Vol 58, No 5 (2016)

Optical properties of semiconductor crystals in the presence of a high magnetic field have been considered. The field turn-on gives rise to oscillations of the optical-absorption edge or, more specifically, the formation of a complex absorption spectrum with a periodic structure, referred to as the spectrum of “diamagnetic excitons.” Such spectra appear a source of the most accurate knowledge about the band structure of semiconductors. Moreover, these spectra can be used for simulating the low-dimensional state in semiconductors and possible interpretation of the emission spectra of neutron stars. The proposed analytical review is based on extensive experimental and theoretical data contained mostly in cited original works of the author with colleagues.

The inelastic electron scattering cross section spectra of Fe have been calculated based on experimental spectra of characteristic reflection electron energy loss as dependences of the product of the inelastic mean free path by the differential inelastic electron scattering cross section on the electron energy loss. It has been shown that the inelastic electron scattering cross-section spectra have certain advantages over the electron energy loss spectra in the analysis of the interaction of electrons with substance. The peaks of energy loss in the spectra of characteristic electron energy loss and inelastic electron scattering cross sections have been determined from the integral and differential spectra. It has been shown that the energy of the bulk plasmon is practically independent of the energy of primary electrons in the characteristic electron energy loss spectra and monotonically increases with increasing energy of primary electrons in the inelastic electron scattering cross-section spectra. The variation in the maximum energy of the inelastic electron scattering cross-section spectra is caused by the redistribution of intensities over the peaks of losses due to various excitations. The inelastic electron scattering cross-section spectra have been analyzed using the decomposition of the spectra into peaks of the energy loss. This method has been used for the quantitative estimation of the contributions from different energy loss processes to the inelastic electron scattering cross-section spectra of Fe and for the determination of the nature of the energy loss peaks.

The temperature dependences of the electrical resistivity of samarium monosulfide single-crystal samples subjected to chemical treatment to remove a metallic phase from their surfaces have been measured in the range of 1.5–400 K at atmospheric pressure and at a pressure of 0.3 GPa. The temperature dependences of the activation energy of conduction electrons at these pressures and the piezoresistance coefficient of uniform compression have been calculated. It has been shown that the known model of the structure of the impurity-level spectrum in SmS remains partially valid at temperatures higher than 15 K. At lower temperatures, the existence of shallow donor centers in SmS and the hopping conduction over them should be taken into account.

The phase stability, electronic structure, and magnetic properties of Al_1–xTi_xN compositions based on the metastable aluminum nitride modification with the rock-salt structure at low (x = 0.03) and high (x = 0.25) concentrations of titanium in the system have been investigated using the results of ab initio band calculations. It has been shown that, at low values of x, the partial substitution is characterized by a positive enthalpy, which, however, changes sign with an increase in the titanium concentration. According to the results of the band structure calculations, the doped compositions have electronic conductivity. For x = 0.03, titanium impurity atoms have local magnetic moments (∼0.6 μ_B), and the electronic spectrum is characterized by a 100% spin polarization of near-Fermi states. Some of the specific features of the chemical bonding in Al_1–xTi_xN cubic phases have been considered.

This paper reports on the results of the experimental investigation of unipolar (diode) current–voltage characteristics of local regions in high-resistance SrTiO₃ crystals that experienced a light-induced drop in electrical resistance. This behavior has been explained by the influence exerted on the electrical conductivity by the irradiated region in the Schottky barrier of one of the contacts. The ideality factor of the Schottky barrier has been determined and the barrier height for a number of regions has been estimated from measurements of the forward branch of the current–voltage characteristics. An analysis of the specific features in the behavior of the reverse branch of the current–voltage characteristics has revealed that, in the SrTiO₃ crystals with p-type conductivity, the resistance switching occurs through a pure electronic mechanism, in contrast to models based on electrochemical processes, in particular, the migration of oxygen vacancies.

Films of composites (Co₄₅Fe₄₅Zr₁₀)_x(Al₂O₃)_100–x, (Co₈₄Nb₁₄Ta₂)_x(SiO₂)_100–x, (Co₄₁Fe₃₉B₂₀)_x(SiO₂)_100–x and multilayer heterogeneous composite–composite structures {[(Co₄₅Fe₄₅Zr₁₀)_x(Al₂O₃)_100–x]/[(Co₄₅Fe₄₅Zr₁₀)_x(Al₂O₃)_100–x + N₂]}_n, {[(Co₄₅Fe₄₅Zr₁₀)_x(Al₂O₃)_100–x]/[(Co₄₅Fe₄₅Zr₁₀)_x(Al₂O₃)_100–x + O₂]}_n, {[(Co₄₁Fe₃₉B₂₀)_x(SiO₂)_100–x]/[(Co₄₁Fe₃₉B₂₀)_x(SiO₂)_100–x + O₂]}_n, and {[(Co₈₄Nb₁₄Ta₂)_x(SiO₂)_100–x]/[(Co₈₄Nb₁₄Ta₂)_x(SiO₂)_100–x + O₂]}_n have been deposited using the ionbeam sputtering method with a cyclic supply of reaction gases during deposition. The structure and magnetic properties of the films have been studied. It has been shown that the introduction of an oxidized interlayer makes it possible to suppress the perpendicular magnetic anisotropy in the (Co₄₅Fe₄₅Zr₁₀)_x(Al₂O₃)_100–x composite with the metallic phase concentration higher than the percolation threshold.

Spectra of absorption, magnetic circular dichroism, and natural circular dichroism of the f–f transitions ⁴I_9/2 → ⁴F_3/2, ²H_9/2 + ⁴F_5/2, ⁴S_3/2 + ⁴F_7/2, ²G_7/2 + ⁴G_5/2, ²K_13/2 + ⁴G_7/2, and ⁴G_9/2 in the Nd³⁺ ions in the Nd_0.5Gd_0.5Fe₃(BO₃)₄ crystal have been measured as a function of the temperature in the interval of 90–300 K. Temperature dependences of the magneto-optical activity (MOA) and natural optical activity (NOA) of the transitions have been obtained. It has been found that, in contrast to allowed transitions, the temperature dependence of the MOA of the f–f transitions does not obey the Curie–Weiss law and the NOA depends on temperature. The NOA of some transitions changes the sign with variation in temperature. These phenomena have been explained by the presence of three contributions to the allowance of the f–f transitions, which lead to three contributions of different signs to the MOA and NOA. The range of the MOA of the f–f transitions in the Nd³⁺ ion has been predicted theoretically and confirmed experimentally.

This paper presents the results of an investigation of changes in the structure, magnetic, electrical, and thermal properties of a nanostructured bismuth ferrite powder prepared by the combustion of nitrateorganic precursors before and after heat treatment at temperatures of 500, 600, 700, and 800°C. It has been shown that there is a dependence of the magnetic properties on the dispersion of the particles. The specific features of the temperature and frequency dependences of the dielectric properties over wide ranges of frequencies and temperatures, as well as near the Néel temperature, have been considered.

The elastic interaction of two point defects in cubic and hexagonal structures has been considered. On the basis of the exact expression for the tensor Green’s function of the elastic field obtained by the Lifschitz–Rozentsveig for a hexagonal medium, an exact formula for the interaction energy of two point defects has been obtained. The solution is represented as a function of the angle of their relative position on the example of semiconductors such as III-nitrides and α-SiC. For the cubic medium, the solution is found on the basis of the Lifschitz–Rozentsveig Green’s tensors corrected by Ostapchuk, in the weak-anisotropy approximation. It is proven that the calculation of the interaction energy by the original Lifschitz–Rozentsveig Green’s tensor leads to the opposite sign of the energy. On the example of the silicon crystal, the approximate solution is compared with the numerical solution, which is represented as an approximation by a series of spherical harmonics. The range of applicability of the continual approach is estimated by the quantum mechanical calculation of the lattice Green’s function.

The crystal structures of solid solutions of hexagonal ferrites BaFe_12–xAl_xO₁₉ (x = 0.1–1.2) have been studied using X-ray diffraction. It has been found that, at normal conditions, the samples are characterized by space group P6₃/mmc. It has been noted that the diamagnetic substitution of Al³⁺ ions for Fe³⁺ leads to a decrease in the unit cell parameters due to the smaller aluminum ion radius. The field dependences of the specific magnetic moment have been studied in fields of ±2 T at 5 and 300 K using vibrating magnetometry. It has been found that the specific magnetic moment decreases from 49.6 emu/g (x = 0.1) to 32 emu/g (x = 1.2) as the concentration of the diamagnetic ions increases. The microstructure has been studied using scanning electron microscopy. The Raman spectra have been measured in the range of 200–800 cm^–1.

The specific features of changes in the electronic structure of multi-walled carbon nanotubes (MWCNTs) due to the interaction with an amorphous tin oxide in the SnO_x/MWCNT composite formed by magnetron sputtering have been investigated using X-ray spectroscopy. It has been shown that the formation of chemical bonds responsible for significant changes in the local and electronic structures of the outer layers of MWCNTs occurs at the boundaries of the “amorphous oxide/MWCNT” contacts. The vacuum annealing of the composite leads to the disturbance of the chemical interaction at interfaces of the composite and to a partial recovery of the local structure of the outer layers of MWCNTs. A decrease in the amount of oxygen in the tin oxide under vacuum annealing conditions causes an increase in the number of unpaired Sn 5s electrons, which, in turn, enhances the charge transfer through the interfaces in the composite and leads to a splitting of the π^*-subsystem of the outer layers of MWCNTs.

A continuous model has been constructed for low-frequency dynamics of a double-walled carbon nanotube. The formation of the low-frequency part of the phonon spectrum of a double-walled nanotube from phonon spectra of its constituent single-walled nanotubes has been considered in the framework of the proposed approach. The influence of the environment on the phonon spectrum of a single double-walled carbon nanotube has been analyzed. A combined method has been proposed for estimating the coefficients of the van der Waals interaction between the walls of the nanotube from the spectroscopic data and the known values of the elastic moduli of graphite. The low-temperature specific heat has been calculated for doublewalled carbon nanotubes, which in the field of applicability of the model (T < 35 K) is substantially less than the sum of specific heats of two individual single-walled nanotubes forming it.

The interatomic interaction and phase formation at interfaces between the metallic layers Co₄₅Fe₄₅Zr₁₀ and nonmetallic interlayers of amorphous silicon or silicon dioxide in multilayered nanostructures (Co₄₅Fe₄₅Zr₁₀/a-Si)₄₀ and (Co₄₅Fe₄₅Zr₁₀/SiO₂)₃₂ have been investigated using ultrasoft X-ray emission spectroscopy (USXES) and X-ray diffractometry. The multilayered nanostructures have been fabricated by ion-beam sputtering of two targets onto the surface of a rotating glass-ceramic substrate. The investigations have demonstrated that, regardless of the expected composition of the interlayer (amorphous silicon or silicon dioxide), d-metal silicides, predominantly lower cobalt silicides, are formed at the metallic layer/interlayer interface. However, in this case, the thickness of silicide interfaces in the multilayered nanostructures with oxide interlayers (series O) has a significantly lower value of ∼0.1 nm, and, therefore, the central layer of the interlayers remains oxide. In the multilayered nanostructures with amorphous silicon interlayers almost all silicon is consumed in the formation of nonmagnetic silicide phases. When the thickness of this interlayer exceeds the thickness of the metallic layer, the multilayered nanostructures become nonmagnetic.

Three-layer Co/Ge/Co films have been studied using electron magnetic resonance. It has been established that the resonance spectrum of the film is a superposition of two Lorentzian lines. It has been found that the anisotropy induced at the cobalt‒germanium interface makes the main contribution to the resonance spectrum and determines its features. The temperature dependences of the anisotropy field and the parameters of the interlayer exchange have been measured. The interlayer interactions exhibit an antiferromagnetic character and have been explained in terms of a model similar to the description of superexchange in magnetic dielectrics.

A thermodynamic analysis of processes of interphase interaction in the Ga–As–O system has been performed and their theoretical laws have been determined, taking into account nonlinear thermal physical properties of the compounds, the oxide film compositions, and modes of molecular-beam epitaxy of GaAs. The processes of interaction of the native oxide of GaAs with the substrate material and also with Ga and As₄ from a vapor gaseous phase have been studied experimentally. The experimental results correlate with the results of the thermodynamic analysis. The laws of influence of the removal of the proper oxide on the evolution of the GaAs surface morphology under conditions of the molecular-beam epitaxy have been proposed.

Thin films Cu₂ZnSnS₄ (up to 0.9 μm thick) with p-type conductivity and band gap E_g = 1.54 eV have been prepared by the spray pyrolysis of 0.1 M aqueous solutions of the salts CuCl₂ · 2H₂O, ZnCl₂ · 2H₂O, SnCl₄ · 5H₂O, and (NH₂)₂CS at a temperature T_S = 290°C. The electrophysical properties of the films have been analyzed using the model for polycrystalline materials with electrically active grain boundaries. The energy and geometric parameters of the grain boundaries have been determined as follows: the height of the barriers is E_b ≈ 0.045–0.048 eV, and the thickness of the depletion region is δ ≈ 3.25 nm. The effective concentrations of charge carriers p₀ = 3.16 × 10¹⁸ cm^–3 and their mobilities in crystallites μ_p = 85 cm²/(V s) have been found using the technique for determining the kinetic parameters from the absorption spectra of thin films at a photon energy hν ≈ E_g. The density of states at grain boundaries N_t = 9.57 × 10¹¹ cm^–2 has been estimated.

This paper presents the results of an experimental investigation of the supramolecular structure of polydiphenylenephthalide thin films that exhibit effects of resistive switching. The supramolecular structure of the polymer has been investigated using small-angle neutron scattering in conjunction with atomic force microscopy. It has been found that the internal structure of polymer films consists of structural elements in the form of spheroids. The sizes of the structural elements, which were obtained from the neutron scattering data and analysis of the atomic force microscopy images, correlate well with each other. A model of the formation of polymer layers has been proposed. The observed structural elements in polymer films are formed due to the association of macromolecules in the initial polymer solution.