Moscow University Physics Bulletin最新文献

The current study presents the results of tunneling spectroscopy measurements of current-voltage characteristics (CVC) and dynamic conductance spectra (dI(V)/dV) of SnS tunneling contacts in single crystals of slightly overdoped superconducting ferro-pnictide of nominal composition BaFe({}_{1.88})Ni({}_{0.12})As({}_{2}) with bulk critical temperature (T_{c}=18) K. Besides the Andreev structure, a strong nonlinearity is observed in the obtained curves both in the superconducting and normal state. The positions and temperature dependences of the observed (dI(V)/dV)-features unrelated to the superconducting state are determined for the temperature range (4.3{-}26.4) K. The paper analyzes possible reasons for the emergence of the observed features.

A method of creating photonic structures whose optical spectrum of the reflection coefficient has an arbitrary shape and predetermined features is presented in the paper. We have developed an algorithm for the construction of a photonic crystal structure, performed numerical simulation of its reflection spectra, and created an experimental sample of a photonic crystal that has a spectral response corresponding to a given shape.

The spectrum transformation of a femtosecond wave packet during filamentation in fused silica under conditions of normal, zero, and anomalous group velocity dispersion has been investigated using numerical simulation methods. It has been shown that the generation of plasma, which induces phase modulation of the light field, leads to rapid anti-Stokes spectral broadening. It has been established that the short-wavelength shift of the broadband supercontinuum spectrum is significantly greater under anomalous group velocity dispersion compared to normal and zero dispersion. The influence of the wave packet energy on the dynamics of spectral broadening has been examined. Estimates of the numerical scheme have been obtained based on dispersion analysis of the broadband supercontinuum propagation process in the medium.

Organic light-emitting transistors are a new type of optoelectronic devices that combine the functionality of organic light-emitting diodes and the control of a transistor. These devices require organic semiconductors as the active layer, which should possess both high charge mobility and high quantum yield of photoluminescence. One promising class of such materials is thiophene-phenylene co-oligomers, whose properties can be tuned by introducing various substituents. In this study, we investigate the effect of hydrogen-to-fluorine substitution on the properties of two model representatives of the thiophene-phenylene co-oligomer class with an annulated central fragment: P–TTA–P and P–BTBT–P. It is demonstrated that fluorination of both molecules lowers the energy levels of the frontier orbitals, which is expected to facilitate electronic transport in their crystals and films. At the same time, fluorination exerts qualitatively different effects on the delocalization of frontier orbitals, optical bandgap, oscillator strength, exciton binding energy, and the Raman spectrum, which can be attributed to the differences in the equilibrium geometry of fluorinated molecules. It is expected that the revealed correlations between the structure and properties of the investigated compounds will contribute to a targeted molecular design of organic semiconductors for efficient light-emitting devices.

To effectively operate many organic electronic devices, organic semiconductors with high charge carrier mobility are required. However, in most known organic semiconductors, charge mobility is limited due to strong local electron–phonon interaction. In this work, using the example of thiophene-phenylene co-oligomers, a class of organic semiconductors that combine high charge mobility with light emission and are therefore promising for light-emitting transistors and electrically pumped lasers, the mechanism of suppression of electron–phonon interaction is studied by introducing electronegative atoms or an additional thiophene ring into the molecule. It is found that such structural modifications lead to changes in the contribution of various vibrational modes to the local electron–phonon interaction, particularly suppressing the contribution of low-frequency torsional mode. Additionally, it is shown that for two modes that contribute the most to the local electron–phonon interaction in the unsubstituted oligomer, this change correlates with their intensity in the combination scattering of light (Raman scattering, RS), confirming the potential of studying electron–phonon interaction using Raman spectroscopy. The obtained results enhance the understanding of the relationship between local electron–phonon interaction and the molecular structure of organic semiconductors, which is crucial for the targeted design of such materials with high charge mobility.

The peculiarities of Fourier spectra of fractal-like systems have been studied. Based on theoretical investigations, the features of scaling invariants of the examined objects in optical fields have been identified, manifesting in the form of stable local formations, or patterns. Special attention has been given to the issue of the stability of scaling relationships between the analyzed systems and their Fourier transforms. The possibility of identifying and systematizing different fractal systems based on the evaluation of their scaling parameters has been explored. The analysis of the stability of scaling parameters of probing radiation with respect to the variations in the parameters of the investigated systems indicates the prospects of a pattern-based approach to improving optical diagnostic methods.

The paper presents the results of test measurements to determine the capabilities of studying the cluster structure of highly excited states of the ({}^{6})Li nucleus using the reaction ({}^{6})Li(n,({}^{3})He n)({}^{3})H on the RADEX neutron channel at the Institute for Nuclear Research, Russian Academy of Sciences. In the experiment, scattered neutrons were detected in coincidence with the decay products of highly excited states (helium-3). The first experimental data on the excitation energy of the ({}^{6})Li nucleus have been obtained.

A theoretical and numerical investigation of the quality criteria for radar absorbing coatings (RACs) has been conducted. Simple analytical expressions for the minimum of the reflection coefficient of a homogeneous absorbing layer have been obtained. It is shown that this value is not a correct characteristic of radar absorption. To describe the quality of RACs, it is necessary to use either the relative bandwidth of its operating waveband or the ratio of the operating waveband to the layer’s thickness. The former of these quantities may be useful when it is important to achieve a high broadband absorbency, while the latter is useful for achieving a small thickness of an absorber.

Ferromagnetics, multiferroics, and other magnetically ordered materials are described by various models of magnetization evolution of the medium. In this work, we develop a many-particle quantum hydrodynamics method for such systems. We derive the evolution equation for macroscopic magnetization, corresponding to the nondissipative version of the Landau–Lifshitz equation for spin-1/2 particles, using the Heisenberg Hamiltonian. It is shown that the well-known form of the exchange interaction contribution in the Landau–Lifshitz equation arises at the third order in the interaction radius. The possibilities for systematic generalization of the obtained result are discussed when considering the fifth order or particles with a large spin.

The paper presents the results of studying magnetic properties of the K({}_{2})O(cdot)Al({}_{2})O({}_{3}cdot)B({}_{2})O({}_{3}) (KAB) glasses with the addition of 2.0 and 3.0 wt (%) Fe({}_{2})O({}_{3}), subjected to radiation treatment in ({}^{60})Co gamma-field at a dose rate of 236 R/s for 2 h at room temperature and when the samples are heated to 423 K. Under both radiation and thermoradiation exposure, the magnetic susceptibility of glasses follows the Curie–Weiss law at 4.2–200 K and slightly deviates from this dependence at 200–340 K. The weakly pronounced magnetic hysteresis with low coercive force was found at low temperatures against the background of magnetization that depends almost linearly on the field. The analysis of data on the temperature and field dependences of magnetization in weak and strong fields, combined with data on structural and optical properties, indi-cates that mainly Fe({}_{2})O({}_{3}) nanoparticles in the uncompensated antiferromagnetic state, as well as an insignificant amount of dissociated Fe and Fe({}_{3})O({}_{4}) ions, are formed in glasses.