Vol 118, No 1-2 (7) (2023)

The focusing of an atomic beam with the use of a two-dimensional magneto-optical trap in order to increase the number of atoms in the region of their laser cooling and localization near an atom chip is discussed. Two regimes of the interaction of atoms with a focusing laser field are considered: (i) the Doppler interaction regime, which occurs at small detunings of the laser field from the atomic resonance, and (ii) the sub-Doppler interaction regime, which occurs at large detunings of the laser field from the atomic resonance. The efficiency of focusing in the first case is low because of the momentum diffusion. It has been shown that the momentum diffusion in the sub-Doppler cooling mechanism is insignificant and, as a result, the broadening of the transverse velocity distribution of atoms is small. The sharp focusing of the atomic beam is possible in this interaction regime.

An algorithm for constructing the structure of a one-dimensional photonic crystal, which forms a femtosecond pulse of an arbitrary given shape when reflecting a Gaussian pulse, is developed theoretically. The method is illustrated by the example of a rectangular pulse. Examples for simple pulse shapes are given, the influence of optical absorption is evaluated, and a method for its compensation is proposed.

The interference contribution to the optical conductance (total transmittance) of a sample of a disordered Faraday medium is calculated. The suppression of wave interference in a magnetic field is shown to be due to helicity-flip scattering events. The magnetic field does not destroy the interference of waves with a given helicity, but suppresses it if the helicity changes along different parts of the wave trajectory. This leads to a decrease in the interference contribution to the conductance with increasing the magnetic field. A similar phenomenon, negative magnetoresistance, is known as a consequence of weak localization of electrons in metals with impurities. It is found that, as the magnetic field increases, the change in the interference correction to the optical conductance tends to a certain limiting value, which depends on the ratio of the transport mean free path to the helicity-flip scattering mean free path. We also discuss the possibility of controlling the transition to the regime of strong “Anderson” localization in the quasi-one-dimensional case by means of the field.

The genesis of the electronic spectrum in SrFe_{1 − x}Al_xO₃, SrFe_{1 − x}Mn_xO₃, SrFe_{1 − x}Co_xO₃, SrFe_{1 − 2x}Al_xCo_xO₃, and SrFe_{1 − y − z}Mn_yCo_zO₃ cubic solid solutions of strontium ferrite, where 0⩽x⩽0.15">$0⩽x⩽0.15$ and 0⩽y">$0⩽y$, z⩽0.125">$z⩽0.125$, is studied in the coherent potential approximation. The inclusion of electron correlations on 3d atoms makes it possible to reproduce the experimental content tendencies in the variation of the electronic and magnetic properties. It is shown that codoping of ferrite with cobalt and aluminum effectively increases the concentration of electron carriers and their degree of localization in SrFe_{1 − x − 0.15}Al_xCo_0.15O₃, which is of interest in the development of oxide thermoelectrics. Due to a relatively large contribution from delocalized states at the Fermi level, SrFe_{1 − y − z}Mn_yCo_zO₃ solid solutions with y=z=0.1−0.12">$y=z=0.1-0.12$ are promising electrode materials.

Plasma and magnetoplasma excitations in high-quality undoped two-dimensional electron systems based on SiGe/Si quantum wells are studied in detail. A two-dimensional electron system is formed by applying a vo-ltage to the top gate, which is partially transparent to subterahertz radiation in the frequency range of 20‒160 GHz. The results for SiGe/Si quantum wells with a Sb δ-doping layer are also presented for comparison. The transport and quantum scattering times for both structures are directly determined. It has been found that the effective electron mass is almost independent of the two-dimensional electron density in a wide density range.

The effect of the quantum decoherence of neutrino mass states on collective oscillations of neutrinos has been studied for the case of three flavors using a method based on the stability analysis of the Lindblad equation with the neutrino evolution Hamiltonian including the effects of the self-interaction. New analytical conditions for the appearance of collective neutrino oscillations in supernova explosions have been obtained taking into account the quantum decoherence of neutrinos.

Infrared absorption spectra of the ErCrO₃ crystal in the region of f−f">$f-f$ transitions in the Er³⁺ ion are recorded for the first time. An analysis of high-resolution temperature-dependent spectra reveals a step at T′=47">$T{}^{\prime }=47$ K on the temperature dependences of the characteristics of spectral lines, in addition to the features at temperatures of magnetic ordering T_N = 133 K and spin-reorientation transition TSR=9.3">$T_{\text{SR}}=9.3$ K. This feature can be associated with either a previously unknown phase transition or with local changes in the crystal structure. The shape of the lines at liquid helium temperatures indicates the presence of additional positions for Er³⁺ ions in the ErCrO₃ crystal. Presumably, these are positions near uncontrolled impurities that enter the crystal during its growth by the solution–melt method and form regions with a distorted structure responsible for the occurrence of polarization.

A coding scheme is introduced to store a set of linked bit strings in planar magnetic nanoelements with holes. Analytical expressions for the corresponding magnetization distributions are developed up to a homotopy and the specific examples are given for doubly- and triply-connected cases. The energy barriers, protecting the information-bearing states, are discussed. Compared to a set of disparate simply-connected nanoelements of the same total connectivity, the nanoelements with holes can hold much more information due to the possibility of linking the individual bits.

The time-resolved magneto-optical Kerr effect in epitaxial thin films of the FePt compound and the FePt_0.84Rh_0.16 solid solution with the perpendicular magnetic anisotropy on MgO (001) substrates has been studied. The evolution of hysteresis loops at short (100 fs–1 ns) and long (1–20 ms) time scales after the excitation by a femtosecond light pulse has been studied. Long-lived nonthermal reduction of the coercive field has been detected. The coercive field is recovered in several milliseconds. It has been proposed to explain the observed phenomenon by the excitation of high-Q-factor acoustic resonances in the substrate/film system and to the strong magnetoelastic interaction in FePt and FePt_0.84Rh_0.16 films.

The formation of topological defects at the nematic–isotropic liquid interface and near satellite droplets has been detected at the breakup and fragmentation of the bridge of the isotropic phase between nematic domains. This process has been implemented in thin optical cells filled with a liquid crystal. The critical width of the bridge at which a universal time dependence of its width is determined by the capillary velocity (ratio of the surface tension to the viscosity) has been determined.

A series of two-particle examples of the Ruijsenaars pq-duality is considered in detail, the dual Hamiltonians are constructed. Of special interest is the case of the sinh-Gordon model.