Vol 58, No 8 (2016)

The magnetization of a layered high-temperature superconductor with different anisotropy parameters has been calculated using the Monte Carlo method in the framework of a modified three-dimensional Lawrence–Doniach model with actual boundary conditions. The penetration of a magnetic flux into a bulk sample from the boundary has been simulated, and the curves of magnetization reversal of a high-temperature superconductor by an external magnetic field have been calculated for different anisotropy parameters γ and types of defects in the sample. It has been found that there are significant differences in the magnetization curves and transport properties of superconductors with different anisotropy parameters γ. The influence of tilted columnar defects on the critical current has been analyzed. A decreasing dependence of the critical current on the tilt angle of defects with respect to the c axis has been obtained. It has been shown that, as the anisotropy parameter increases, this dependence weakens and, for a specific value of γ, disappears. An explanation of the mechanism responsible for the disappearance of the dependence has been proposed.

The conductivity of single-phase ceramic materials based on proton-conducting perovskite La_0.9Sr_0.1ScO_3–α containing from 0.3 to 47 at % Fe in the scandium sublattice has been studied. Synthesis has been performed by burning with ethylene glycol. Measurements have been carried out by the four-probe (500–900°C) and impedance (100–500°C) methods in oxidizing and reducing atmospheres, as well as at different pressures \({p_{{O_2}}}\) (2.1 × 10⁴–10^‒15 Pa) and \({p_{{H_2}O}}\) (0.04–2.5 kPa). Substitution of scandium with iron significantly decreases the proton conductivity.

The effect of electron and phonon scattering on nanometer-sized pores on the thermoelectric properties of lead telluride has been studied theoretically. Estimations show that the thermoelectric efficiency can increase by 20–25% at room temperature and by 5–10% at 600 K at the optimal pore size of several nanometers and the porosity of ~10%. An analysis shows that the increase in the thermoelectric efficiency due to additional scattering in the porous material is related to the decrease in the thermal conductivity of the lattice and the increase in the thermoelectric coefficient due to the change in the energy dependence of the relaxation time. To estimate the multiple scattering at high pore concentration, the lattice thermal conductivity by the molecular-dynamics method and the electron free paths in a coherent potential approximation were calculated. It is shown that the inclusion of the multiple scattering slightly influences the thermoelectric properties at noted sizes and pore concentrations.

The complex permittivity ε = ε′–iε″ of manganite bismuth–lanthanum Bi_0.5La_0.5MnO₃ ceramics has been measured at temperature T = 78 K in the frequency range f = 200–10⁵ Hz and in the magnetic induction range B = 0–5 T. Dielectric relaxation and the pronounced magnetodielectric effect have been detected. The explanation based on the superposition of Maxwell–Wagner relaxation and the magnetoresistance effect has been proposed.

The results of examination of AC dependences of capacitance and dielectric loss tangent of sillenite Bi₁₂TiO₂₀ crystals doped with ruthenium on frequency are presented. Non-Debye dispersion of dielectric coefficients is found in the frequency interval of 5 × 10²–10⁵ Hz, and a resonance phenomenon is observed. Polarization processes in the studied samples are attributed to relaxators associated with metal–oxygen vacancies and structural elements incorporating 6s² lone-pair electrons.

Methods for magneto-optical detection of the in-plane magnetization in a magnetic film due to the deposition of a one-dimensional metallic diffraction grating on the film have been considered. This structure is a magnetoplasmonic crystal, in which the excitation of the waveguide and plasmon modes leads to the appearance of five resonant magneto-optical effects that consist in a change of the intensity, the polarization, and the phase of the transmitted and reflected waves. The conditions responsible for the origin of these effects and their magnitude are determined by the configuration of the incident light, the parameters of the metallic grating, and the chemical composition of the magnetic layer. It has been found that the magnetophotonic intensity effects are the most optimal for the detection of the in-plane magnetization. The influence of the parameters of the metallic grating on the magneto-optical effects has been analyzed and the most optimal conditions for the observation of these effects have been determined. It has also been found that an increase in the concentration of bismuth in a magnetic dielectric material can lead to a weakening of the optical and magneto-optical responses.

Small Sm additions (~1–3 at %) have been found to slow down the relaxation of the magnetization in a magnetic field in the (NdDy)(FeCo)B alloy by several times. The effective “freezing” of the spontaneous relaxation of the magnetic moment is related to the substantial increase in the potential barriers for motion of domain walls when introducing Sm ions that have other symmetry of the single-ion anisotropy than that of Nd and Dy ions. The results can be used to stabilize the properties of hard magnets.

The crystal structure and piezoelectric and magnetic properties have been studied in Bi_1‒xSm_xFeO₃ solid solutions with the compositions near the morphotropic boundary between rhombohedral and orthorhombic (Rh–Orh) phases. The coexistence areas of rhombohedral and antipolar orthorhombic phases, as well as the evolution of structural phases at the interface, have been established. A maximum piezoelectric signal is found for the two-phase composition with the dominating rhombohedral phase, and an increase in the piezoresponse is caused by the decreasing structural stability of the sample. The evolution of magnetic properties in Bi_1–xSm_xFeO₃ compounds has been elucidated depending on the substitutional ion concentration. The orthorhombic phase composites are the weak ferromagnetics with the residual magnetization of ~0.2 emu/g.

The kinetics of the formation and growth of 180° domains in a weak quasi-stationary external electric field has been considered in the framework of the phenomenological Ginzburg–Landau model using the example of sodium nitrite (NaNO₂) crystals that undergo a first-order ferroelectric phase transition of the order–disorder type. The influence of the rate and temperature of quenching, as well as the strength of an external electric field, on the subsequent evolution of the system toward the thermodynamic equilibrium state has been analyzed. It has been shown that, by varying a weak external electric field applied to the ferroelectric crystal after quenching, it is possible to obtain both single-domain and multi-domain ordered structures. It has been established that the formation of nonequilibrium (“virtual”) multi-domain structures of the asymmetric type is possible for particular strengths of the electric field applied to the ferroelectric after quenching. A similar effect can be achieved by varying the depth of quenching of the sample. It has been found that, if the size of the order parameter inhomogeneities formed at the stage of quenching does not exceed a critical value, they can be reoriented partially or completely into domains of opposite sign. For this purpose, the relaxation after quenching should be performed in an external electric field of the appropriate sign.

The structure and properties of a polycrystalline Ni–Mn–In Heusler alloy have been studied after a plastic deformation by upsetting. An analysis of points of a martensitic and magnetic phase transformations shows that the martensite transformation takes place at temperatures lower than the Curie point. At high temperatures in the range 930–1110 K the alloy undergoes the phase transformation of ordered phase L2₁ to disordered phase B2, and the melting temperature of the alloy is 1245 K. The flow curves of the alloy cylindrical samples at temperatures 773, 873, and 973 K have been built. An analysis of the alloy microstructure after upsetting at a temperature of 773 K leads to the conclusion that many macrocracks are initiated in the sample. The treatment at 873 and 973 K causes a fragmentation of the grains with grain sizes from several to several dozen micrometers. However, the upsetting at 873 K leads to insignificant scatter in the grain sizes, and the microstructure is more homogeneous and worked out.

Xerogel reactor powders and supramolecular polyethylene fibers with various degrees of hood have been studied via differential scanning calorimetry. A higher strength of laboratory fibers in comparison with industrial ones is found to be achieved due to a multistage band high-temperature hood that causes the thermodynamic parameters of supramolecular polymer structure.

Electron-spin resonance spectra of Eu²⁺ and Gd³⁺ centers substituting Y³⁺ ions in single-crystal yttrium–aluminum garnet have been studied and the parameters of their rhombic spin Hamiltonian have been determined. The fine-structure parameters of the above ions have been calculated in the superposition model disregarding changes in the angular coordinates of the ligand environment of the impurity defect thus demonstrating the necessity of taking these changes into account.

The dimension D of a polycrystalline film and the optical anisotropy m = ε_z/ε_x of uniaxial crystallites with the principal components ε_x = ε_y and ε_z of the tensor of the dielectric constant have been shown to produce a strong influence on the effective dielectric constant ε_D^* and the effective refractive index n_D^* = (ε_D^*)^1/2 of the film in the optical transparency region, as well as on the boundaries of the intervals B_Dl ≤ ε_D^* ≤ B_Du. The intervals Δ₂(m) = B_2l–B_2u and Δ₃(m) = B_3l–B_3u are separated by a gap for m in the range 1 < m < 2, whereas the theoretical dependence ε₂^*(m) is separated by a gap from the interval Δ₃(m) for m in the range 1 < m < 4. This is confirmed by a comparison of the experimental (n_oP) and theoretical (n_D^*) ordinary refractive indices for uniaxial polycrystalline films of the conjugated polymer poly(p-phenylene vinylene) (PPV) with uniaxial crystallites and appropriate values of m. In the visible transparency region of the PPV films with a change in m(λ) in the range 2 < m(λ) < 3 due to the dependence of the components ε_x,z(λ) on the light wavelength λ, the refractive indices n_oP²(λ) = ε_oP(λ) are consistent with the theoretical values of ε₂^*(λ) and lie outside the interval Δ₃(m). For m(λ) > 3 near the electronic absorption band of the crystallites, the values of ε_oP(λ) lie in the region of the overlap of the intervals Δ₂(m) and Δ₃(m). The boundaries mc of the range 1 < m < m_c are determined, for which the interval Δ₂(m) is separated by a gap from the dependences ε₃^*(m) corresponding to the effective medium theory with spherical crystallites and hierarchical models of a polycrystal, as well as from the proposed new dependence ε₃^*(m).

The ab initio calculation of the crystal structure and the phonon spectrum of crystals RFe₃(BO₃)₄ (R = Pr, Nd, Sm) has been performed in the framework of the density functional theory. The ion coordinates in the unit cell, the lattice parameters, the frequencies and the types of fundamental vibrations, and also the intensities of lines in the Raman spectrum and infrared reflection spectra have been found. The elastic constants of the crystals have been calculated. For low-frequency A₂ mode in PrFe₃(BO₃)₄, a “seed” vibration frequency that strongly interacts with the electronic excitation on a praseodymium ion was found. The calculation results satisfactory agree with the experimental data.

Phase transformations, electrical transport and thermal properties of the systems K_1‒xCs_x(H₂PO₄)_1–x(HSO₄)_x (x = 0.01–0.95) and Cs(H₂PO₄)_1–x(HSO₄)_x (x = 0.01–0.30) have been studied in detail. It has been shown that the mixed compounds Cs(H₂PO₄)_1–x(HSO₄)_x are characterized by an increase in the low-temperature electrical conductivity by one to five orders of magnitude depending on the composition, as well as by the disappearance of the superionic phase transition at x ≥ 0.15. The partial substitution of HSO₄^- ions for the anions in CsH₂PO₄ at x = 0.01–0.10 leads to the formation of Cs(H₂PO₄)_1‒x(HSO₄)_x solid solutions isostructural with the CsH₂PO₄ (P2₁/m) phase. For Cs(H₂PO₄)_1–x(HSO₄)_x with x = 0.15–0.30 at room temperature, there is a stabilization of the high-temperature cubic phase isostructural with the CsH₂PO₄ (\(Pm\overline 3 m\)) phase existing in CsH₂PO₄ at temperatures above 230°C. The stability of the \(Pm\overline 3 m\) cubic phase at room temperature has been investigated using X-ray powder diffraction, ¹H NMR spectroscopy, and impedance spectroscopy. In the K_1–xCs_x(H₂PO₄)_1–x(HSO₄) system, there are two regions of compositions with x = 0.05–0.50 and 0.60–0.95, where the proton conductivity and thermal properties are determined respectively by the formation of the CsH₅(PO₄)₂ phase, which is stoichiometrically different from the initial salts, and the potassium-containing phase, which is isostructural with the superionic salt Cs₃(HSO₄)₂(H₂PO₄).

The fine structure of Ni–Mn–In alloys has been studied when manganese atoms are substituted for nickel atoms in an annealing state. The concentration dependence of the critical temperatures and the structures of the alloys have been discussed. It has been found that, as manganese atoms replace nickel atoms, the structure after annealing is changed from a two-phase (L2₁ + martensite) to single-phase L2₁ structure. The martensitic transformation in Ni₄₇Mn₄₂In₁₁ alloy is accompanied by the formation of modulated 14M martensite.

A new method for generating spin-polarized currents in topological insulators has been proposed and investigated. The method is associated with the spin-dependent asymmetry of the generation of holes at the Fermi level for branches of topological surface states with the opposite spin orientation under the circularly polarized synchrotron radiation. The result of the generation of holes is the formation of compensating spin-polarized currents, the value of which is determined by the concentration of the generated holes and depends on the specific features of the electronic and spin structures of the system. The indicator of the formed spin-polarized current can be a shift of the Fermi edge in the photoelectron spectra upon photoexcitation by synchrotron radiation with the opposite circular polarization. The topological insulators with different stoichiometric compositions (Bi_1.5Sb_0.5Te_1.8Se_1.2 and PbBi₂Se₂Te₂) have been investigated. It has been found that there is a correlation in the shifts and generated spin-polarized currents with the specific features of the electronic spin structure. Investigations of the graphene/Pt(111) system have demonstrated the possibility of using this method for other systems with a spin-polarized electronic structure.

Thin polyaniline films were fabricated by thermal vacuum evaporation from a Knudsen effusion cell. The conducting properties of films synthesized under different evaporation conditions were studied. The enhancement of the emission capacity of a wolfram tip coated with a polyaniline film of a nanometer thickness was demonstrated experimentally. A model of the discovered effect was proposed. The obtained Fowler–Nordheim current–voltage characteristics were used to estimate the change in the electronic work function occurring when a thin film is deposited on the tip surface. The effective temperature of electrons emitted from the polyaniline film was determined based on the results of analysis of energy distributions, and the specific features of charge transport in the metal–polyaniline–vacuum system were examined. A model of energy bands of the metal–polymer film contact was also constructed.

Isomerically pure endohedral metallofullerenes Gd@C82(C2v), Ho@C₈₂(C_2v), and their monoanions have been synthesized and separated. The optical absorption spectra of solutions of obtained compounds in o-dichlorobenzene have been studied. Within the Hubbard model, the energy spectrum of isomer of C_2v symmetry (no. 9) of fullerene С₈₂ has been calculated. Based on the obtained spectrum, optical absorption spectra of endohedral metallofullerenes Gd@C₈₂ and Ho@C₈₂ and their monoanions have been simulated. The calculated optical absorption spectra have been compared with experimental ones; it has been found that qualitative agreement between them is observed.

Various Stone–Wales defects in phagraphene, which is a graphene allotrope, predicted recently are studied in terms of the nonorthogonal tight-binding model. The energies of the defect formation and the heights of energy barriers preventing the formation and annealing of the defects are found. Corresponding frequency factors in the Arrhenius formula are calculated. The evolution of the defect structure is studied in the real-time mode using the molecular dynamics method.

Joule heat release during current flow through a fine wire connecting two bulk electrodes (two contacts) is considered. The irreversible heat release symmetric in the absence of electron–phonon coupling becomes asymmetric if this coupling is taken into account. The Joule heat release appears larger in that symmetric contact which lies along the carrier drift velocity.