Vol 58, No 12 (2016)

The results of the investigation of the electronic structure and optical properties of the TbMn_0.33Ge₂ compound have been presented. The spin-polarized calculations of the band spectrum have been performed within the framework of the local spin density approximation (LSDA) with a correction for strong correlations in the 4f shell of the rare-earth ion (the LSDA + U method). The optical constants have been measured using the ellipsometric method and a number of spectral and electronic characteristics of the compound under investigation have been determined over a wide range of wavelengths. The interband part of the experimental dependence of the optical conductivity has been interpreted using the results of the calculation of the electron density of states.

The influence of the concentration of lithium ions on the phase composition, the electrical conductivity, and the thermoelectric power of La_1–xLi_xCoO_3–δ (0 ≤ x ≤ 0.1) oxides synthesized by the ceramic method has been investigated. It has been found that the region of the existence of perovskite-type La_1–xLi_xCoO_3–δ solid solutions does not exceed x = 0.05. The doping with lithium leads to an increase in the electrical conductivity of single-phase samples in comparison with that of the LaCoO₃ compound. As the temperature increases from 300 to 400 K, the thermoelectric power of the LaCoO₃ compound increases from the negative to positive values and then decreases, but remains positive in the temperature range from 400 to 1020 K. The thermoelectric power of the other samples has a positive sign. The results obtained have been discussed based on the models of the electron density of states in LaCoO₃ and La_1–xSr_xCoO_3–δ, proposed in the studies of Señarís-Rodríguez and Goodenough, as well as in the framework of the theory of non-crystalline materials, developed by Mott.

We report the possibility of autocatalytic synthesis of highly crystalline perfect CdTe nanowires by magnetron presputtering deposition through the windows in ultrathin layers of SiO₂. The photoluminescence spectra of obtained CdTe nanowires exhibit an emission band in the 1.4–1.7 eV region, indicating crystalline perfection of the nanowires.

The phase composition and the structural properties of potassium nitrate KNO₃ and its heterogeneous composites with nanometer-sized powder of aluminum oxide Al₂O₃ have been studied by X-ray diffraction at various concentrations of an Al₂O₃ nanopowder. It is found that, in the (1–x)KNO₃ + xAl₂O₃ nanocomposites, additional high-temperature rhombohedral phase of potassium nitrate (phase III) with lattice parameters a = 5.4644 Å and c = 9.0842 Å. With increasing concentration of Al₂O₃ nanopowder, the content of the main potassium nitrate phase (phase II) is found to significantly decrease, and the relative fraction of the phase III in the total content of the nitrate in the composite composition increases. This phase is assumed to be “frozen” in the nanocomposite at the KNO₃–Al₂O₃ interface. The estimated size of KNO₃ crystallites in the phase III is more than 20 nm.

The temperature dependences of the magnetization of ferromagnetic Heusler alloys Co₂YAl, where Y = Ti, V, Cr, Mn, Fe, and Ni have been studied at H = 50 kOe in the range 2 K < T < 1100 K. It is shown that the high-field (H ≥ 20 kOe) magnetization is described within the Stoner model.

The band structure and distributions of the electron and spin densities of samarium orthoferrite have been calculated within the framework of the first-principles density functional theory in the LSDA + U approximation taking into account the collinear antiferromagnetic ordering of the magnetic moments of iron and samarium cations. The possibility of inducing a ferroelectric state at temperatures below the antiferromagnetic ordering temperature of the magnetic sublattice formed by samarium cations has been considered using the results of the group-theoretical analysis. In the high-temperature range, the formation of regions with a spontaneous electric polarization is possible in the presence of additional factors that reduce the symmetry of the crystal.

Thin film single-layered (Co₄₁Fe₃₉B₂₀)_x(SiO₂)_100–x nanocomposites at x = 30–80 at % and multilayered nanocomposites composed of 176 pairs of [(Co₄₁Fe₃₉B₂₀)₆₀(SiO₂)₄₀]/[(Co₄₁Fe₃₉B₂₀)₆₀(SiO₂)₄₀ + O₂] have been prepared via ion-beam sputtering of the complex target. The concentration dependences of the magnetic permeability of single-layered films at a frequency of 50 MHz are characterized by maximum losses near x = 60 at %, whereas the percolation threshold with respect to the electric conductivity is x = 50 at %. The high-frequency magnetic permeability of films has been measured by the resonator method in the frequency range of 0.1—10 GHz. As is shown, while the single-layer film passes to the multilayered structure, the ferromagnetic resonance frequency shifts from 1.5 to 2.5 GHz, and the imaginary part of the magnetic permeability attains 200 that is presumably due to the inhibition of the perpendicular magnetic anisotropy component.

This paper presents the results of the investigation of the energy spectrum of electronic states due to trapping centers, the role of which in CuInAsS₃ is played by lattice defects. The results of the analysis of the thermally stimulated current curves of CuInAsS₃ demonstrate that the energy spectrum of trapping centers is localized under the bottom of the conduction band in the energy range E_C–(0.14–0.35) eV.

The numerical analysis of a nonlinear equation set has revealed a possible increase in the ferroelectric phase transition point at the presence of the intermediate layer at the ferroelectric–dielectric interface. The order parameter distribution in the ferroelectric particles, as well as the effect of the intermediate layer thickness on the Curie temperature, has been studied.

ZnS crystals grown form the vapor phase and ZnS/(001)GaAs epitaxial structures grown by metalorganic vapor phase epitaxy are studied by transmission electron microscopy after in situ irradiation in an electron microscope at an electron energy of 400 keV and intensity of (1–4) × 10¹⁹ electrons/cm² s. It is shown that irradiation leads to the formation of small (2.5–45 nm) dislocation loops with a density of 1.4 × 10¹¹ cm^–2, as well as voids and new phase inclusions ≤10 nm in size. Using the moire fringe contrast analysis, these inclusions were identified as ZnO and ZnO₂.

The microstructure and amplitude dependences of the Young’s modulus E and internal friction (logarithmic decrement δ), and microplastic properties of biocarbon matrices BE-C(Fe) obtained by beech tree carbonization at temperatures T_carb = 850–1600°C in the presence of an iron-containing catalyst are studied. By X-ray diffraction analysis and transmission electron microscopy, it is shown that the use of Fe-catalyst during carbonization with T_carb ≥ 1000°C leads to the appearance of a bulk graphite phase in the form of nanoscale bulk graphite inclusions in a quasi-amorphous matrix, whose volume fraction and size increase with T_carb. The correlation of the obtained dependences E(Т_carb) and δ(T_carb) with microstructure evolution with increasing Т_carb is revealed. It is found that E is mainly defined by a crystalline phase fraction in the amorphous matrix, i.e., a nanocrystalline phase at Т_carb < 1150°C and a bulk graphite phase at T_carb > 1300°C. Maximum values E = 10–12 GPa are achieved for samples with Т_carb ≈ 1150 and 1600°C. It is shown that the microplasticity manifest itself only in biocarbons with T_carb ≥ 1300°C (upon reaching a significant volume of the graphite phase); in this case, the conditional microyield stress decreases with increasing total volume of introduced mesoporosity (free surface area).

The ¹⁵¹Eu²⁺ orthorhombic centers in yttrium aluminum garnet crystals have been investigated. The initial splitting and hyperfine interaction parameters have been obtained taking into account the positions of the hyperfine structure components. The relative signs of the fine and hyperfine structure parameters have been determined from the analysis of the type of the hyperfine structure formed by the allowed and forbidden electron–nuclear transitions.

An experimental study of structural and optical properties of silver and gold thin films, as well as bilayer films based on these metals, has been carried out. To study properties of nanofilms, reflection spectra upon excitation of surface plasmons in the Kretschmann geometry and spectra of ellipsometric parameters have been investigated. The reflection spectra were analyzed using a theoretical model for determining (effective) dielectric constants of the films. The calculated dielectric constants of the films differ from data obtained by ellipsometry. The features in determining dielectric constants by different methods and characteristics of manufactured films in relation to the influence of the substrate are discussed.

The results are presented on phonon excitations and the electronic structure of Co₂TiO₄ inverse spinel in which magnetically ordered cobalt ions Co²⁺ (3d⁷) are in equal amounts in tetrahedral and octahedral sublattices below T_C = 56 K. Single crystals are studied using optical reflection and absorption in a wide spectral range, Raman scattering, and dielectric spectroscopy methods. The dynamics of infrared and Raman-active phonons are studied, and the features associated with disordering in tetrahedral sites are detected. The d–d electronic transitions recorded in the regions of 3800 and 6300 cm^–1 confirm the coordination features of Co²⁺ ions. An increase in the permittivity in the temperature region below 130 K is detected.

The crystal structure of rubidium gallate RbGaO₂ in the temperature range of 300–853 K has been investigated using high-temperature neutron diffraction. The channels available for the motion of rubidium cations in the low-temperature and high-temperature modifications of RbGaO₂ have been determined using the computer simulation with the TOPOS program. A correlation between the radius of the migration channel cross section and the rubidium cation conductivity has been established.

A theory of plasmonic differential anisotropic reflection of light from nanoparticles located near the interface between media is developed. The model of a monolayer consisting of identical ellipsoidal metal particles occupying sites of a rectangular lattice is investigated. Effective plasmonic polarizabilities of nanoparticles in the layer are calculated self-consistently using the Green’s function technique in the quasipoint dipole approximation. The local-field effect caused by anisotropic dipole plasmons of particles in the layer and their image dipoles is taken into account. The lately observed resonant reflectance anisotropy spectra of indium nanoclusters on InAs surface are explained by the difference between frequencies of plasmons with the orthogonal polarizations in the surface plane. The difference between the plasmon frequencies is attributed to anisotropy of the particles shape or/and the layer structure; the signs of frequency difference for the two types of anisotropy being different.

A comparative investigation of graphene prepared by cracking of propylene (C₃H₆) on nickel surfaces with different orientations, Ni(111) and Ni(100), has been carried out using angle-resolved photoemission spectroscopy. It has been shown that the graphene formed on the Ni(111) surface is well ordered on a large surface area, whereas the graphene on the Ni(100) surface has a well-defined domain structure. It has been found that the electronic structures of the two systems are similar to each other, and graphene is strongly bound to the nickel substrate. It has been demonstrated that the intercalation of a gold monolayer for the two systems leads to the formation of an electronic structure that is characteristic of quasi-free-standing graphene.

Structure of 40-nm thick La_0.67Ca_0.33MnO₃ (LCMO) films grown by laser evaporation on (001) and (110) LaAlO₃ (LAO) substrates has been investigated using the methods of medium-energy ion scattering and X-ray diffraction. The grown manganite layers are under lateral biaxial compressive mechanical stresses. When (110)LAO wafers are used as the substrates, stresses relax to a great extent; the relaxation is accompanied by the formation of defects in a (3–4)-nm thick manganite-film interlayer adjacent to the LCMO–(110)LAO interface. When studying the structure of the grown layers, their electro- and magnetotransport parameters have been measured. The electroresistance of the LCMO films grown on the substrates of both types reached a maximum at temperature T_M of about 250 K. At temperatures close to T_M magnetoresistance of the LCMO/(110)LAO films exceeds that of the LCMO/(001)LAO films by 20–30%; however, the situation is inverse at low temperatures (T < 150 K). At T < T_M, the magnetotransport in the grown manganite films significantly depends on the spin ordering in ferromagnetic domains, which increase with a decrease in temperature.

The resistive switching effects in composite films containing polyfunctional polymers, such as derivatives of carbazole (PVK), fluorene (PFD), and polyvinyl chloride (PVC), and also graphene particles (Gr) and graphene oxide (GO), the concentration of which in the polymer matrices varied in the range from 1 to 3 wt % corresponding to the percolation threshold in such systems, have been studied. The analysis of the elemental composition of the investigated composites by means of X-ray photoelectron spectroscopy have shown that the oxidation degree of Gr in GO is about 9 to 10%. It has been established that a sharp conductivity jump characterized by S-shaped current-voltage curves and the presence of their hysteresis occurs upon applying a voltage pulse to the Au/PVK (PFD; PVC): Gr (GO)/ITO/PET structures, where ITO is indium tin oxide, and PET is poly(ethylene terephthalate), with the switching time, t, in the range from 1 to 30 μs. The observed effects are attributed to the influence of redox reactions taking place on the Gr and GO particles enclosed in the polymer matrix, and the additional influence of thermomechanical properties of the polymer constituent of the matrix.

Theoretical description is proposed for new modes of reorientation of the director field \(\widehat n\) and the evolution of the NMR spectrum I(ν) of a nematic liquid crystal (LC) formed by molecules of deuterated 4-n-pentyl-4'-cyanobiphenyl encapsulated in a rectangular LC cell under the action of crossed electric, E, and magnetic, B, fields directed at an angle of α to one another. Numerical calculations in the framework of the nonlinear generalization of the classical Ericksen–Leslie theory show that, under certain relations between the forces and moments acting upon a unit volume of the LC phase upon the reorientation of \(\widehat n\), transient periodic structures can arise if the corresponding distortion mode is characterized by the fastest response and thus suppresses all other modes, including uniform ones. It is shown that the rise of these periodic structures leads to a reduction in the time for the reorientation of the director field.