Vol 59, No 2 (2017)

The influence of spin fluctuations on the thermodynamic properties of a helical ferromagnet MnSi has been investigated in the framework of the Hubbard model with the electronic spectrum determined from the first-principles LDA + U + SO calculation, which is extended taking into account the Hund coupling and the Dzyaloshinskii–Moriya antisymmetric exchange. It has been shown that the ground state of the magnetic material is characterized by large zero-point fluctuations, which disappear at the temperature T* (<T_c is the temperature of the magnetic phase transition). In this case, the entropy abruptly increases, and a lambdashaped anomaly appears in the temperature dependence of the heat capacity at constant volume (C_V(T)). In the temperature range T* < T < T_c, thermal fluctuations lead to the disappearance of the inhomogeneous magnetization. The competition between the increase in the entropy due to paramagnon excitations and its decrease as a result of the reduction in the amplitude of local magnetic moments, under the conditions of strong Hund exchange, is responsible for in the appearance of a “shoulder” in the dependence C_V(T)).

The character of temperature dependences of the electric conductivity of MgB₂ granular BCS superconductors at temperatures of ~35–45 K in external magnetic fields H_ext of up to ~2 kOe is studied. An increase in the superconducting transition width ΔT_c with an increase in Hext is found. The presence of a system of weak links in MgB₂-based granular superconductors is established. On the basis of experimental data, MgB₂ granular superconductor is assigned to two-level superconducting systems and the H–T phase diagram is constructed.

The luminescence excitation spectra of localized excitons in GaSe_0.85Te_0.15 solid solutions have been investigated at the temperature T = 2 K. It has been shown that the excitation spectra of excitons with the localization energy ε > 10 mV exhibit an additional maximum M_E located on the low-energy side of the maximum corresponding to the free exciton absorption band with n = 1. It has been found that the shift in the position of the maximum M_E in the excitation spectrum with respect to the energy of detected photons increases as the energy of detected photons decreases, i.e., with an increase in the localization energy of excitons. Under the resonant excitation of localized excitons by a monochromatic light from the region of the exciton emission band, in the exciton luminescence spectrum on the low-energy side from the excitation line, there is also a maximum of the luminescence (M_L). The energy distance between the position of the excitation line and the position of the maximum in the luminescence spectrum increases with a decrease in the frequency of the excitation light. The possible mechanisms of the formation of the described structure of the luminescence excitation and exciton luminescence spectra of GaSe_0.85Te_0.15 have been considered. It has been concluded that the maximum M_E in the excitation spectrum and the maximum M_L in the luminescence spectrum are attributed to electronic–vibrational transitions with the creation and annihilation of localized excitons, respectively.

We present a heterostructure consisting of anodic copper oxide Cu₂O on a copper substrate and a transparent Cd–Sn–O conducting film for use in solar cells. Focusing on simplicity and the availability of film fabrication techniques, we chose anodic oxidation for forming the Cu₂O film and the extraction-pyrolysis technique for forming the transparent Cd–Sn–O conducting layer. We demonstrate the possibility of considerable enhancement of the phototransformation efficiency in the Cu–Cu₂O/Cd–Sn–O structure over this parameter in the Cu–Cu₂O structure.

The effect of pressure on the structural and electronic properties of lithium, sodium, potassium, and ammonium perchlorates have been studied in terms of the density functional theory with allowance for the Van der Waals dispersion interaction. The pressure dependences of the geometric parameters, the band gaps, the densities of states, the charge distributions, and the atomic charges are calculated. The compressibilities of the perchlorates are found to be anisotropic, which is due to the differences of the lattice parameters and the nature of interatomic bonds. Ammonium cation is rotated under pressure around axis b at an angle of ~9°. The band gaps of the perchlorates are ~4.5–4.7 eV and increase with pressure.

Zero spin-wave mode in inhomogeneous magnetic films with orthorhombic anisotropy has been found to exhibit a change of its localization region in two of three typical angular dependences of resonance fields of high-intensity modes. It has been shown that the anisotropy fields on both sides of the film can be determined from the resonance fields of the zero and uppermost high-intensity spin-wave modes of spin-wave resonance spectra.

The temperature dependences of the velocity of longitudinal sound waves and the internal friction in a La_0.82Ca_0.18MnO₃ single crystal with the Curie temperature T_C = 181 K have been studied. As temperature decreases, the single crystal is shown to undergo the transition from the pseudocubic O* to the Jahn–Teller O’ phase at T ~ 254 K and the reverse transition from O’ to O* phase at T ~ 84 K. The velocity of sound and the internal friction in the O’ phase are found to be significantly smaller than those in the O* phase.

Results of studies of magnetooptical Kerr effect and magnetoreflection of natural light in La_2/3Ba_1/3MnO₃/SrTiO₃ films of different thickness are presented. The Kerr effect was shown to be the most prominent in visible and near IR range; magnetoreflection was found to achieve its maximum of about 10% in the mid-IR range near the room temperature. Physical mechanisms defining the value and sign of the effects and the influence of the thin-film state on the magnetooptical properties are discussed. Magnetoreflection is estimated in the framework of the magnetorefractive effect theory.

The electric field-induced dynamics of the phase transition to a ferroelectric phase in PbIn_1/2Nb_1/2O₃–PbMg_1/3Nb_2/3O_3–xPbTiO₃ (x = 25 and 32%) crystals have been studied in different geometries of applying an external electric field E|| [001] and E || [011]. The optical transmission, the sound velocity, and the sound damping have been measured simultaneously at room temperature. It was found that the change in the elastic properties follows the change in the optical transmission, which is related to different polar region sizes and relaxation times, to which light and sound are responsive. It was found that, in the field range 0–2 kV/cm at E|| [001], both the crystals demonstrate only a partial polarization as a result of the phase transition, while the phase transition in the PT32 crystal at E|| [011] is finished by almost complete polarization of the sample in the same field range.

A method for processing the results of dynamic spall fracture tests, based on the exact solution of the wave equation, and its commonly used simplified version based on the assumed unique relation between the free surface velocity drop and the ultimate medium fracture stress, are analyzed. Using the considered exact solutions of the wave technique, tensile stress pulses during spalling are determined. The obtained stress levels at the fracture point are compared with the spall strength calculated by the velocity drop technique. The cases of agreement and disagreement of the results obtained using both techniques are shown. By the example of differently shaped loading pulses, possible scenarios of sample fracture are presented, in particular, the probability of the fracture delay effect is shown, which can be lost in the simplified processing method.

The effect of decrease in the strength of submicron-sized specimens of face-centered cubic (fcc) metals with a nanocrystalline structure and a cross-sectional size D < 5d, as compared to the strength of the specimens with D ≫ 5d (where d is the grain size), has been considered theoretically on the basis of the dislocation–kinetic equations and relationships. Previously, it has been found that this decrease is caused by the escape of a part of the dislocations through the surface of the specimen under the action of single-pole dislocation sources in grains adjacent to the surface. In this study, it has been shown that the absorption of lattice dislocations by grain boundaries and its accompanying grain boundary sliding lead to a further decrease in the flow stress of specimens, which is equally related to both thin (D < 5d) and thick (D ≫ 5d) specimens.

The structural changes in crystalline lamella cores of tridecanoic acid CH₃(CH₂)₁₁COOH during heating in the range from the temperature T₁ = 13.5°C to T₂ > T_m = 41.6°C have been investigated using Fourier transform infrared spectroscopy. The behavior of the bands of rocking (in the region of 720 cm^–1) and bending (in the region of 1470 cm^–1) vibrations of CH₂ groups in tridecanoic acid methylene segments has been analyzed. It has been shown that, in the first-order phase transition region (T_s–s ~ 36°C) within a narrow temperature range (ΔT₁ ≤ 1 K), there is a gradual transformation of the initial triclinic subcell into the hexagonal subcell. The mechanism of this transition has been considered in terms of the theory of diffuse first-order phase transitions.

The tangential G modes in individual semiconducting double-walled nanotubes have been examined via Raman spectroscopy over a wide laser excitation wavelength range. Individual suspended nanotubes have been synthesized via chemical vapor deposition. The (n, m) chirality indices are determined via electron diffraction and high-resolution transmission electron microscopy. The pronounced shift in the tangential modes compared to the analogous modes of single-walled nanotubes has been observed in Raman spectra of double-walled nanotubes. The shift value is shown to depend on the interlayer distance and on the van der Waals interaction between the layers in a double-walled tube.

The recently obtained equation for the dependence of the glass transition temperature on the cooling rate of a melt has been considered and compared with the experimental data. The kinetic criterion of glass transition has been discussed.

The simulation of the decomposition of a three-dimensional fragment of a solid solution satisfying the regular solution approximation has been carried out based on the Cahn–Hilliard equation taking into account the Gaussian fluctuations of the initial state of the alloy. The simulation has been performed for several temperatures and revealed the existence of four stages (nucleation, growth, coagulation, and coalescence) of the process. The influence of the temperature on the distribution of phases during the decomposition of binary alloys has been established, and the specific features in the change of stages of the decomposition process have been revealed.

The nuclear spin–lattice relaxation and Knight shift of ⁷¹Ga, ⁶⁹Ga, and ¹¹⁵In nuclei in a ternary liquid gallium–indium–tin alloy of the eutectic composition, which was introduced into pores of an opal matrix and porous glasses with pore sizes of 18 and 7 nm, have been investigated and compared with those for the bulk melt. It has been found that longitudinal relaxation is accelerated and the Knight shift is decreased, depending on the size of pores. The correlation time of the atomic motion has been calculated for the nanostructured melt in porous matrices. It has been shown that the atomic mobility in the melt decreases with decreasing size of pores in the glasses.

The internal structure of empty porous micro- and macroporous magnetic glasses and the related nanocomposites containing NaNO₂ and KNO₃ embedded in pores of the glasses has been investigated using small-angle neutron scattering. The characteristic sizes of magnetite particles in matrices and the sizes of nanoparticles embedded in ferroelectric materials have been estimated. It has been shown that, for microporous glasses in the momentum transfer range 0.35 nm^–1 < Q < 1.7 nm^–1, the dependence of the small-angle neutron scattering intensity on the momentum transfer has two characteristic regions: the first region corresponds well to the Porod’s law Q^–4 (smooth surface), and the second region is consistent with the mass fractal reflecting the internal structure of channels in the microporous glass. For macroporous glasses over the entire momentum transfer range 0.35 nm^–1 < Q < 1.7 nm^–1, the scattering intensity is described by the dependence I(Q) ~ Q^–n with α = 3.96 ± 0.02; i.e., in these glasses, there is a system of channels with sufficiently smooth surfaces.

The structural and magnetic properties of Fe₈₇Pt₁₃ films synthesized by solid-state reactions and Fe₈₇Pt₁₃–Al₂O₃ composite films fabricated by aluminothermy are investigated. It is shown that the synthesized samples of both types are characterized by the rotational magnetic anisotropy, when the easy magnetization axis in the film plane can be set by a magnetic field. It is established that the value of rotational magnetic anisotropy in the Fe₈₇Pt₁₃–Al₂O₃ composite films is higher than in the Fe₈₇Pt₁₃ samples by an order of magnitude. The rotational magnetic anisotropy is assumed to be caused by the exchange coupling of the L1₀–FePt phase with the L1₂–Fe₃Pt phase in the Fe₈₇Pt₁₃ films and magnetic iron oxides in the Fe₈₇Pt₁₃–Al₂O₃ samples.

The results of the investigation of the density of unoccupied electronic states (DOUS) in the energy range from 5 to 20 eV above the Fermi level (E_F) in dioctyl-substituted perylenedicarboximide (PTCDI-C₈) and diphenyl-substituted perylenedicarboximide (PTCDI-Ph) ultrathin films have been presented. The experimental results have been obtained from measurements of the secondary low-energy electron current with the use of the total current spectroscopy (TCS) technique. A theoretical analysis has been performed, including the density functional theory calculation of the energies and spatial distribution of the orbitals of the molecules under investigation and the subsequent scaling of the calculated orbital energies according to the procedure well-proven previously in studies of small conjugated organic molecules. It has been found that, for each of the two types of the studied films, at energies below 8 eV above the Fermi level E_F, there are two main maxima of the density of unoccupied electronic states predominantly formed by the π*-orbitals of the molecules. The higher-lying maxima have essentially a σ*-character. The influence of dioctyl- and diphenyl-substituent groups on the density of unoccupied electronic states has been analyzed in comparison with the results obtained for the studied types of films. In the case of the π*-maxima, the relative shift has been observed at an energy of approximately 1 eV. In the region of σ*-electronic states, there is a small transformation of the structure of the maxima.

The applicability of individual Ni, Cu, Au, Pt, and Pd nanoclusters as data bits in next generation memory devices constructed on the phase-change carrier principle is studied. To this end, based on the modified tight-binding potential (TB-SMA), structure formation from the melt of nanoparticles of these metals to 10 nm in diameter was simulated by the molecular dynamics method. The effect of various crystallization conditions on the formation of the internal structures of Ni, Cu, Au, Pt, and Pd nanoclusters is studied. The stability boundaries of various crystalline isomers are analyzed. The obtained systematic features are compared for nanoparticles of copper, nickel, gold, platinum, and palladium of identical sizes. It is concluded that platinum nanoclusters of diameter D > 8 nm are the best materials among studied metals for producing memory elements based on phase transitions.

In the framework of the Hubbard model in the static fluctuation approximation, the energy spectrum of fullerene C₇₀ with allowance for different lengths of the bonds between nonequivalent nodes is calculated. On the basis of the calculated energy spectrum, the optical absorption spectrum in the ultraviolet and visible region is simulated. A good qualitative agreement between the calculated and measured absorption spectra and between the measured and theoretical values of the gap width between the highest occupied and the lowest unoccupied molecular orbital is found.