Vol 61, No 4 (2019)

Based on the nonlinear Schrödinger equation, new types of localized states near the contact of defocusing and self-focusing media with the Kerr nonlinearity separated by a thin defect layer with the similar nonlinearity have been described. The occurrence of the localized states of two types is demonstrated. The dispersion relations between the excitation localization energy and characteristics of the media and defect layer have been obtained. The localization energy levels for particular cases have been obtained in the explicit analytical form.

The electronic and hole states in quantum dots (QD) of cubic II–VI semiconductors with a spheroidal shape and uniaxial anisotropy have been studied theoretically. The smooth potential energy profiles simulated by the Gauss function in all three spatial directions are considered. The energy level lowering and the energy splitting of the hole state from the valence band top Γ₈ by momentum 3/2 into the states with projections ±3/2, ±1/2 on the anisotropy axis are analyzed. The QD anisotropies of three types are considered: the QD size anisotropy, the QD potential barrier anisotropy, and the combined anisotropy. In the first case, flattened quantum dots, in which the characteristic size in the structure plane is larger than the size along the anisotropy axis, are considered. In the second case, QDs, in which the potential barrier height in the plane is lower than that along the anisotropy axis, are considered. In the third case, flattened quantum dots with the anisotropy of the size and the potential barrier are considered. The conditions of the charge carrier localization inside QD have been found, and the influence of the form and composition anisotropies on the energies of exciton transitions in the structures with Cd_xZn_1 – xSe quantum dots are discussed.

The total, bulk, and grain boundary conductivities of proton-conducting zirconates of general formula AZr_0.95Sc_0.05O_3 – α (AZS), where A = Ca, Sr, Ba, are measured in air with different humidity levels. The conductivity is measured using the four-probe dc method (600–900°C) and impedance spectroscopy (30–800°C). The impact of humid atmosphere on the total, bulk, and grain boundary conductivities of the AZS system is studied at different humidity levels: \({{p}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\) = 0.04, 0.61, and 2.5 kPa. Humidity is found to have a considerable effect on the conductivity of our CaZS and BaZS at lower temperatures, suggesting the likelihood of hydronium ion-mediated transport.

Using ab initio technique the physical properties of ScIr₂ superconductor have been investigated with T_c 1.03 K with a MgCu₂-type structure. We have carried out the plane-wave pseudopotential approach within the framework of the first-principles density functional theory (DFT) implemented within the CASTEP code. The calculated structural parameters confirm a good agreement with the experimental and other theoretical results. Using the Voigt-Reuss-Hill (VRH) averaging scheme the most important elastic properties including the bulk modulus B, shear modulus G, Young’s modulus E and Poisson’s ratio ν of ScIr₂ are determined. At ambient condition, the values of Cauchy pressure and Pugh’s ratio exhibit ductile nature of ScIr₂. The electronic and optical properties of ScIr₂ were investigated for the first time. The electronic band structure reveals metallic conductivity and the major contribution comes from Ir-5d states. In the ultraviolet region the reflectivity is high up to 50 eV as evident from the reflectivity spectrum.

Objective factors that allow the experiment on growing crystals of a Ge–Si–Sb solid solution on the Soyuz–Apollo spacecraft to take a special place among many experiments on growing single crystals aboard spacecrafts are analyzed. In the study of crystals grown on board a spacecraft, anomalous and unexpectedly high segregation of the solid solution components in the direction transverse to the direction of crystallization is found. An analysis of the obtained results allowed us, for the first time, to establish a causal relationship between anomalous segregation and microgravity features on board a spacecraft. Determining the physical nature of anomalous segregation have affected the direction of further research and give rise to an in-depth study of specific features of the new technological environment, the rapid development of numerical methods for studying heat and mass transfer processes in melts, and the expanding of the range of studied crystals and methods for studying them. A great contribution to the development of this area was made by Professor V.S. Zemskov.

The variation of a defect structure of a lithium tetraborate single crystal under the influence of a high-strength external electric field applied along polar direction [001] has been studied by the X-ray diffraction (XDR) method. The conductivity kinetics has been measured; it is found to agree with changes in the diffraction peak parameters. Application of the electric field with the strength of 300–500 V/mm leads to a sharp broadening of the rocking curve and the increase in the integral intensity by several times, but the curve position and shape are only slightly changed. At higher electric fields from 500 to 1500 V/mm, the process of broadening the curve slows down; however, the shape asymmetry appears and the peak shifts to smaller angles, which is due to an increase in the lattice parameter along axis c. In this case, the changes become irreversible, since the distorted structure is partially recovered with a very low rate (for several months). Two types of the dependences of the rocking curves parameters variation under an external field are interpreted as the manifestation of two mechanisms of the ionic conduction due to mobile lithium (Li⁺) ions at low fields and oxygen vacancies (\({\text{V}}_{{\text{O}}}^{{2 + }}\)) at higher fields. The charge carrier migration leads to an increase in the defect concentration and structural changes in a near-surface crystal region. The obtained results have practical importance from the point of view of the controlled change in the defect structure in the crystals with ionic conductivity.

The magnetic properties of substitutional solid solutions Eu_0.9Yb_0.1B₆ have been investigated in the temperature range of 2–300 K in fields up to 5 T. The data obtained confirm that the state with the electronic and magnetic phase separation (typical of europium hexaboride) is implemented in Eu_0.9Yb_0.1B₆ when the system metallization (T_M ≈ 15 K) precedes the ferromagnetic ordering (T_C ≈ 11.4 K). An analysis of the curves M(H) in the Belov–Arrott coordinates makes it possible to evaluate spontaneous magnetization M_sp and zero field susceptibility χ₀ and determine the character of their critical behavior near the Curie point. The calculated critical indices (γ ≈ 1.28 and β ≈ 0.34) are in agreement with the predictions of the three-dimensional Heisenberg model.

A trilayer structure model has been investigated, in which uniaxial ferromagnetic layers and thin-film interfaces between them are characterized by the magnetic anisotropy. The magnetization perturbation induced in this structure is described by the nonlinear Schrödinger equation with a nonlinear potential, which simulates the interfaces between ferromagnetic layers. It is shown that, in this structure, two types of nonlinear spin waves can propagate along the layers, which are induced by the nonlinearity of the interfaces caused by their magnetic anisotropy. The frequencies of such localized stationary states of spin waves have been obtained in the explicit analytical form. The conditions for the existence of these states, depending on the characteristics of the layers and interfaces between them, have been established.

The time changes of permittivity, damping and velocity of sound are studied for PbFe_0.5Nb_0.5O₃ and PbFe_0.5Nb_0.5O₃–7PbTiO₃ single crystals in the electric fields of 0 < E < 6 kV/cm. The room-temperature local symmetry of these compounds is shown to be rather more monoclinic than rhombohedral, which is typical of other similar systems. No abrupt sound attenuation anomalies are observed upon the phase transition into the ferroelectric phase in the electric field. The effects seem to be far from the crystal symmetry changes and are attributed to the gradual transition of near-range monoclinic domains into long-range monoclinic domains, as well as to the emergence of the ferroelectric phase. The polarized phase induced in the electric field is only partially stable.

The micro- and nanoreliefs of the loaded surfaces of a Fe₇₇Ni₁Si₉B₁₃ metal glass are probed via scanning tunneling and atomic force microscopies, scanning electron microscopy, and X-ray fluorescence. The fractal characteristics of surfaces are evaluated via the multifractal approach. As found, the variations in a singularity spectrum width, calculated from the tunneling and atomic force microscopies, may be useful for predicting the forthcoming fracture. An increase in the breaking strength of ribbons subjected to hydrostatic compression is due to decreasing microporosity of a near-surface layer, which corresponds to the surface smoothing.

A model and a method are proposed to explain the origin and main specific features of the dynamic patterns of different types, which were previously observed on the free surface of solids under load in experiments. It is shown that the pattern is formed due to the dynamic instability of the flat surface of the solid under load. This instability develops at the relaxation mechanism caused by the dynamic atomic displacements due to a change in the interatomic interaction during the electron-density redistribution.

Electron scattering by short-range defects in planar monolayer graphene is considered. This interaction is approximated by the delta-like potential concentrated on a circumference with a small radius, which provides the suppression of nonphysical shortwave modes. The contribution of this scattering to the graphene resistance is analyzed. The obtained results agree well with the experiment on suspended annealed monolayer graphene. This makes it possible to determine parameters of the approximating potential based on experimental data about the graphene resistivity, which is important for applications.

Polycrystalline samples of La_0.67Sr_0.33Mn_0.65Fe_0.35O₃ (LSMFO) were synthesized using the standard ball mill method with different calcination temperatures ranging from 800 to 1100°C for 7 h. The phase purity of these samples was confirmed using X-ray diffraction (XRD) patterns. All samples were found to have rhombohedral crystal structure with \(R\bar {3}c\) space group. The lattice parameters, cell volume, bond angle and bond length have been obtained using the Rietveld refinement by FullProf software. The average crystallite size calculated using the Debye-Scherrer formula was found between 27 and 60 nm. Surface morphology of the prepared samples has been examined using a scanning electron microscope (SEM). SEM images show the formation of well-arranged grain sizes distributed from 240 to 400 nm, much larger than one estimated using the Scherrer formula. All tiny particles are highly agglomerated with the increasing temperature and porosity decreases with increasing temperature. An analysis of the frequency and peak broadening of Raman modes as a function of temperature clearly shows the significant temperature effect on the A_1g and E_g modes of LSMFO. The shifts and broadening of the A_1g and E_g modes are discussed in light of the oxygen sublattice distortion. Our study shows the reduction in distortion with increasing calcination temperature, which suggests a decrease in the JT effect.

The structure, the photoluminescence, and IR absorption spectra of solid solutions (Lu_1 ‒ xEu_x)₂(MoO₄)₃ have been studied over a wide range of europium concentrations (0 ≤ x ≤ 1). It has been found that there is a correlation between the structure and the spectral characteristics of these compounds. Three types of crystal phases are sequentially changed as the europium concentration increases. The photoluminescence and IR absorption spectra of the monoclinic phase with space group P2₁/a have been studied for the first time. The glow maximum at a resonant excitation of Eu³⁺ ions is shown to be observed in the samples with orthorhombic structure Pba2 at x ~ 0.8. The samples existing in monoclinic P2₁/a phase at x ~ 0.2 demonstrate the maximum luminescence intensity upon excitation in the band with the charge transfer.

Equation of state P(ν/ν_o) and the baric dependences of the lattice and surface properties of silicon macro- and nanocrystals have been calculated using the method of calculation of crystal properties from the pair Mie–Lennard-Jones interatomic potential and the RP-model of nanocrystal. The isothermal dependences of P(ν/ν_o) for the macro- and the nanocrystal are shown to be intersected at a certain value of relative volume (ν/ν_o)₀. The surface pressure becomes zero at the intersection point (at (ν/ν_o)₀). The value of (ν/ν_o)₀ decreases upon isomorphic–isomeric increase in temperature and also at isomorphic–isothermic decrease in the number of atoms N in the nanocrystal, or at isomeric–isothermic deviation of the nanocrystal shape from the most energetically optimal shape (in the RP-model, this shape is a cube). The obtained equation of state is used to study the changes of the silicon properties at isochoric (ν/ν_o = 1) and also isobaric (P = 0) decrease in N at temperatures 300 and 1000 K.

The structure, electric properties, and magnetocaloric effect in Ni₄₇Mn₄₂In₁₁ ferromagnetic alloy in which the martensite transformation temperature is close to room temperature and nearly coincides with the austenite Curie temperature are studied. It is revealed that the spontaneous transformation of martensite to austenite is accompanied with the decrease by 45% in its resistivity. In the martensite transformation induced by a magnetic field, a negative magnetoresistance is observed; it reaches ≈15% in the magnetic field with a strength of 18 kOe. The temperature dependence of the maximum change in the entropy in the martensite transformation induced by the magnetic field was calculated by using the Clausius–Clapeyron equation. It is shown that the maximum values of the magnetoresistance and magnetocaloric effect are observed near the temperature of the spontaneous martensite transformation.

The electrical properties of composite films based on conductive polymer PEDOT: PSS, graphene oxide (GO), and titanium dioxide nanoparticles (TiO₂) (PEDOT: PSS– TiO₂ and GO–TiO₂) used as contact layers of organic and perovskite photovoltaic structures have been studied. As a result of the study of morphology by atomic force microscopy, it was found that the PEDOT: PSS–TiO₂ and GO–TiO₂ films have a globular structure with a grain size of ~200–300 nm. The current–voltage characteristics of the PEDOT: PSS–TiO₂ and GO–TiO₂ films are measured in the temperature range of 80–300 K, the dependences of the resistivity versus temperature, ρ(T), which have an activation character, are obtained. It is established that as the temperature decreases, the ρ(T) dependences show a transition from large values of the activation energy (570 meV and 329 meV) to lower values (25 meV and 2.2 meV) for the PEDOT: PSS–TiO₂ and GO–TiO₂ films, respectively. The mechanisms of transport of charge carriers in the materials studied are discussed.

It has been shown by means of numerical methods, in the framework of the classical Ericksen-Leslie theory, together with accounting the entropy balance equation, how a temperature gradient, in the initially uniformly heated a hybrid-oriented liquid crystal (HOLC) channel, may set up under action of a shear stress (SS). The cases of complete and partial thermal insulation of one of the boundary surfaces of the HOLC are analyzed under the condition that a constant temperature is kept on the rest boundaries. It also has been shown how to heat up the planarly oriented upper boundary of the HOLC channel under the influence of the SS and, thereby, to form the temperature gradient across the HOLC channel.

The first-principle calculations of the atomic and electronic structures of fullerene-like Zn_nSe_n and Cd_nSe_n have been carried out for n = 12, 36, 48, and 60. A model of two-layer fullerene-like (ZnSe)₆₀ and (CdSe)₆₀ clusters with mixed sp²/sp³ bonds has been built for the first time. Ab initio calculations are performed in terms of the electron density functional and the hybrid B3LYP functional theory. The stability and the energy gap width of the clusters are estimated in the dependence on the number of atoms in a cluster and its geometry. It is shown that the relaxation of 1.7–1.8-nm two-layer (ZnSe)₆₀ and (CdSe)₆₀ clusters with mixed sp²/sp³ bonds is accompanied by splitting out of the external layer.

The effect of a weak nonbonded layers interaction on the bending resistance of a multilayered graphene nanoribbon is studied. A numerical simulation of bending of a finite multilayered nanoribbon and analysis of its bending vibrations show that interaction of layers significantly increases bending stiffness normalized to a number of layers only for nanoribbons with length L > 12 nm. The greater the length, the stronger this increase. Thus, at length L = 24 nm, layers interaction increases bending stiffness three times for a two-layer nanoribbon and six times for a nine-layer nanoribbon. Therefore, the use of multilayered nanoribbons can significantly increase the bending resistance of extended nanoconstructions.