Vol 59, No 12 (2017)

A model of fast formation of high-contrast periodic structure appearing on a semiconductor surface under action of laser radiation is proposed. The process of growing a surface structure due to the interaction surface plasmon- polaritons excited on nonequilibrium electrons with incident laser radiation are considered in the framework of a medium with nonlinear diffusion of nonequilibrium carriers (defects). A resonance effect of superfast pico- and subpicosecond amplification of the plasmon-polariton structure generated on the surface, the realization of which can result in a high-contrast defect lattice.

Titanates Sm₂Ti₂O₇ and Er₂Ti₂O₇ with pyrochlore structure have been prepared by solid-phase synthesis in air from stoichiometric Sm₂O₃ (Er₂O₃)–TiO₂ mixtures sequentially at 1673 and 1773 K. Hightemperature heat capacity of the oxide compounds has been determined by differential scanning calorimetry. Their thermodynamic properties have been calculated from experimental temperature dependence C_p = f(T).

The structural and electrophysical characteristics of a number of solid solutions of layered perovskite-like oxides Bi_3–xY_xTiNbO₉ (x = 0, 0.1, 0.2, 0.3) have been studied. According to the data of powder X-ray diffraction, all the compounds are single-phase with the structures of two-layer Aurivillius phases (m = 2) with the orthorhombic crystal lattice (space group A2₁am). The temperature dependence of the relative permittivity ε/ε0(T) compounds have been measured and showed that the Curie temperature of the perovskite-like oxides Bi_3–xYxTiNbO₉ linearly increases with the substituting parameter x up to T_C = 965°C.

The evolution of the ground state of the manganese spin ensemble in the (Sm_1–yGd_y)_0.55Sr_0.45MnO₃ in the case of isovalent substitution of rare-earth samarium ions with large radii with gadolinium ions with significantly smaller radii is studied. The measured temperature dependences of the ac magnetic susceptibility and the field dependences of the dc magnetizations are analyzed using the Heisenberg–Kitaev model describing the transition from the ordered spin state with classical isotropic AFM exchange to the frustrated spin state with quantum highly anisotropic FM exchange. A continuous transition from the 3D ferromagnetic state of manganese spins in the initial sample with y = 0 to zigzag AFM ordering of CE-type spins in ab planes for y = 0.5, coexisting in samples with y = 0.5, 0.6, and 0.7 at temperatures below T_N ≅ 48.5 K with a disordered phase such as a quantum Griffiths phase is identified. As the gadolinium concentration further increases, the CE-type zigzag AFM structure is molten, which leads to the appearance of an unusual phase in Gd_0.55Sr_0.45MnO₃ in the temperature range close to the absolute zero. This phase has characteristic features of a gapless Z₂ quantum spin liquid in zero external magnetic field. The step changes in the magnetization isotherms measured at 4.2 K in the field range of ±75 kOe are explained by quantum phase transitions of the Z₂ spin liquid to a phase with topological order in weak magnetic fields and a polarized phase in strong fields. The significant difference between critical fields and magnetization jumps in isotherms indicates the existence of hysteretic phenomena in quantum spin liquid magnetization–demagnetization processes caused by the difference between localization–delocalization of 2D vortex pairs induced by a magnetic field in a quantum spin liquid with disorder.

Samples of BiFe_0.93Mn_0.07O₃ with different specific surface area were synthesized for the first time by ultrasonic spray pyrolysis. The resulting powders consist of porous particles of a spherical shape of medium size ~0.5 μm and have record values of residual magnetization and coercive force. It is found that the magnetic properties of the porous powder particles are determined by the distortion of the crystal lattice and the presence of uncompensated magnetic moments of iron ions on the surface.

Electrodynamic properties of Pb(Fe_0.95Sc_0.05)_2/3W_1/3O₃ solid solution belonging to A(B'B'')O₃ perovskite structural family have been investigated by broadband dielectric spectroscopy in a wave-number range of (4 × 10^–9–4 × 10³) cm^–1 and a temperature range of 100–600 K. The influence of low-frequency relaxations on the vibrational spectrum is determined within the four-parameter factorized dispersion model. Anomalies in the behavior of the dielectric response function are found near the temperature-diffuse maximum of permittivity.

The variation of the internal friction, Young’s modulus, and electrical resistivity of two grades of polycrystalline titanium (VT1-0 and Grade 4) in the area of low temperatures (100–300 K) as depending on the initial structure and subsequent severe plastic deformation converting the material into the submicrocrystalline structural state in relation to the grain size is studied. The maximum of the internal friction is detected in submicrocrystalline titanium, which is interpreted as a Bordoni peak. All the studied characteristics are sensitive indicators for a nonequilibrium state of the grain boundaries after the deformation. The effect of the initial structure of the metal on its properties after the severe deformation is revealed.

Behavior of threading dislocations in porous heteroepitaxial gallium nitride (GaN) films has been studied using computer simulation by the two-dimensional discrete dislocation dynamics approach. A computational scheme, where pores are modeled as cross sections of cylindrical cavities, elastically interacting with unidirectional parallel edge dislocations, which imitate threading dislocations, is used. Time dependences of coordinates and velocities of each dislocation from dislocation ensembles under investigation are obtained. Visualization of current structure of dislocation ensemble is performed in the form of a location map of dislocations at any time. It has been shown that the density of appearing dislocation structures significantly depends on the ratio of area of a pore cross section to area of the simulation region. In particular, increasing the portion of pores surface on the layer surface up to 2% should lead to about a 1.5-times decrease of the final density of threading dislocations, and increase of this portion up to 15% should lead to approximately a 4.5-times decrease of it.

The spectral characteristics of the photoluminescence lines detected for nanodiamonds obtained by the reactive ion etching of diamond particles in oxygen plasma, deposited by chemical vapor deposition on a silicon substrate, are studied. At room temperature, narrow lines are observed in the visible and infrared spectral regions, with a full width at half-maximum in the range of 1–2 nm at an almost complete absence of a broadband photoluminescence background signal. At decreasing temperature, the lines narrowed to 0.2–0.6 nm at T = 79 K, and the minimum line width was 0.055 nm at T = 10 K. With increasing temperature, the narrow lines shifted to the long-wavelength region of the spectrum, and their intensity decreased.

A gallium nitride crystal 5 mm in thickness was grown by chloride–hydride vapor-phase epitaxy on a sapphire substrate, from which the crystal separated during cooling. At an early stage, a three-dimensional growth mode was implemented, followed by a switch to a two-dimensional mode. Spectra of exciton reflection, exciton luminescence, and Raman scattering are studied in several regions characteristic of the sample. Analysis of these spectra and comparison with previously obtained data for thin epitaxial GaN layers with a wide range of silicon doping enabled conclusions about the quality of the crystal lattice in these characteristic regions.

Using infrared (IR) spectroscopy and spectral ellipsometry, we experimentally confirmed the previously predicted mechanochemical effect of the stoichiometric composition disorder leading to the formation of carbon-vacancy structures in silicon carbide (SiC) films grown on silicon substrates by the atom substitution method. It was found that a band at 960 cm^–1 in the IR spectra of SiC films on silicon, corresponding to “carbon-vacancy clusters” is always present in SiC films grown under pure carbon monoxide (CO) or in a mixture of CO with silane (SiH₄) on Si substrates of different orientation and doping level and type. There is no absorption band in the region of 960 cm–1 in the IR spectra of SiC films synthesized at the optimum ratio of the CO and trichlorosilane (SiHCl₃) gas pressures. The previously predicted mechanism of the chemical reaction of substitution of Si atoms for carbon by the interaction of gases CO and SiHCl₃ on the surface of the silicon substrate, which leads to the formation of epitaxial layers of single-crystal SiC, is experimentally confirmed.

Temperature dependences of the electrical conductivity of thin vanadium sesquioxide V2O3 films obtained by using the laser sputtering technique have been studied. A significant decrease (by four–five orders of magnitude) in the electrical conductivity has been observed below 150 K as a result of a metal–insulator phase transition. It is shown that hydrogenation of films lowers the temperature of this phase transition.

Using Monte Carlo calculations, we study the thermodynamic and magnetic properties of 2D structures that can be described by a three-state Potts model on a triangular lattice with nearest- and secondnearest- neighbor interactions characterized by the coupling constants J₁ and J₂, respectively. Analyzing the thermodynamic parameters of heat capacity, the order parameter, the susceptibility, and fourth-order Binder cumulants, we show that the three-state Potts model with coupling constants J₁ > 0 and J₂ < 0 predicts a phase transition of the second kind for 0 ≤ |r| ≤ 1/3, where r = J₂/J₁.

Based on the free-energy density functional method (the Cahn–Hilliard equation), a phenomenological model that describes the influence of grain boundaries on the distribution of components in binary alloys has been developed. The model is built on the assumption of the difference between the interaction parameters of solid solution components in the bulk and at the grain boundary. The difference scheme based on the spectral method is proposed to solve the Cahn-Hilliard equation with interaction parameters depending on coordinates. Depending on the ratio between the interaction parameters in the bulk and at the grain boundary, temperature, and alloy composition, the model can give rise to different types of distribution of a dissolved component, namely, either depletion or enrichment of the grain-boundary area, preferential grainboundary precipitation, competitive precipitation in the bulk and at the grain boundary, etc.

The structure of surface layers of thin metal inverse opals has been studied first by the grazing-incidence small-angle X-ray scattering technique. Contributions of the form factor and structure factor to the small-angle diffraction pattern have been separated using a numerical model of the scattering process. The complementary use of the small-angle X-ray scattering and grazing-incidence small-angle X-ray scattering techniques has provided independent information about the bulk and surface properties of the samples and allowed a type of defect in the investigated structures to be determined. The measurement results have been verified by atomic force microscopy.

NMR spectrum and recovery of longitudinal nuclear magnetization after inverting pulses were measured for isotopes ⁶⁹Ga and ⁷¹Ga in a liquid eutectic alloy Ga–In–Sn introduced in porous glasses having pores with the sizes of 25 and 7 nm. The measurements were conducted in the fields of 9.4, 11.7, and 17.6 T and compared with those for the bulk melt sample. Differences between the shape and position of the NMR lines in various fields and for different gallium isotopes were revealed in the melt introduced into the porous glass with the pores size of 7 nm. Data obtained for this sample were interpreted based on the theoretical model of dynamic quadrupolar shift, and correlation time of atomic motion was found within this model. Characteristics of the atomic motion calculated based on the dynamic shift model and from the spin relaxation data were shown to agree with each other.

Field-effect transistor (FET) structures based on soluble organometallic perovskites, CH₃NH₃PbBr₃, were obtained and their electrical properties were studied. FETs made of CH₃NH₃PbBr₃ films possess current- voltage characteristics (IVs) typical for ambipolar FETs with saturation regime. The transfer characteristics of FETs based on CH₃NH₃PbBr₃ have an insignificant hysteresis and slightly depend on voltage at the source-drain. Mobilities of charge carriers (holes) calculated from IVs of FETs based on CH₃NH₃PbBr₃ at 300 K in saturation and weak field regimes were ~5 and ~2 cm²/V s, respectively, whereas electron mobility is ~3 cm²/V s, which exceeds the mobility value ~1 cm²/V s obtained earlier for FETs based on CH₃NH₃PbI₃.

The effect of solid-state phase transitions on the kinetic parameters of creep deformation in polytetrafluoroethylene has been studied by differential scanning calorimetry and a laser-interferometric creep rate meter. It has been shown that the deformation activation volumes and the elementary phase transition volume differ from each other by more than an order of magnitude. The crystalline component of this polymer becomes a liquid-crystal phase coexisting with an amorphous phase.

Within the framework of the generalized mean-field model that takes into account the anisotropic interactions between the nearest neighbors of molecules forming freely suspended smectic films (FSSFs) and the stabilizing effects of the smectic-A (SmA)–air interface, a numerical study was performed of the structural, thermodynamic, and optical properties of these systems in the process of their layer-by-layer thinning. The results of calculating the disjoining pressure P, the average thickness of the smectic layers L, and the reflectivity index R of a FSSF formed by 5-n-alkyl-2-(4-n-(perfluoroalkyl-methylene oxide)-pentyl) (H10F5MOPP) molecules showed that these values undergo precipitous changes in the process of layer-bylayer thinning of the film. Calculations of R(T) as a function of temperature T exceeding the phase transition temperature of SmA into an isotropic state in the bulk of the liquid crystal material are in good agreement with the experimentally obtained data for the reflectivity of the FSSF formed by H10F5MOPP molecules.

The catalytic activity of Pt clusters is dependent not only on the nanoparticle size and its composition, but also on its internal structure. To determine the real structure of the nanoparticles used in catalysis, the boundaries of the thermal structure stability of Pt clusters to 8.0 nm in diameter interacting with carbon substrates of two types: a fixed α-graphite plane and a mobile substrate with the diamond structure. The effect of a substrate on the processes melting of Pt nanoclusters is estimated. The role of the cooling rate in the formation of the internal structure of Pt clusters during crystallization is studied. The regularities obtained in the case of “free” Pt clusters and Pt clusters on a substrate are compared. It is concluded that platinum nanoparticles with diameter D ≤ 4.0 nm disposed on a carbon substrate conserve the initial fcc structure during cooling.

Vanadates Cd₂V₂O₇ and CdV₂O₆ have been prepared from CdO и V₂O₅ by three-phase synthesis with subsequent burning at 823–1073 K and 823–853 K, respectively. The molar heat capacity of these oxide compounds has been measured by differential scanning calorimetry. The enthalpy change, the entropy change, and the reduced Gibbs energy are calculated using the experimental dependences C_p = f(T). It is shown that there is a correlation between the specific heat capacity and the composition of CdO–V₂O₅ oxide system.