Vol 60, No 7 (2018)

Structural percolation transitions in ultrathin metal films at thermally activated granularity was studied. The degree of granularity of an Au/GGG film is shown to increase with decreasing its effective thickness. The film structure peculiarities are visualized upon the percolation transition via scanning electron microscopy. The feasible causes, mechanisms and models of activation conductivity in islet metal nanofilms are discussed and the statistical width distribution of inter-islet gaps is considered, as well.

The technique for estimating the interfacial free energy of transition-metal nanocrystals and its anisotropy at the interface with their melts has been developed. The expression for the coordinate of the Gibbs’ interface, which takes into account the size dependence, has been derived. The interfacial free energy of crystal faces at the interface with the related melts of monomorphic 4d and 5d metals decreases nonlinearly with a decrease in the nanocrystal size and, at a certain size, disappears. At the nanocrystal radius of more than 10 nm, the interfacial free energy of the faces approaches that for a macrocrystal. The temperature dependence of the interfacial free energy at the crystal–melt interface is almost linear. The technique developed is shown to be in agreement with the known experimental data for mono- and polycrystals and applicable for estimating the orientational, temperature, and size dependences of the interfacial free energy at the interfaces of nano-, micro-, and macrocrystals with their melts.

The effect of bismuth on the structural perfection and the luminescent properties of Al_xIn_yGa_1–x–yBi_zSb_1–z/GaSb heterostructures has been studied. The optimal parameters of the process of zone recrystallization with temperature gradient at which epitaxial AlInGaBiSb layers have the minimum roughness and high structural perfection have been revealed: temperature gradient 1 ≤ G ≤ 30 K/cm, the liquid zone thickness 60 ≤ l ≤ 100 μm, the temperature range 773 K ≤ T ≤ 873 K, and bismuth concentration 0.3–0.4 mol fraction.

The method of the density functional theory is used to study structural transformations between graphites and diamond-like phases. The calculations have been carried out in two approximations: a local density approximation and a generalized gradient approximation. It is found that the phase transitions of hexagonal graphene layers to a cubic diamond and diamond-like phases must occur at uniaxial compressions of ~57–71 GPa, whereas some diamond-like phases can be obtained from tetragonal graphene layers at significantly lower pressures of 32–52 GPa. The X-ray diffraction patterns have been calculated for the phase transition of graphite I4₁/amd to tetragonal LA10 phase that takes place at the minimum pressure that can be used for experimental identification of these compounds.

The recrystallization of silver sulfide Ag₂S nanoparticles has been studied and the range of the thermal stability of the nanoparticle sizes has been determined. Nanopowders Ag₂S with particle sizes of 45–50 nm were obtained by chemical deposition from aqueous solutions. To study the thermal stability of the Ag₂S nanoparticle sizes, the nanocrystalline powders have been annealed in a vacuum of 0.01 Pa on heating from room temperature to 493 K and in argon at 623 K. Annealing up to a temperature of 453 K leads to insignificant nanoparticle growth and annealing of microstrains, which allows one to consider this temperature range as the region of thermal stability of the silver sulfide nanostate. The temperature range from 450 to 900 K, in which the particle size increases by a factor of 3–6, corresponds to the temperature of collective recrystallization of the silver sulfide nanopowder.

Using the Monte Carlo method, critical behavior of the one-dimensional ferromagnetic Ising model has been investigated with allowance for the interaction of the second and third neighbors and four-particle interaction. The obtained results on the critical temperature were compared with the critical temperature of the quasi-one-dimensional Ising magnetic [(СН₃)₃NH] · FeCl₃ · 2H₂O and with the magnitude of the exchange interaction J/k_B = 17.4 K. Within the scope of the finite-dimensional scaling theory, the critical susceptibility exponent has been calculated. It has been shown that values of the susceptibility exponent for the one-dimensional Ising model with periodic boundary conditions are considerably less than the known values of the exponents for three-dimensional systems. The critical susceptibility exponent strongly depends on energy parameters; namely, it decreases with an increase in them.

It is shown in terms of the phenomenological Landau theory of phase transitions that a phase transition to an inhomogeneous polar phase preceding in temperature a phase transition to a homogeneous polar state is possible. As a result of solving a boundary eigenvalue problem for the polarization equilibrium equation and electrostatics equations, wave vector k_⊥ characterizing the inhomogeneous phase has been determined and the temperature boundaries of its existence in the dependence on the film thickness and its surface properties have been found.

The mechanism of dynamic instability of a solid surface under a load, determined by perturbations of the electron density and the change in the interatomic interaction, is considered. This instability is manifested upon the excitation of dynamic displacements of atoms in the surface layer, which can lead to formation of finite-lifetime structures in the form of waves with a large amplitude. Such structures were earlier observed experimentally on a stretched germanium (111) surface.

The interaction of a structural subsystem with dislocations under the superposition of an elastic torsion strain and an additional uniaxial spatially nonuniform time-constant pressure is studied. The analysis is carried out within of Landau’s phenomenological theory with refusal of the approximation of constant magnitudes of irreducible vectors. The rise of spatial resonances of the dislocation density in these conditions is shown.

The features of the macroscopic inhomogeneity of plastic deformation in the form of autowaves with a pulsating amplitude are analyzed, and data on the localization of sources of acoustic emission at different stages of plastic flow in the stretching of fcc mono- and polycrystals are presented. The relationship between the local components of the plastic distortion tensor in the strain localization zone is traced. The role of acoustic phenomena accompanying the localization of plastic strain in the development of the process of plastic deformation is considered.

Paramagnetic resonance of ZnSe: Fe single crystals was studied at 120 K. Parameters of the spin Hamiltonian were determined for the monoclinic dimer complex Fe³⁺–Cu⁺ and a second discovered Fe³⁺ center, which appears due to association of the iron ion with a K⁺ ion in a Zn²⁺ position or with a zinc vacancy. Effect of illumination of the zinc selenide samples by blue, green and red light on the paramagnetic resonance spectrum was investigated.

The mechanism of the upconversion processes in Y₆O₅F₈: 2%Er³⁺/X%Yb³⁺ (X = 3, 10, 20) microtubes has been explored. The luminescent properties of the as prepared sample is investigated by utilizing up- /downconversion, decay and time resolve spectra. The results indicate that the red and green emission are clearly competitive depending on the Yb³⁺ concentration. High Yb³⁺ concentration induces the enhancement of the energy-back-transfer (EBT), process, which leads to the quenching of green emission and enhances the red emission. So it is possible to utilize the temporal evolutions of emission bands to deeply understand the color change UC mechanisms.

The Nd_0.67Sr_0.33MnO₃ manganite is a material promising for application as a cathode for medium-temperature solid oxide fuel elements. A high electrical conductivity of such a cathode is the parameter determining the efficiency of the operation of a fuel element. In this report, the effect of influence of excess manganese on the structure and the conductivity of manganite ceramics with compositions (Nd_0.67Sr_0.33)_1–xMn_{1 + x}O_{3± Δ} (x = 0, 0.2) sintered at temperatures 1273–1673 K is presented for the first time. The existence of 20% excess manganese in the initial manganite powder after sintering is shown to lead to that the conductivity of the obtained ceramics in the temperature range 823–1073 K is several times higher than the conductivity of the ceramics without excess manganese.

General analytical expressions for densities of states of a lateral heterostructure, formed by the contact of two square semi-infinite lattices with single-band and two-band spectra, were obtained in the tight-binding approximation by the Green’s function method. The semi-elliptical density of states was used for numerical estimates, and the model of two interacting dimers was proposed to estimate the charge transfer. Application of this approach to description of lateral epitaxial and graphene-like heterostructures is discussed.

A sequence of phases forming during the solid-phase reaction in Al/Pt bilayer thin films has been investigated by in situ electron diffraction. It is shown that the amorphous PtAl₂ phase forms first during the solid-phase reaction initiated by heating. Upon further heating, PtAl₂, Pt₂Al₃, PtAl, and Pt₃Al crystalline phases sequentially form, which is qualitatively consistent with an effective formation heat model. The content of phases forming during the reaction has been quantitatively analyzed and the structural phase transformations have been examined.

The results of structural and magnetic investigations of nanogranular Co–Al₂O₃ films formed from Co₃O₄/Al thin-film layered structures upon vacuum annealing are reported. The Co₃O₄/Al films have been obtained by sequential reactive magnetron sputtering of a metallic cobalt target in a medium consisting of the Ar + O₂ gas mixture and magnetron sputtering of an aluminum target in the pure argon atmosphere. It is shown that such a technique makes it possible to obtain nanogranular Co–Al₂O₃ single- and multilayer thin films with a well-controlled size of magnetic grains and their distribution over the film thickness.

The intercalation of iron under a graphene monolayer grown on 4H-SiC(0001) is studied. The experiments have been carried out in situ under conditions of ultrahigh vacuum by low-energy electron diffraction, high-energy-resolution photoelectron spectroscopy using synchrotron radiation, and near carbon K-edge X-ray absorption spectroscopy. The deposited iron film thicknesses have been varied within 0.1–2 nm and the sample temperatures from room temperature to 700°C. It is shown that the intercalation process begins at temperatures higher than ~350°C. In this case, it is found that intercalated iron atoms are localized not only between graphene and a buffer layer coating SiC, but also under the buffer layer itself. The optimal conditions of the intercalation are realized in the range 400–500°C, because, at higher temperatures, the system becomes unstable due to the chemical interaction of the intercalated iron with silicon carbide. The inertness of the intercalated films to action of oxygen is demonstrated.

A theoretical description of the process of formation of vortex flows v(t, r) and the evolution of the director field \(\hat n\) in microliter liquid crystal (LC) volumes with a free surface under the influence of a temperature gradient ∇T(t, r), which is initiated by focused laser radiation, has been proposed. Thermomechanical contributions to both the stress tensor and viscous moment which are acting per unit volume of the LC phase were taken into account in the framework of the nonlinear generalization of the classical Ericksen−Leslie theory, which allowed describing the origin and formation of vortex flows in nematics formed by 4-n-pentyl- 4'-cyanobiphenyl molecules. Various hydrodynamic modes of vortex formation in microsized LC volumes under the action of focused laser radiation have been investigated by numerical methods.

The magnetic Freedericksz transition in a ferronematic representing a diluted suspension of magnetic nanoparticles in an N-(4-methoxybenzylidene)-4-butylaniline (MBBA) nematic liquid crystal was experimentally studied. The transition point was determined by means of dielectric capacitance measurements in cells of different thickness for samples with various volume fractions of ferroparticles. The effect of weak coupling between the magnetic and liquid-crystal subsystems was studied. Low concentrations of quasispherical magnetic particles were shown to decrease the magnetic Freedericksz transition threshold in comparison with a pure liquid crystal. The obtained results were theoretically substantiated. The phase diagram of the suspension was plotted, and the method of estimating the energy of coupling between magnetic particles and a liquid-crystal matrix was proposed.

Polyhydroxylated fullerene C₆₀(OH)_n (with an estimated number of hydroxyl groups n = 38–44) synthesized from pure fullerene by mixing a benzene solution of C₆₀ with a NaOH aqueous solution in the presence of a catalyst was studied with ¹H and ¹³C solid-state nuclear magnetic resonance. Possible features of the structure of a molecule shell were revealed from ¹H NMR data. The ¹³C spectrum showed a peak splitting with an increase in temperature, which is probably due to fullerenol isomers.