Volume 61, Nº 11 (2019)

Based on the solution of the hydrodynamic equations and Maxwell’s equations, we show that an external quasi-homogeneous magnetic field leads to the emergence of a secondary electric field that is resulted from a nonlinear effect over magnetic potential A. This field is proved to exist in the region with a depth of \(\delta {\text{/}}2\), where δ is the London penetration depth. The hydrodynamic flow velocity is estimated.

Based on density functional theory, structural, electronic, and thermoelectric properties of TiXSb (X = Si, Ge) at pressures of 0, 5, 10, and 15 GPa have been investigated. Structural properties at 0 GPa are in accord with other theoretical and experimental works. In electronic properties at 0 GPa, Ti-d² orbitals have main contributions near the Fermi energy in valence band and in conduction band. According to our calculations, as shown in diagrams of electronic density of states at different pressures, peaks in the valence band move to more negative energies when the pressure increases. However, in the conduction band, they move to more positive energies. This occurs as a result of decreasing stability of the system due to increase in pressure. In this study, we also calculated the thermoelectric properties such as Seebeck coefficient, thermal conductivity divided by relaxation time, electrical conductivity divided by relaxation time, and figure of merit, at pressures of 0, 5, 10, and 15 GPa, in the temperature range of 100–900 K (contribution of phononic thermal conductivity was ignored as low).

Problems of the dipole ordering and ionic conductivity of Na₃Sc₂(PO₄)₃ polycrystal that has three polymorphic phases—α, β, and γ—are studied. The features of the α-Na₃Sc₂(PO₄)₃ crystal structure and the dipole ordering and relaxation polarization in the α and β phases are specified. The origin of the dipole ordering in the α phase and the partial dipole disordering in the β-Na₃Sc₂(PO₄)₃, as well as the high ionic conductivity in the β and γ phases of the Na₃Sc₂(PO₄)₃ polycrystal are associated with phase transformations α → β, β → γ that result in structural changes of the rhombohedral {[Sc₂(PO₄)₃]^3–}_3∞ crystalline framework. A model explaining the emergence of the dipole ordering and the ionic conductivity phenomena in Na₃Sc₂(PO₄) is proposed.

The frequency dependences of the real (ε') and imaginary (ε'') parts of the complex dielectric permittivity, the dielectric loss tangent (tanδ), and the ac conductivity (σ_ac) in the frequency range f = 5 × 10⁴–3.5 × 10⁷ Hz have been studied in the TlIn_1 – xEr_xS₂ (0 ≤ x ≤ 0.01) crystals synthesized in this work. It is found that, in TlIn_1 – xEr_xS₂, the relaxation dispersions of ε' and ε'' take place. The effect of the erbium concentration (Er) in the TlIn_1 – xEr_xS₂ crystals on their dielectric coefficients has been studied. At high frequencies, the ac conductivity of the TlIn_1 – xEr_xS₂ crystals obeys the relationship σ_ac ~ f ^0.8, which is characteristic of the hopping mechanism of the charge transfer over states localized near the Fermi level. The parameters of the states localized in the forbidden band of TlIn_1 – xEr_xS₂ and the influence of the chemical composition of the crystals on these parameters are estimated.

Low-temperature luminescence spectra of Cu₂O crystals grown by different methods (copper oxidation, induction charge melting, magnetron sputtering, and hydrothermal), as well as natural crystals, have been compared. It is suggested that the high quality of natural crystals and crystals grown by the hydrothermal method is due to the fact that they grow for a long time at relatively low temperatures, which facilitates the minimization of stress.

A crystallophysical model of ion transfer in the superionic Pb_1 – xSn_xF₂ conductor with a fluorite (CaF₂) structure is proposed. The concentration dependence of the ionic conductivity of Pb_1 – xSn_xF₂ single crystals and poly- and nanocrystals is analyzed. The single-crystal form of the superionic conductor is characterized by the highest conductivity. The mobility and concentration of anionic charge carriers in a single crystal and ceramics of Pb_1 – xSn_xF₂ (x = 0.2) is calculated on the basis of structural and electrophysical data. The mobility of carriers μ_mob = 2.5 × 10^–6 cm²/s V (at 293 K) in a single crystal is seven times higher than in nanoceramic. The concentration of carriers n_mob = 1.7 × 10²¹ and 3.6 × 10²¹ cm³ (4.5 and 9.5% of the total number of anions) for a single crystal and nanoceramic, respectively. The comparison of isostructural Pb_0.8Sn_0.2F₂, Pb_0.67Cd_0.33F₂, and Pb_0.9Sc_0.1F_2.1 single crystals shows that anionic carriers have a maximum mobility in the β-PbF₂ and SnF₂ based solid solution.

The observed specific features of the orbital phase transition in AMnO₃ (A = La, Pr, …, Lu) are shown to be due to the appearance, at high temperatures, of rotational distortions of the cubic lattice, which cause anion displacements, which induce an “internal field” coinciding in symmetry with the order parameter, which describe the ordering of octahedra distorted due to the Jahn–Teller effect (orbital ordering). The effect of this “field” substantially determines the orbital ordering type, its temperature, and the phase transition order. The comparative analysis of the orbitally ordered states is performed for the entire AMnO₃ series in both the framework of the phenomenological theory of phase transitions and on the base of the three-minimum multi-sublattice model proposed in this report. The peculiarities of magnetic transitions in manganites determined by the specific features of their structures are discussed.

Using three-dimensional micromagnetic simulation, the dynamic processes occurring in a domain wall (DW) that moves under the influence of a constant magnetic field in a magnetically soft uniaxial film with planar anisotropy are studied. It is shown that the general picture of the dynamics of topological transformations taking place in DWs can significantly vary with the introduction of perturbing factors that randomly depend on the coordinates or time (spatially heterogeneous anisotropy; a magnetic field fluctuating in time). Moreover, the characteristic configurations of a vector magnetization field near the intrafilm vortex cores and singular (Bloch) points preserve their shape. The dependences of the energy and DW shifts on the time are determined for a wide range of film thickness values, temperatures, and magnetic field intensities. Imaging techniques based on calculating two types of topological charges are used when analyzing magnetization configurations.

In this paper, we obtained α-Fe(50 at %)(PrDy)(FeCo)B(48 at %) microwires by extraction of a hanging drop of the (PrDy)(FeCo)B melt in the electron beam. A single microwire with a diameter of 50 μm and a length of 0.8–6 mm and with an amorphous (PrDy)(FeCo)B content of ~48% and a polycrystalline α‑Fe phase of ~52% is shown to have a rectangular narrow magnetic hysteresis loop and, accordingly, a bistable state with a switching field of ~100 Oe. Shortening the wire to ~0.6 mm leads to a sharp deviation from the loop rectangularity and a decrease in the slope of the dependence of magnetization on the field and coercive force to 20 Oe. In the near-surface layers consisting of an amorphous phase (PrDy)(FeCo)B, oriented regions of reverse magnetization are observed. The role of the magnetic dipole interaction in the formation of the magnetic hysteresis loop of chaotic ensembles of microwires with various compositions is discussed.

This article describes the experimental and theoretical studies of the mechanisms of failure of ABS polymer in the range of strain rates of 10^–4–10³ s^–1 at different temperatures of testing (from an ambient temperature of 23 to 100°C). The alteration of the type of failure as a function of the strain rate and temperature is analyzed. Based on the structure–time approach, a model of the temperature–rate superposition is proposed.

It has been shown that the nonequilibrium correlations in hopping conductivity can be taken into account by generalizing the Miller–Abrahams network of resistors. The resistances of resistors between nodes in such a network are related to their environment. Moreover, some nonlocal elements exist, which create the voltage on one resistor proportional to the current through the other resistor. These elements are required to make controllable the approximation used to construct the network. The greatest change in the resistance takes place in the resistors, whose resistance is lower than the critical resistance of the percolation theory.

The bra + ket quantum language, introduced earlier by the authors to explain quantum correlation at macroscopic distances, makes it possible to understand the physical mechanism of interference of individual quantum particles without the idea of the transformation of a wave into a particle and vice versa, as well as to avoid claims of retrocausality, nonlocality, instantaneous interaction, and other similar phenomena supposedly inherent in quantum mechanics. Within the bra + ket approach, double- and triple-slit experiments, delayed choice experiments, and some more complex interference experiments with variable parameters were considered, the results of which has provoked a lively discussion over many years with opposite conclusions about particle trajectories in interferometers.

The structure, luminescence, and IR absorption spectra of (Lu_1 – xEu_x)₂(WO₄)₃ solid solutions are studied in a wide range of europium concentrations (0 ≤ x ≤ 1). As the europium concentration increases, two types of crystalline phases sequentially alternate. An orthorhombic phase (space group Pbcn) of tungstate solid solutions is observed at 0 ≤ x < 0.5. In the range 0.5 ≤ x ≤ 0.8, along with the orthorhombic phase, a monoclinic phase (space group C2/c) appears; at x > 0.8, the solid solution has a monoclinic structure. The correspondence between the structure and spectral characteristics of these compounds is found. A change in the structure state leads to not only changes in luminescence spectra, but also in tungstate luminescence excitation spectra. In connection with the hydrophilicity of (Lu_1 – xEu_x)₂(WO₄)₃ solid solutions, the water adsorption isotherm for tungstate samples and its effect on their spectral and structural characteristics is studied. It is found that the luminescence maximum under resonant excitation of Eu³⁺ ions is observed in samples with monoclinic structure C2/c at x ~ 0.9.

ZnO together with TiO₂ is a main photocatalyst for various redox reactions to convert light energy into a chemical one and to purify the environment. Intrinsic surface defects in ZnO—the vacancies in anionic and cationic sublattices (F-type and V-type centers)—allow creation of long-lived (up to 10³ s) photocatalysis centers and, therefore, tenfold increase in quantum yield of reactions. Slow surface states—the photocatalysis centers—appear via diffusion of electrons and holes generated during the interband transitions in the bulk of a photoactivated sample. The transfer efficiency, however, decreases sharply because of recombination of charge carriers and losses during overcoming the surface Schottky barrier. Neutral energy carriers—excitons—were used in this work to decrease these losses during the energy transfer to a surface. High exciton binding energy in ZnO (60 meV) allows it to move at room temperature without decay. The exciton energy loss for radiation is effectively decreased in our experiments via formation of a 2D surface structure. The results confirm high efficiency of exciton channel to form surface long-lived photocatalysis F-centers and V‑centers during the photoadsorption and photodesorption processes of oxygen, which simulate full cycle of a redox photocatalytic reaction.

The dynamics of a three-component nonlinear delocalized vibrational mode in graphene is studied with molecular dynamics. This mode, being a superposition of a root and two one-component modes, is an exact and symmetrically determined solution of nonlinear equations of motion of carbon atoms. The dependences of a frequency, energy per atom, and average stresses over a period that appeared in graphene are calculated as a function of amplitude of a root mode. We showed that the vibrations become periodic with certain amplitudes of three component modes, and the vibrations of one-component modes are close to periodic one and have a frequency twice the frequency of a root mode, which is noticeably higher than the upper boundary of a spectrum of low-amplitude vibrations of a graphene lattice. The data obtained expand our understanding of nonlinear vibrations of graphene lattice.

The lattice structure and dynamics of single-crystal Bi₄Ti₃O₁₂ with a thickness from 4 to 430 nm on a (001)MgO substrate with a previously deposited Ba_0.4Sr_0.6TiO₃ sublayer (4 nm) have been studied. The two-layer structures were prepared by radio-frequency sputtering of ceramic targets of the corresponding compositions. The X-ray diffraction studies performed at room temperature have shown that, in this heterostructure, axis c of the Bi₄Ti₃O₁₂ unit cell is perpendicular to the substrate, and direction [100] has an angle of ±45° to the [100]MgO direction. At the thicknesses of Bi₄Ti₃O₁₂ to ~40 nm, the film unit cell is compressed in the direction of a normal to the substrate plane and extended in the conjugate plane, and the sign of the deformation is changed at large thicknesses. It is found that the phonon mode frequency in the Bi₄Ti₃O₁₂ film shift and additional peaks appear in the Raman spectra, which demonstrates an increase in the degree of monoclinic distortion of the film crystal structure as compared to the crystal structure.

The independent components of the tensor of piezoelectric moduli are calculated for various 2D nanoallotropes of boron nitride. The principle of the proposed approximation calculation method consists in the fact that the effective dipole moment of the unit cell of the 2D structure normalized to the unit area is expressed through the tensor of elastic rigidities and relative cell deformations. It is shown that, in addition to well-known graphite-like boron nitride h-BN, its other hexagonal and tetragonal nanoallotropes possessing higher piezoelectric properties in comparison to h-BN can be of practical interest as well.

Temperature dependences of longitudinal sound wave velocities and internal friction of ferromagnetic La_0.7 – yPr_yCa_0.3MnO₃ single crystals (0 ≤ y ≤ 0.3) with the first-order magnetic phase transition have been analyzed. In the paramagnetic region, the temperature dependences of the sound velocity and internal friction exhibit extended temperature hysteresis, which is indicative of inhomogeneity of the paramagnetic state. Structural phase stratification is likely the main reason for the inhomogeneous paramagnetic state of La_0.7 – yPr_yCa_0.3MnO₃ single crystals (0 ≤ y ≤ 0.3).

Results of the study of the effect of various types of plastic deformation on microstructural features and change in physical and mechanical properties of the nonstoichiometric Heusler alloy Ni₄₇Mn₄₂In₁₁ are shown. It was demonstrated that the deformation by rolling and upsetting leads to an increase in the microhardness and to an embrittlement of the investigated alloy. Severe plastic deformation by torsion under high pressure of 8 GPa at room temperature was found to strongly refine initially coarse grain and to contribute to the formation of a nanocrystalline structure with grain fragments up to 10 nm. In this case, the fraction of viscous constituent on the fracture and the microhardness increased, while the magnetic susceptibility decreased.

Two-dimensional hexagonal boron nitride (h-BN) as a graphene-like material was investigated due to its impending applications in electronics. The h-BN band gap E_g as an important factor and its variation between bilayer ZrSe₂ sheets were explored under an external electric field. The initially indirect band gap is found to convert to direct band gap by means of density functional theory. Additionally, the band gap is modulated by van der Waals corrections from 0.21220 to 0.01770 eV. Based on the results, the proposed heterostructure is converted to the direct band gap, and band gap smoothly decreased from 0.25440 to 0.0436 eV following the application of external electric field from 0.2 to 0.6 eV. Moreover, ZrSe₂|h-BN|ZrSe₂ is investigated under the applied biaxial compressive strain from 1 to 4%. The findings demonstrated that the gap was decreased by any compressive strain amplification, while the semiconducting behavior in the heterostructure attained to the semi-metallic performance under the increasing strain.

Excitons and trions in bilayer structures based on transition metal dichalcogenides are theoretically studied. Expressions for the effective potential of interparticle interaction in such a system with allowance for a significant dielectric contrast in the heterostructure are obtained. Simple and physically justified variational functions for electron–hole complexes are proposed. A variational calculation of the exciton and trion binding energies as functions of the interlayer distance, taking into account the specific features of the screening of the Coulomb interaction, is performed. The accuracy of the variational approach is confirmed by direct numerical calculation of the exciton binding energy as a function of the interlayer distance.

Thin carbon films prepared by pulsed plasma ion-assisted deposition of graphite in an atmosphere of a mixture of argon and nitrogen are studied. The results of characteristic electron energy loss spectroscopy and electron diffraction indicate the increase in the graphite component with increasing ion assistance energy. The use of ion assistance during the film deposition makes it possible to control their resistivity by changing it from 10⁵ to 10² Ω cm.

A new electron donating polymer poly(2-(cyclopent-2-enyl)aniline) is synthesized, and synthesis parameters such as monomer-to-oxidant molar ratio and reaction duration are optimized. The separability of the target product (i.e., functionalized polyaniline) and its solubility in a series of conventional organic solvents with different polarity are estimated. The synthesized high-molecular weight product is studied by thermogravimetry analysis, the temperature dependence of its electrical conductivity is investigated, and the charge carrier mobilities are calculated.

Stationary states of multi-walled carbon nanotubes have been studied. The numerical simulation showed that nanotubes become bistable systems at rather large diameters. They can be in two stable states in spite of the interaction with substrates: in an open state with a hollow internal cavity and in a collapsed state. The interaction with a plane substrate leads to the nanotube flattening, making its transition to a collapsed state more energetically preferable (the stronger the interaction, the more preferable is the transition). It is shown that a change in the multilayer nanotube shape due to its interaction with the substrate or due to its collapse leads to a sharp increase in the number of its collective eigenmodes, in which all its layers take part. The energy profile of the transition is found between the steady states of the nanotube; the profile is a strongly asymmetric double-well potential with the first narrow minimum, which corresponds to the collapsed state and the second broad minimum, which corresponds to the nanotube open state.