Vol 58, No 9 (2016)

Based on the modification of the linear part of the Granato–Lücke dislocation theory of absorption, the equation of state of polycrystalline solids with dissipative and reactive nonlinearity has been derived. The nonlinear effects of the interaction and self-action of longitudinal elastic waves in such media have been theoretically studied.

The effect of external mechanical stresses on the parameters of photoacoustic signals within Vickers indents in steel and nanocopper has been experimentally revealed. It has been shown that changes in photoacoustic signals can be reversible and irreversible, depending on the indent orientation and the stress applied to the sample. In this case, reversible changes can reach significant values at the level of tens of percent of the average signal from the sample. The relative changes in the photoacoustic signal amplitudes have been theoretically evaluated for indented and unindented areas, taking into account the temperature dependence of the elastic modulus of metals. It has been shown that its consideration allows qualitative explanation of the differences in the behavior of photoacoustic signals under stresses in indented and unindented areas.

The effect of hydration at T = 150 and 200°C on the structures of YBa₂Cu₃O_y (123) and a number of binary cuprates has been studied. It has been shown that the compounds containing oxygen vacancies in their structures interact with hydrogen significantly more strongly than cuprates without vacancies. Depending on the cuprate structure, hydrogen can be embedded in interstitial sites with the formation of hydrides and be attached to oxygen with the formation of hydroxides. The phase transition of the 123 phase to a defect tetragonal 124-type phase occurs only for the compounds with a high oxygen content. All the cuprates under study are more stable to reduction as compared to CuO.

The results on the growth of GaInPSbAs isoperiodic solid solutions on indium arsenide substrates from the liquid phase in a field of temperature gradient have been discussed. The heterophase equilibria in the Ga–In–P–Sb–As system have been analyzed in the framework of the regular solution model. The kinetics of the growth, the composition, the structural perfection, and the luminescence properties of Ga_zIn_1–zP_xSbyAs_1–x–y/InAs isoperiodic heterostructures have been investigated.

The photothreshold of a β-GaS layered crystal has been calculated as a function of the thickness of the crystal using the first-principles density functional method. The final thickness of the crystal has been simulated using the periodic plate method. Two adjacent crystal plates consisting of several layers are separated by a vacuum gap with a thickness of two layers, which corresponds to the size of the unit cell of the bulk crystal. It has been shown that, in the crystal with more than 10 layers in thickness, the photothreshold almost ceases to depend on the thickness of the crystal.

The microstructure of lithium fluoride thin films deposited on a glass substrate by thermal vacuum evaporation was investigated. An additional annealing of the films in air at temperatures of 400, 500, and 600°C resulted in an increase in the size of grains forming the film, as well as in the transformation of luminescence spectra of the initial and annealed samples before and after their X-ray irradiation used to generate in the films color centers that played the role of luminescent probes. It was found that an increase in the annealing temperature with an increase in the size of grains leads to a decrease in the intensity of the characteristic photoluminescence bands of color centers in lithium fluoride and to the appearance of new luminescence bands that are not characteristic of this material. Based on the results of the X-ray powder diffraction analysis, it was concluded that the increase in the grain size is caused by the change in the chemical composition of the films primarily due to the interaction of lithium fluoride with the substrate material.

The crystal structure and the magnetic properties of multiferroics Bi_1–xCa_xFe_1–xMn_xO₃ (x ≤ 0.22) have been studied. It has been found that the stoichiometric compositions undergo a crystal-structure transformation from the rhombohedral (space group R3c) polar phase (x ≤ 0.18) to the orthorhombic (space group Pnma) nonpolar phase (x ≥ 0.20) via a two-phase structural state. The polar phase is antiferromagnetic at x < 0.10 and exhibits a metamagnetic behavior. The polar (x ≥ 0.10) and nonpolar phases are weak ferromagnets at room temperature with a spontaneous magnetization close to 0.07 emu/g (x = 0.18 and 0.22). A decrease in temperature leads to the transition to a state close to an antiferromagnetic one.

The crystal structure of solid solutions of an M-type hexagonal barium ferrite BaFe_1–xGa_xO₁₉ (x = 0.1–1.2) with the isostructural diamagnetic substitution of Ga³⁺ ions has been studied by X-ray diffraction. The unit cell parameters have been calculated for all compositions. The field and temperature dependences of the specific magnetization of these solid solutions have been measured by vibrational magnetometry. The microwave properties of BaFe_12–xGa_xO₁₉ (x = 0.1–1.2) have been studied in a bias field. It has been shown that the frequency of natural ferromagnetic resonance decreases as the Ga concentration increases from x = 0.1 to x = 0.6 and it increases once again as the Ga concentration increases to x = 1.2. As the Ga concentration increases, the natural ferromagnetic resonance lines broaden. This indicates an increase in the frequency range of strong absorption of electromagnetic radiation. In this case, the amplitude of the resonance curve peak changes insignificantly. The frequency shift of the natural ferromagnetic resonance in an external magnetic field increases as the gallium ion concentration in the sample decreases.

Changes in the temperatures and entropy of a piezoelectric layer under the electric field and stresses applied to its boundaries (a multicaloric effect) have been investigated. It has been shown that these changes are composed of three terms, which describe the electrocaloric, elastocaloric, and piezoelectrocaloric effects, respectively. If the influence of the strain gradient on the polarization is taken into account, one more term associated with the flexoelectrocaloric effect arises in the entropy change. The influence of the multicaloric effect on losses and motion of domain walls in ferroelectrics has been discussed.

In this work, the semiempirical equation of state of quartz glass and polycrystalline γ-cerium are plotted and verified at the compressive pressures to 3 GPa. The proposed equations are shown to uniformly describe the thermophysical and physicomechanical properties of quartz glass and polycrystalline γ-cerium at their abnormal compressibility in compression. The room isotherms of these materials are discussed, as well.

A theoretical model that effectively describes stress-driven migration of low-angle tilt grain boundaries in nanocomposites with nanocrystalline or ultrafine-grained metallic matrices containing ensembles of coherent nanoinclusions has been developed. Within this model, low-angle tilt boundaries have been considered as walls of edge dislocations that, under the influence of stress, slip in the metallic matrix and can penetrate into nanoinclusions. The dislocation dynamics simulation has revealed three main regimes of the stress-driven migration of low-angle grain boundaries. In the first regime, migrating grain boundaries are completely retarded by nanoinclusions and their migration is quickly terminated, while dislocations forming grain boundaries reach equilibrium positions. In the second regime, some segments of the migrating grain boundaries are pinned by nanoinclusions, whereas the other segments continue to migrate over long distances. In the third regime, all segments of grain boundaries (except for the segments located at the boundaries of inclusions) migrate over long distances. The characteristics of these regimes have been investigated, and the critical shear stresses for transitions between the regimes have been calculated.

Results of the investigation of optical density spectra of Cr₅S₆ single crystals in the infrared region have been presented. A comparative analysis of the temperature dependences of the magnetization and the light absorption has been performed. Physical mechanisms to explain the specific features of the spectral and temperature dependences of the optical density have been proposed.

The continual approximation of the ground state of the discrete Frenkel–Kontorova model is tested using a symmetric algorithm of numerical simulation. A “kaleidoscope effect” is found, which means that the curves representing the dependences of the relative extension of an N-atom chain vary periodically with increasing N. Stairs of structural transitions for N ≫ 1 are analyzed by the channel selection method with the approximation N = ∞. Images of commensurable and incommensurable structures are constructed. The commensurable–incommensurable phase transitions are stepwise.

Mechanical properties and stability of two layers of defect silicene supported by graphene sheets, between which the lithium ion passes under an electrostatic field, are studied by the molecular dynamics method. Defects are mono-, di-, tri-, and hexavacansies. Graphene and silicene edges are rigidly fixed. Graphene sheets contacting with silicene take a convex shape, deflecting outward. Mono- and divacancies in silicene tend to a size decrease; larger vacancies exhibit better stability. The ion motion control using an electric field becomes possible only using perfect silicene or silicene with mono- and divacancies. The ion penetrated through larger defects, and its motion in the silicene channel becomes uncontrolled. When the ion moves in the channel, the most strong stress spikes appear in silicene containing monovacancies. In the case of fixed edges, perfect silicene intercalated with a lithium ion is inclined to accumulate larger stresses than silicene containing defects.

Nanostructure Nd–Ho–Fe–Co–B alloys have been probed via atomic force microscopy and magnetic force microscopy (AFM and MFM, respectively). The ribbon samples with a thickness of ~30 μm are prepared via the rapid solidification on a rotating copper barrel. A part of samples has been subjected to hydration, whereas another one has undergone severe plastic deformation. AFM was mainly used to study the contact and free surface of ribbon samples. This has enabled us to establish the topography, structure, defects of both sides, morphology of magnetic inclusions of the initial quenched samples and the materials subjected to the subsequent external effects. The AFM and MFM data allowed the magnetic hysteresis properties of the bulk samples with the identical composition to be interpreted.

Well-textured gallium oxide β-Ga₂O₃ layers with a thickness of ~1 μm and a close to epitaxial layer structure were grown by the method of chloride vapor phase epitaxy on Si(111) wafers with a nano-SiC buffer layer. In order to improve the growth, a high-quality silicon carbide buffer layer ~100 nm thick was preliminarily synthesized by the substitution of atoms on the silicon surface. The β-Ga₂O₃ films were thoroughly investigated using reflection high-energy electron diffraction, ellipsometry, X-ray diffraction, scanning electron microscopy, and micro-Raman spectroscopy. The investigations revealed that the films are textured with a close to epitaxial structure and consist of a pure β-phase Ga₂O₃ with the (\(\overline 2 01\)) orientation. The dependence of the dielectric constant of epitaxial β-Ga₂O₃ on the photon energy ranging from 0.7 to 6.5 eV in the isotropic approximation was measured.

Organic field-effect transistor (OFET) structures with the active layers on the basis of composite films of semiconductor polymer poly(3-hexylthiophene) (P3HT), fullerene derivatives [60]PCBM, [70]PCBM, and nickel (Ni) nanoparticles are obtained, and their optical, electrical, and photoelectrical properties are studied. It is shown that introducing Ni nanoparticles into P3HT: [60]PCBM and P3HT: [70]PCBM films leads to an increase in the absorption and to quenching of photoluminescence of the composite in the 400–600 nm spectral band due to the plasmon effect. In P3HT: [60]PCBM: Ni and P3HT: [70]PCBM: Ni OFET structures at the P3HT: [60]PCBM and P3HT: [70]PCBM concentrations of ~1: 1 and Ni concentrations of ~3–5 wt %, current–voltage (I–V) characteristics typical of ambipolar OFETs with the dominant hole conduction are observed. The charge-carrier (hole) mobilities calculated from the I–V characteristic at V_G =–10 V were found to be ~0.46 cm²/(V s) for P3HT: [60]PCBM: Ni and ~4.7 cm²/(V s) for P3HT: [70]PCBM: Ni, which means that the mobility increases if [60]PCBM in the composition is replaced with [70]PCBM. The effect of light on the I–V characteristics of P3HT: [60]PCBM: Ni and P3HT: [70]PCBM: Ni OFETs is studied.

The results of the investigation of the electronic structure of the conduction band and the interfacial potential barrier during the formation of interfaces of dioctyl-substituted perylenedicarboximide (PTCDI-C₈) and diphenyl-substituted perylenedicarboximide (PTCDI-Ph) ultrathin films with the oxidized germanium surface have been presented. The experimental results have been obtained using the very low energy electron diffraction (VLEED) technique in the total current spectroscopy (TCS) mode at energies in the range from 5 to 20 eV above the Fermi level E_F. The positions of the maxima of the fine structure of total current spectra (FSTCS) of the PTCDI-C₈ and PTCDI-Ph films differ significantly in the energy range from 9 to 20 eV above the Fermi level E_F, which can be associated with the difference between the substituents of the chosen molecules, dioctyl- and diphenyl-, respectively. At the same time, the positions of the lowenergy maxima in the FSTCS spectra at an energy 6–7 eV above the Fermi level E_F for the PTCDI-C₈ and PTCDI-Ph films almost coincide with each other. It has been suggested that these maxima are attributed to the electronic states of the perylene core of the molecules under investigation. The process of the formation of interfacial potential barriers of the PTCDI-C₈ and PTCDI-Ph films with the oxidized germanium surface has been analyzed. It has been found that the work functions of the surface, E_vac–E_F, differ little from 4.6 ± 0.1 eV over the entire range of organic coating thicknesses from 0 to 6 nm.

The orientational phases in an antiferromagnetic liquid crystal (ferronematic) based on the nematic liquid crystal with the negative anisotropy of diamagnetic susceptibility are studied in the framework of the continuum theory. The ferronematic was assumed to be compensated; i.e., in zero field, impurity ferroparticles with the magnetic moments directed parallel and antiparallel to the director are equiprobably distributed in it. It is established that under the action of a magnetic field the ferronematic undergoes orientational transitions compensated (antiferromagnetic) phase–non-uniform phase–saturation (ferrimagnetic) phase. The analytical expressions for threshold fields of the transitions as functions of material parameters are obtained. It is shown that with increasing magnetic impurity segregation parameter, the threshold fields of the transitions significantly decrease. The bifurcation diagram of the ferronematic orientational phases is built in terms of the energy of anchoring of magnetic particles with the liquid-crystal matrix and magnetic field. It is established that the Freedericksz transition is the second-order phase transition, while the transition to the saturation state can be second- or first-order. In the latter case, the suspension exhibits orientational bistability. The orientational and magnetooptical properties of the ferronematic in different applied magnetic fields are studied.

The formation of endohedral metallofullerene clusters with Y, Gd, and Ho in an N,N-dimethylformamide solution and on a mica substrate surface has been investigated using static and dynamic light scattering and atomic force microscopy, respectively. It has been found that the size distribution of the clusters depends on the concentration of endohedral metallofullerenes and on the exposure time of the solution. It has been shown that the clusters are resistant to high temperatures and ultrasound effects. The concentration of endohedral metallofullerenes at which only single clusters are formed in the solutions has been determined. It has been established that an increase in the concentration of endohedral metallofullerenes leads to the agglomeration of single clusters. The fractal dimension has been estimated, and the zeta potential of endohedral metallofullerene clusters has been measured.

The structural and the thermodynamic properties of potassium nitrate KNO₃ and its composites with nanosized aluminum oxide Al₂O₃ have been studied by differential scanning calorimetry. It has been found that an amorphous phase forms in composites (1–x)KNO_3–xAl₂O₃. The thermal effect corresponding to this phase has been observed at 316°C. It has been found that the phase transition heats of potassium nitrate decreased as the aluminum oxide fraction increased.