_II, and L_III X-ray levels on the atomic number different from Moseley’s law have been revealed. The simplest cubic polynomial approximations of the found dependences make it possible not only to describe experimental data with an accuracy better than one percent but also to control their reliability.