Vol 61, No 3 (2019)

Porous silicon is formed using pulsed current with a pulse modulation varied in a range spanning five orders of magnitude, starting from a hundredth of a second, in order to achieve modulation of the properties of PS on the nanoscale. The PS is characterized by performing photoluminescence (PL), paramagnetic, and charge transport measurements. We find that the properties of prepared PS are greatly affected by the modulation period applied and depend on it in a nonmonotonic way. The intensity of orange-red PL exhibits a resonance-like behavior reaching a maximum at the modulation period in the range of 0.1–0.25 s. A correlation between the variation in PL intensity and the electron paramagnetic resonance signal is noticed. We show that PS with a porosity of 50% and a distance between pores of 10 nm is mainly consists of air and silicon oxide. In the prepared PS, the silicon, in the form of granules with a diameter of ≈1.5 nm arranged in chainlets, has a volume fraction on the order of 1%.

The effect of the anisotropy of elastic properties on the thermal conductivity of GaAs/AlGaAs heterostructures at low temperatures is investigated. The effect of phonon focusing on the anisotropy of the thermal conductivity is analyzed. The parameters of the specular reflection of phonons from the boundaries of the heterostructures, which characterize the heat flux in the Knudsen mode of the phonon gas flow, are determined. The angular dependences of the mean free paths of phonons of different polarizations, which determine the thermal conductivity anisotropy of heterostructures with orientations {100} and {110}, are calculated.

The optical and luminescence properties of La₃Ga₅SiO₁₄ lanthanum–gallium silicate crystals grown in atmospheres of argon and argon with the addition of oxygen are investigated. The results of calculations of the structure of energy bands are presented, obtained using the CASTEP module in the framework of the generalized gradient approximation and the local density approximation. The width of the optical band gap of the crystal is determined to be \(E_{g}^{{{\text{opt}}}}\) = 5.1 eV. Upon interband excitation, La₃Ga₅SiO₁₄ crystals grown in argon atmosphere show a luminescence band with a maximum at 430 nm, whereas for a crystal grown in argon with addition of oxygen, two luminescence bands with maxima at 470 and 530 nm dominate in the luminescence spectrum. The nature of the luminescence centers responsible for these bands is discussed with the help of the data for electronic structure calculations. The effect of temperature on the luminescent properties of La₃Ga₅SiO₁₄ is demonstrated. The presence of traps in La₃Ga₅SiO₁₄ is shown using thermally stimulated luminescence, and their activation energy is determined.

Magnetic inhomogeneities formed at columnar defects of potential-well type in uniaxial films are theoretically studied. It is shown that under some conditions, vortex-like structures occur at such defects; they have a magnetization distribution with three segments of magnetic moments’ rotation. It follows from the analysis of the structure and properties of the vortex-like inhomogeneities, depending on the material parameters, that they are determined mainly by the defect sizes and the depth of the potential well. The experimental data concerning their existence are provided.

In this paper, we present the results of studies of the spectral, temperature, and field dependences of the transversal Kerr effect in Ga_1 – xMn_xAs (x = 0.0066–0.033) layers produced by ion implantation and subsequent pulsed laser annealing. The complicated nonmonotonous nature of the temperature dependences of the transversal Kerr effect and its dependence on the measurement range indicate a magnetic inhomogeneity of the layers. The reasons for the inhomogeneity can be the Gaussian distribution of Mn over the thickness of the layers and the electron phase separation in them. The appearance of new features in the spectra of the transversal Kerr effect is explained by the presence in the doped semiconductor matrix of nanoregions with a higher carrier concentration and a higher Curie temperature and a shift of the Fermi level into the valence band leading to an increase in the energy of optical transitions.

The trivalent iron oxide ε-Fe₂O₃ is a fairly rare polymorphic iron oxide modification, which only exists in the form of nanoparticles. This magnetically ordered material exhibits an intriguing magnetic behavior, specifically, a significant room-temperature coercivity H_C (up to ~20 kOe) and a magnetic transition in the temperature range of 80–150 K accompanied by a sharp decrease in the H_C value. Previously, the temperature of the transition to the paramagnetic state for ε-Fe₂O₃ was believed to be about 500 K. However, recent investigations have shown that the magnetically ordered phase exists in ε-Fe₂O₃ also at higher temperatures and, around 500 K, another magnetic transition occurs. Using the data on the magnetization and temperature evolution of the ferromagnetic resonance spectra, it is shown that the temperature of the transition of ε-Fe₂O₃ particles 3–10 nm in size to the paramagnetic state is ~850 K.

The periodic motion of the interacting vortex domain walls in a pair of nanostripes has been theoretically investigated. As a model, two parallel nanostripes with magnetization inhomogeneities in the form of magnetic vortices have been examined. The magnetic subsystems of the stripes are magnetostatically coupled. The quasi-elastic coupling between vortices ensures the existence of normal modes of the periodic magnetization motion. The frequencies of these modes have been calculated. It is shown that not any combination of the vortex topological charge leads to the resonant behavior of magnetization in ac fields. The effect of the static component of a magnetic field on the frequency of the periodic motion of vortex domain walls is discussed.

Electric polarization in ErCrO₃ single crystals has been investigated in the temperature range of 5‒370 K. Ferroelectric ordering has not been found in any of the directions. However, electric polarization induced by restricted polar domains of structural origin has been observed. These domains are formed in the crystal matrix near impurity Bi³⁺ ions partially substituting Er³⁺ ions during the growth of single crystals by the method of spontaneous crystallization using solvent Bi₂O₃. The restricted polar domains form the superparaelectric state. Hysteresis loops with remanent polarization, both along the c axis and in the [110] directions, have been observed below some temperatures T_fr (in the frozen superparaelectric state). The polarization exists up to certain temperatures, which depend on the applied electric field orientation with respect to the crystal axes and exceed significantly temperature T_N of magnetic ordering. These temperatures correspond to the condition kT_fr ≈ E_A for activation barriers at the boundaries of the restricted polar domains.

The design of cantilevers and microelectromechanical systems based on carbon nanotubes necessitates a thorough study of their thermal fluctuations and resonance properties. This work aims at the simulation of thermal fluctuations and power spectral density for single-walled carbon nanotubes in the context of the continuous model of elastic cylindrical membranes with nonmacroscopic thicknesses. The conditions imposed on the geometry of this probe and providing its stable operation, as well as the high lateral resolution determined by only the radius of the nanotube independently on its thermal vibrations, are found, as well. The most intensive resonance modes, making the greatest contribution to the thermal fluctuations of carbon nanotubes, are established, and their frequencies are found analytically.

The local structure and charge state of the cobalt impurity in SrTiO₃ is studied by X-ray absorption fine structure (XAFS) spectroscopy. The synthesis conditions, under which cobalt predominantly (up to 76%) substitutes the atoms at the A site of the perovskite structure, is found for SrTiO₃(Co) samples. By varying the synthesis conditions, it is possible to appreciably change the ratio between the concentrations of cobalt atoms incorporated into the A and B sites. It is established that the charge state of cobalt is +2 at the A site and +3 at the B site. It is revealed that the Co impurity at the A site is off-center, and its deviation from the lattice site is 1.0 Å. First-principles calculations show that the isolated Co³⁺ ion at the B site is diamagnetic, whereas the Co²⁺ ion at the A site is in a high-spin state (S = 3/2).

Multi-layer film structures BaTiO₃/SiO₂ with a thickness of ~1 μm containing up to 14 pairs of layers were synthesized by the sol–gel method with sequential heat treatment. It is shown that the synthesized structures are X-ray amorphous. The formation of bands in the transmission and reflection spectra caused by interference effects is demonstrated. A more regular structure exhibits a photon band gap (opacity band) in the visible range with main minimum at 636 nm and corresponding maximum in the reflection spectra. Dispersion characteristics of barium titanate films with different concentrations of initial sols were studied and an increase in the refractive index with an increase in the concentration of sol was demonstrated. For a sol with a concentration of 60 mg/mL, the refractive index in the spectral range of 390–1600 nm is 1.88–1.81. The prospects of sol–gel technology for the formation of BaTiO₃/SiO₂ structures for nanophotonics and solar radiation converters are discussed.

The studies of magneto-optical effects in epitaxial films of magnetic semiconductors—europium chalcogenides EuX (X = O, Se, Te) are presented: linear and quadratic magneto-optical Kerr effects, photo-induced Faraday effect, magnetically-induced optical second and third harmonics generation, the inverse Faraday effect, and the phenomenon of optical orientation. A large quadratic magneto-optical Kerr effect was found in EuO and (Eu, Gd)O films. Photo-induced spin polarons with a huge value of the magnetic moment were studied in europium chalcogenides EuTe and EuSe using the optical pump-probe technique. In the same materials, a new type of nonlinear magneto-optical susceptibility, which is responsible for the optical second harmonic generation and associated with the magnetic ordering of the spins of Eu²⁺ ions, is revealed.

The thermodynamic description of the thermoelastic effect in anisotropic materials is given. Simple examples of using this apparatus are presented.

A symmetry analysis of possible magnetic structures in an incommensurate magnetic phase in FeGe₂ compound, resulted from phase transitions from the paramagnetic phase, was performed based on a phenomenological consideration. It is shown that two possible approaches to a such an analysis, the first of which uses the magnetic representation of the space group, and the second one is based on the expansion of the magnetic moment in basis functions of irreducible representations of the space group of the paramagnetic phase, yield the same results. Space group irreducible representations are determined, according to which the transition to an incommensurate structure can occur. The set of these representations appears identical in both approaches. Ginzburg–Landau functionals for analyzing the transitions according to these representations are written. A renormalization group analysis of the second-order phase transitions from the paramagnetic state to the incommensurate magnetic structure is performed. It is shown that a helical magnetic structure can arise in the incommensurate phase as a result of two second-order phase transitions at the transitions temperature.

The morphology, structure, and electrophysical characteristics of multiwall carbon nanotubes (MWCNTs), as affected by defects induced by ion bombardment, are studied using Raman spectroscopy and scanning and transmission electron microscopies. Annealing in an inert atmosphere results in partial recovery of the nanotube structure: the nanotube walls contain regions of recovered graphene structure and regions plagued with extended defects that result in corrugation of graphene layers comprising the walls of MWCNTs. These structural alterations result in a marked decrease in conductivity of the processed MWCNTs.

The influence of the atmosphere on the electrical conductivity of polydiphenylenephthalide (PDP) films included in the ITO–PDP–Sb sandwich structure is discussed. On the basis of electrophysical measurements and quantum-chemical simulation, a conclusion is drawn that the main contribution to the electrical conductivity of the PDP films comes from oxygen.

Three-layer epitaxial SrRuO₃/SrTiO₃/SrRuO₃ heterostructures with the 900-nm-thick intermediate layer of strontium titanate have been grown on single-crystal (001)La_0.29Sr_0.71Al_0.65Ta_0.35O₃ substrates by the laser evaporation method. Plane-parallel film capacitors have been formed on the basis of the grown heterostructures using photolithography and ion etching. Temperature dependences of the dissipation factor have been measured for these capacitors at different bias voltages applied to strontium ruthenate electrodes. Temperature dependences of the permittivity of the intermediate SrTiO₃ layer in the formed capacitor structures are visualized with compensation of the internal electric field and without it. The reasons for the sharp increase in the dielectric loss in the formed film capacitors at temperatures below 50 K are analyzed.

The atomic composition of films of a polyphenol antioxidant, namely, resveratrol (RVL), with a thickness of up to 50 nm thermally deposited on an oxidized silicon surface is studied by the method of X-ray photoelectron spectroscopy (XPS). It is found that the surface area of pores in the RVL film is about 15% of the total surface area. The results of studying the stability of the RVL films when their surface is treated with Ar⁺ ions of 3 keV under the electric current of 1 μA passing through the sample for 30 s are given. The treatment gives rise to an increase in the area of pores to 30–40%, while the ratio of the concentration of C atoms to the concentration of O atoms in the RVL film both before and after the treatment of the surface with ions does not correspond to the chemical formula of RVL molecules. Using the method of atomic force microscopy (AFM) in contact mode with a scanning area size of about 10 × 10 μm, RVL coatings deposited on the oxidized silicon and polycrystalline Au surfaces are studied. It is found that the RVL films produce grainy and porous coatings on the substrate surfaces. The typical size of grains in the sample surface plane is 150–300 nm, and the characteristic elevation reaches 30 nm.

The thermal stability of recently predicted quasi-fullerenes С₂₀, С₄₂, С₄₈, and С₆₀ is studied by the method of molecular dynamics. The routes of their decomposition and the temperature dependences of their lifetimes are determined. The activation energy and frequency factor values that appear in the Arrhenius law are found. New isomers are detected.

A double-chain model of an epitaxial graphene nanoribbon, the zigzag edges of which are decorated with foreign adparticles, has been proposed. The substrate is assumed to be a metal. Analytical expressions for the Green’s functions of carbon adatoms and adparticles are obtained. The band spectrum for the free state is determined, and the approximation of the density of states is proposed. Analytical expressions for the occupation numbers in the mode of tight binding between the adsorption complex and the substrate are presented. A chain of carbon adatoms decorated with adparticles (epicarbyne) is considered.