Volume 118, Nº 11-12 (12) (2023)

We study optical properties of linear dye aggregates in which the transition dipole matrix elements of two monomer molecules forming their unit cell are not coplanar with the aggregate axis, and the Frenkel exciton is delocalized along this axis. An analytical model has been developed for the description of polarization effects in the light absorption spectra of such aggregates. It is shown that the nature of their optical spectra differs drastically from previously studied linear aggregates with a single molecule per unit cell. The developed theory contains simple formulas of the well-known Davydov–McRae–Kasha model for conventional linear aggregates as a particular case. A quantitative explanation of the experimental data is given for the absorption spectra of the pseudoisocyanine bromide dye aggregate.

The evolution of an initially planar monolayer of charged microparticles in a complex plasma (plasma crystal) in horizontal (in the plane of the monolayer) and vertical parabolic confinements has been considered. The separation (buckling instability) of a Yukawa system into several layers at the weakening of the vertical confinement, as well as structural changes in such a plasma crystal, has been studied using the molecular dynamics method. In particular, it has been shown that the radial inhomogeneity of the plasma crystal qualitatively changes the character of the separation compared to homogeneous systems. Indeed, the separation begins in the center of the crystal, where the average distance between particles is minimal, and propagates in the form of a wave towards the periphery of the system at the weakening of the vertical confinement. This explains features of the behavior of plasma crystals in recent experiments with the complex plasma.

The possibility of coding the response of the electron–nuclear system of the tetracyanoethylene radical under microwave pulse irradiation in combination with a pulsed magnetic field gradient in the nanosecond timescale by a binary code is demonstrated. To this end, the tetracyanoethylene radical, which has a well-resolved equidistant electron paramagnetic resonance spectrum due to the interaction of the electron with equivalent magnetic nuclei, is used. The aim is to demonstrate the possibility of implementing this procedure physically rather than to encode the longest possible sequence.

The features of electronic states on the surface of an intrinsic antiferromagnetic topological insulator (AFM TI) containing defects are theoretically investigated. Our approach takes into account the role of the electrostatic potential and the variation in the orientation of magnetic moments in the near-surface layers. A change in the spectral characteristics of the surface states under the transformation of magnetization from an equilibrium AFM phase of A-type to a ferromagnetic phase through a noncollinear texture is described. It is shown that in AFM TI with uniaxial anisotropy, an external magnetic field applied along the easy axis can cause a significant modulation of the exchange gap size in the spectrum of surface states and even invert the gap sign. Modeling the single defect effect as a surface potential perturbation over a finite scale, we analytically investigate the formation of a bound state and its behavior depending on the strength of potential and exchange scattering by the defect and the exchange gap size. The energy level of the bound state is demonstrated to experience a sharp shift in the vicinity of the spin-flop transition. The theoretical results obtained allow us to provide a consistent explanation of recent experimental data on scanning tunneling spectroscopy of antisite defects on the surface of the prototype AFM TI MnBi2Te4 in an external magnetic field.

In this study, for the first time, we present a systematic study of the critical current density J_c at two magnetic field orientations B∥ab">$B\parallel ab$ and B∥c">$B\parallel c$ in a single crystal of the superconducting pnictide EuCsFe₄As₄, which shows a magnetic ordering below T_c and belongs to the 1144 family. The anisotropy of J_c and its temperature dependence are determined. The exponents α">$\alpha$ of the Jc(B)">$J_c(B)$ dependences from 0.1 to 1 T are in a good agreement with the theoretical predictions for strong Abrikosov vortex pinning, which is also confirmed by the values of the pinning strength estimated using the Dew-Hughes model.

Spectral studies of the photoconductivity in the temperature range of T = 5–70 K, as well as studies of the magneto-absorption and magnetotransport at T = 4.2 K, have been performed in a HgTe/CdHgTe heterostructure with a double quantum well under an “optical gate” effect. Studies of magneto-absorption spectra under the controlled optical exposure have made it possible to observe absorption lines caused by both the cyclotron resonances of electrons and holes simultaneously. The coexistence of electrons and holes in the HgTe/CdHgTe double quantum well with a relatively large bandgap (~80 meV) indicates the appearance of a strongly inhomogeneous light-induced distribution of charge carriers in the plane of the structure. Experimental results obtained clearly demonstrate disadvantages of the control of the Fermi level positions in heterostructures with HgTe/CdHgTe quantum wells by means of the optical gate.

The behavior of the interlayer magnetoresistance Rzz is analyzed in quasi-two-dimensional layered metals in
a magnetic field tilted at Yamaji angles at which the minimum of the interlayer conductivity is observed. The
cases of the Lorentzian line shape of Landau levels and of the shape corresponding to the self-consistent Born
approximation are studied. At high fields, the behavior Rzz B3/2  is theoretically predicted, which agrees well
with experimental data.

The dynamic conformal transformation method has been generalized for the first time to numerically simulate the capillary wave turbulence of a liquid surface in the plane symmetric anisotropic geometry. The model is strongly nonlinear and involves effects of surface tension, as well as energy dissipation and pumping. Simulation results have shown that the system of nonlinear capillary waves can pass to the quasistationary chaotic motion regime (wave turbulence). The calculated exponents of spectra do not coincide with those for the classical Zakharov–Filonenko spectrum for isotropic capillary turbulence but are in good agreement with the estimate obtained under the assumption of the dominant effect of five-wave resonant interactions.

Systematic studies of magneto-transport properties of the whole (MnBi₂Te₄)(Bi₂Te₃)_m family of magnetic topological insulators (m=0,1,...,6)">$m=0,1,...,6)$ have been carried out. Temperature dependences of the resistivity, magnetoresistance and the Hall effect at low temperatures have been studied. When m increases, i.e., when the separation between 2D MnBi₂Te₄ magnetic layers becomes larger, the transition from antiferromagnetic to ferromagnetic state takes place. We have found that ferromagnetic state survives even in the samples with m=6">$m=6$, when 2D magnets are separated by six non-magnetic Bi₂Te₃ blocks.

The paper describes results of the band structure measurements of SnAs single crystals by the ARPES technique. We performed detailed analysis of isoenergetic surfaces in the vicinity and below the Fermi energy. The ARPES experimental data are consistent with theoretically predicted shape of the SnAs Fermi surface. The determined type of the Fermi surface provides the basis for estimating the Ginzburg–Landau parameter, from which it follows that SnAs is a type-I superconductor. In addition, our results of ARPES measurements confirm the presence of band splitting in the energy spectrum at the Γ¯">$\overline{\mathrm{\Gamma }}$ point at electron binding energies in the range of 0.6–1.2 eV associated with spin–orbit interaction.

A method has been proposed to reconstruct at arbitrary time the spectral–temporal characteristics of a
14.4-keV single-photon wave packet that is emitted by a 57Co source and is resonantly absorbed in the
medium of 57Fe nuclei. The method is based on the frequency separation of the field emitted by the source
and resonance nuclear polarization induced by this field by means of delayed acoustically induced transparency
of the absorber, which appears after the activation of oscillations of the absorber at the corresponding
frequency and amplitude. The proposed method has been compared to the known quantum-optical memory
methods and methods of nuclear polarization control in the gamma range. Experimental conditions have
been proposed to implement the method. It has been shown that this method allows the implementation of
the time-resolved Mössbauer spectroscopy of various media.