Vol 59, No 7 (2017)

The results of investigation of the electronic structure and optical properties of ErRhGe are presented. The band spectrum of this compound is calculated in the local electron spin density approximation with correction for strong electron interactions in the 4f shell of the rare-earth metal (LSDA + U method) with allowance for the spin polarization. The optical constants of the compound are measured, and a number of spectral and electronic characteristics are determined by the ellipsometric method in a wide range of wave-lengths. Structural features of the optical conductivity spectrum in the interband absorption region are interpreted on the basis of the calculated electron state density.

Temperature variations of the amplitude of zero-point and thermal spin fluctuations in a helicoidal ferromagnetic (MnSi) are characterized using the electronic structure model that follows from ab initio LDA + U + SO calculations. It is found that a drastic reduction in the amplitude of zero-point spin fluctuations at temperature T_S (in the vicinity of the magnetic phase transition) leads to ferromagnetic solution instability (a change in the sign of the intermode interaction parameter). The observed magnetovolume effect and a sharp change in the radius of spin correlations have the same underlying cause. The results of calculation of the volumetric coefficient of thermal expansion agree well with the observed anomaly in the region of the magnetic phase transition.

To elucidate the origin of the well-known anisotropy of the magnetoresistive properties of granular high-temperature superconductors (HTSs), which is related to the mutual orientation of magnetic field H and transport current j, we investigate the hysteretic dependences of magnetoresistance R(H) of the yttrium HTS sample at the perpendicular (H ⊥ j) and parallel (H || j) configurations. The hysteretic R(H) dependences are analyzed using the concept of the effective field in the intergrain boundaries through which superconducting current carriers tunnel. The effective degree of magnetic flux compression in the intergrain medium at the perpendicular configuration was found to be twice as much as at the parallel one. This approach explains well the anisotropy of the magnetoresistive properties of granular HTSs, which was previously reported by many authors, and the temperature dependences of the resistance in the resistive transition region.

The influence of optical illumination on transmission of THz radiation through a bulk crystal of semi-insulating GaAs is experimentally studied. It is established that, without additional illumination, absorption of electromagnetic waves with a frequency of about 1 THz in the studied crystal is almost absent. Optical illumination in the spectral range of fundamental absorption of the crystal does not affect the transmission of THz waves. At the same time, if the illumination photon energy is a little below the edge of fundamental absorption, i.e., actually in the transparency region, the transmission of THz radiation drops sharply. At liquid helium temperature, the maximum effect is achieved for the energy of optical photons lower by approximately 30 meV than the crystal band gap. Further shift of the illumination toward lower photon energies is accompanied by almost complete recovery of the transmission. With increasing sample temperature, the spectral range of efficient action of the illumination shifts together with the edge of fundamental absorption toward lower photon energies.

Paramagnetic layered semiconductor NbS₂ doped with some transition metals can transform into ferromagnetic material. That is why such materials are promising candidates for spintronic devices. It is found that only at certain concentrations of a doping metal T crystallographic ordering is possible, which is essential for magnetic ordering of ternary compounds TNbS₂. In particular, CrNb3S6 crystals are studied, which form almost completely ordered superstructure with intercalated Cr between NbS₂ layers. The main difficulty in crystal growth is reaching stoichiometry of the compound. This problem is solved in the developed method of two-staged gas transport chemical reaction. This new approach provides growth of CrNb₃S₆ single crystals of several millimeters in diameter and 0.3–0.5 mm thickness. X-ray phase analysis (XRD) of powders is performed to identify all phases involved in synthesis and growth of the crystals. High frequency absorption in external periodic magnetic field as a function of temperature and intensity of magnetic field is used to estimate the temperature of ferromagnetic transition in CrNb₃S₆ single crystals. The Curie temperature is estimated as 115 K. Growth of CrNb₃S₆ crystals from vapor phase is studied in detail and full analysis of phase transitions during growth is given. It has been shown that using of high frequency absorption in the crystal provides reliable estimation of the point of ferromagnetic transition in this semiconductor. The authors are grateful to the Physical Science Department of Russian Academy of Sciences for financial support of the studies in the frameworks of the program “Physics of new materials and structures” (project no. 00-12-10).

It is established that, in ceramics based on solid solutions of BiFeO₃–SrTiO₃ and BiFeO₃–BaTiO₃ multiferroics, antiresonant dielectric spectra—a kind of spectra not described in literature—are possible in addition to relaxation and resonant spectra. Antiresonant dielectric spectra are simulated, and experimental data confirming the simulation results are obtained. It is shown that the antiresonant spectra are characterized by a large activation energy (U > 1 eV). A relationship between the antiresonant spectra and hopping conductivity is supposed.

Comparative studies of physical characteristics (the electrical resistivity, the magnetic susceptibility, the magnetization, the bending deformation, and the degree of shape recovery during subsequent heating) of the Ni₅₄Mn₂₁Ga₂₅ ferromagnetic alloy as-cast and rapidly quenched from melt have been performed in the temperature range 2–400 K. The results are compared to the results of studying the structural–phase transformations by transmission and scanning electron microscopy and X-ray diffraction. It is found that the rapid quenching influences the microstructure, the magnetic state, the critical temperatures, and the specific features of thermoelastic martensite transformations in the alloy. It is found that the resource of the alloy plasticity and thermomechanical bending cyclic stability demonstrates a record-breaking increase in the intercritical temperature range and during subsequent heating.

The effect of the nanopore size on the mechanical properties of a porous carbon material with the density of 1.4 g/сm³ is discussed. The atomistic models of porous carbon materials depending on the nanopore size are constructed. The numerical experiments are implemented with using the molecular mechanical method based on the Brenner potential. The Young’s moduli are evaluated for porous carbon structures at certain nanopore dimensions and are found to decrease with the enlarging nanopores.

The structure of surface layers with a thickness of ~1 μm formed at destruction of gneiss is studied by means of photoluminescent and infrared spectroscopy. It is found that, in this layer, feldspar (plagioclase and microcline) crystals are completely destroyed and replaced by montmorillonite.

Electron paramagnetic resonance spectra of Eu²⁺ and Gd³⁺ centers in Lu₃Al₅O₁₂ single crystals have been studied and the parameters of their rhombic spin Hamiltonian have been determined.

The structure and optical diffraction properties of monolayers of monodisperse spheres crystallized on transparent dielectric substrates are studied. Two types of diffraction phenomena are considered: surface light diffraction on the lattice of spheres and waveguide resonances in the monolayer plane. For experimental study of these phenomena, optical retroreflection and transmission spectra are measured as functions of the light incidence angle and azimuthal orientation of the incidence plane. The monolayer structures determined by scanning electron microscopy and light diffraction methods are in quantitative agreement. It is concluded that one-dimensional Fraunhofer diffraction is applicable to describe surface diffraction in the hexagonal lattice of spheres. In the case of oblique light incidence, anisotropy of diffraction and transmission spectra depending on the light incidence plane orientation with respect to the sphere lattice and linear polarization of incident light is detected. Waveguide resonances of the planar two-dimensional photonic crystal are approximated within the light diffraction model in the “empty” hexagonal lattice. The best approximation of the waveguide resonance dispersion is achieved using the effective refractive index, depending on the wavelength. Surface diffraction suppression by waveguide resonances of the photonic crystal is demonstrated. Surface diffraction orders are identified as diffraction at singular points of the Brillouin zone of the planar twodimensional photonic crystal.

New Rb₃PO₄-based ceramic materials with high rubidium-cation conductivity in the Rb_3–2xPb_xPO₄ system have been synthesized and studied. Introduction of Pb²⁺ cations leads to a sharp increase in the conductivity of rubidium orthophosphate due to formation of cation vacancies and, at temperatures 350–550°C, also due to the stabilization of high-temperature cubic modification Rb₃PO₄. At high temperatures, the electrolytes prepared have very high ion conductivity higher than 10^–1 S cm^–1 at 700°C, which is higher than the values previously obtained in similar systems with additions of tin and cadmium ions. The factors influencing the transport properties of the materials under study are discussed.

Thermal effects in some nanoporous silicate matrices (with different pore sizes) loaded with ferroelectric NaNO₂ from both a saturated solution and from a melt have been studied in a wide temperature range including the phase transition temperatures. All the samples reliably demonstrate maxima of the heat capacity, corresponding to first-order ferroelectric phase transitions. The characteristics of the maxima (intensity, half-width, phase transition temperature, etc.) have been determined. A more complex situation is the observation of an incommensurable phase (sinusoidal antiferroelectric), in particular, in the case of pore sizes comparable to the period of an “incommensurable” wave, the manifestation of which can be explained by the appearance of a corresponding orientation of sodium nitrite nanocrystals in pores of these matrices. It is found that the characteristics of above noted effects depend on the prehistory of the samples under study.

New relationships have been obtained in an integral form for binary systems in the case when one of the phases is dispersed (to nanometer sizes) inside another phase (matrix) in the framework of the thermodynamics of phase equilibrium, including surface phenomena. These relationships are used to construct a size-dependent phase diagram in the binary Mo–Ru system containing solid nanoparticles with bcc and hcp structures and a liquid phase in equilibrium conditions. The calculations are performed with the successive inclusion of the size dependences of the characteristics of pure components and the parameter of the interparticle interaction in the phases. The calculated results agree with the experimental data in the microscopic case.

Chemical carbonization of polyvinylidene fluoride is necessary for the synthesis on its basis of quasi-one-dimensional carbon structures having a prospect of use in microelectronics. To determine the details of the carbonization process, the concentrations of ¹⁹F and ¹H nuclei were measured and the changes in the concentration of CF₂, CH₂, and OH groups in partially dehydrofluorinated polyvinylidene fluoride samples were studied using nuclear magnetic resonance and infrared spectroscopy. The efficiency of substitution of fluorine and hydrogen atoms by OH groups in the process of dehydrofluorination of polymer chains is estimated.

The experimental results of investigation of the nickel single crystal surface morphology after compression deformation are presented. The quasi-periodic character of the deformation profile, common for shear deformation of different types of relief structural elements, is found. It is demonstrated that the morphological manifestation of these structural elements is determined by local shear systems along octahedral planes. The regularities of the deformation structure in these regions defining the material extrusion and intrusion regions and the specific features of disorientation accumulation are established. If reorientation of local regions takes part in the relief element formation, along with octahedral slip, much stronger growth of the surface area is observed. The possibility of application of two-dimensional and three-dimensional surface roughness parameters for description of deformation relief is considered.

Introducing the filler nanoparticles into a polymer matrix is shown to not necessarily favor the formation of a true nanomaterial (nanocomposite). For this, a certain nanofiller structure has to be achieved in the polymer matrix. In polymer/organoclay nanocomposites it can be attained only by forming an exfoliated organoclay structure. A transition from micro-to nanocomposite under other similar conditions is accompanied by a drastic increase (by several times) in the elastic modulus.

The temperature gradient formation mechanism in an initially uniformly heated hybrid-oriented liquid-crystal (HOLC) channel of microscopic sizes upon exposure to a steady hydrodynamic flow is theoretically studied within the nonlinear generalization of the Ericksen–Leslie theory, taking into account the heat conduction equation. The case of total thermal insulation of one of the HOLC channel is considered provided that a constant temperature is maintained on the other surface. It is shown that the temperature difference χ_max(ζ) in the HOLC channel section, caused by the horizontal steady flow with the “triangular” velocity profile u(z, ζ) is significantly affected by the position ζ of the maximum velocity. It is shown that, in the case of the LC system formed by 4-n-pentyl-n'-cyanobiphenyl molecules, the hydrodynamic flow characterized by the peak position ζ = 0.98 of the velocity u(z, ζ = 0.98) ~ 0.9 μm/s forms a maximum temperature difference χ_max(ζ) = 0.03 (~9 K) over the HOLC channel section.

Kinetic processes have been studied in composites based on fullerenes and exfoliated graphite at the initial proportions of components from 1: 16 to 16: 1 in mass. The samples are produced by heat treatment of initial dispersed mixtures in vacuum in the diffusion–adsorption process, their further cold pressing, and annealing. It is shown that the annealing almost does not influence the conduction mechanisms and only induces additional structural defects acting as electron traps. As a whole, the results obtained at the noted proportions of components make it possible to consider the material as a compensated metallic system with a structural disorder in which the charge transfer at temperatures from 4.2 K to room temperature is controlled by quantum interference phenomena. At low temperatures, the effect of a weak localization is observed, and the electron–electron interactions take place at medium and high temperatures.

The method for calculating the thermal conductance of “metal–dielectric” interfaces, which takes into account the adhesive interaction and anharmonicity of atomic vibrations at the interface within the acoustic mismatch model and allows achieving better agreement of calculated and experimental data.