Volume 58, Nº 6 (2016)

The specific features of quantum oscillations of the magnetization in quasi-two-dimensional wide-band-gap antiferromagnetic semimetals with a low concentration of charge carriers have been considered theoretically. It has been shown that, in these systems, the Fermi energy determined from the analysis of the frequency of the de Haas–van Alphen oscillations according to the standard procedure can differ significantly from the true value. For the correct determination of the Fermi energy in the canted phase, it has been proposed to analyze quantum oscillations of the magnetization M not as a function of the inverse magnetic field 1/H, but as a function of 1/cosγ, where the angle γ characterizes the inclination angle of the magnetic field with respect to the plane of the quasi-two-dimensional semimetal.

The previously published photoluminescence spectra of bulk germanium single crystals with orientations (100), (110), and (111) under different biaxial tensile strains have been investigated using the differential method proposed by the author for the analysis of luminescence spectra of semiconductors. An increase in the strain for all these orientations of the single crystals leads to a shift in the maxima of the differential spectra in the region of direct radiative transitions toward lower photon energies due to the narrowing of the germanium direct band gap. At the same time, the positions of the maxima of the differential spectra in the region of indirect radiative transitions remain almost unchanged. This indicates that the germanium indirect band gap does not depend on the tensile strains, at least for their values of ∼0.2–0.3%.

The integrated intensity of X-ray diffraction reflections has been measured for a series of epitaxial layers of AIII nitrides (GaN, AlN, AlGaN) grown on different substrates (sapphire, SiC) and characterized by different degrees of structural perfection. It has been shown that, despite a high density of dislocations and a significant broadening of the diffraction peaks, the obtained values are not described by the kinematic theory of X-ray diffraction and suggest the existence of extinction. The results have been analyzed on the basis of the Darwin and Zachariasen extinction models. The secondary extinction coefficients and the thicknesses of epitaxial layers have been determined using two orders of reflection both in the Bragg geometry (0002 and 0004) and in the Laue geometry (\(10\bar 10\)) and \(10\bar 20\)). It has been demonstrated that the secondary extinction coefficient is the greater, the smaller is the broadening of the diffraction peaks and, consequently, the dislocation density. It has been found that, for epitaxial layers with a regular system of threading dislocations, the secondary extinction coefficient for the Laue reflections is substantially greater than that for the Bragg reflections.

The EPR spectra of a system of two coupled spins with strongly anisotropic g factors have been calculated. It has been shown that the spectra can exhibit an additional absorption line in the vicinity of the energy level anticrossing. The emergence of this line is caused by strengthening of the off-resonance absorption in the anticrossing region.

The magnetic susceptibility, electrical resistivity, and thermoelectric power of Ti₅₀Ni_50‒xCu_x alloys with copper concentrations x ⩽ 25 at % have been measured in the temperature range of 2–500 K. The change in the electronic band structure near the Fermi level upon thermoelastic martensitic transformations, such as B2 ↔ B19′, B2 ↔ B19 ↔ B19′, and B2 ↔ B19, has been considered.

It has been found that CoFeB–SiO₂/C and CoFeB–SiO₂/Bi₂Te₃ multilayer heterostructures with a cluster structure of CoFeB layers feature a long-range magnetic order in the entire temperature range from 2 to 300 K. At high temperatures (T = 300 K), CoFeB clusters exhibit magnetic properties characteristic of superparamagnets. At low temperatures (T = 5 K), clusters are ferromagnetic, and the easy magnetization axis is in the film plane. The temperature of the ferromagnetic-to-superparamagnetic state transition of clusters depends on a dielectric interlayer material: the use of Bi₂Te₃ instead of C as a spacer layer leads to an increase in the transition temperature by a factor of 4 and an increase in the magnetization blocking temperature of CoFeB clusters in a field of 100 Oe by a factor of 3.

The effect of small copper additions (to ∼6 at %) on the distribution of iron ions in six crystallographic sites of the unit cell of the main magnetic phase (PrDy)₂(FeCo)₁₄B has been detected. An increase in the copper concentration leads to a decrease in the 8j₁ site occupation by iron ions. Independently of the presence of copper, the temperature dependences of the saturation magnetization of all samples have a minimum which can correspond to the presence of a low-temperature phase or a compensation point in grain boundary regions of the (PrDy)₂(FeCo)₁₄B main magnetic phase. The alloys under study are not additive sets of all phases in their composition, but behave as new materials with the mutual influence of phases on each other.

The capacitance and dielectric loss tangent of Gd_xMn_1–xS (x ≤ 0.2) solid solutions have been measured at a frequency of 10 kHz without magnetic field and in a magnetic field of 8 kOe in the temperature range of 90–450 K. An increase in the permittivity and a dielectric loss maximum have been detected in the low-temperature region. It has been found that the temperature of the maximum of the imaginary part of the permittivity shifts to higher temperatures with increasing concentration. The magnetocapacitance effect has been revealed for two compositions. The dielectric loss has been described in the Debye model with “freezing” dipole moments and in the orbital-charge ordering model.

The dielectric spectra of Pb_1–xBa_x(Mg_1/3Nb_2/3)_m(Zn_1/3Nb_2/3)_y(Ni_1/3Nb_2/3)_nTi_zO₃ (x = 0–0.15, m = 0.4541, y = 0.0982, n = 0.1477, and z = 0.3) ceramic samples have been studied in wide intervals of temperature (10–873 K) and frequency of the measurement electric field (0.1–1000 kHz). It has been found that an increase in the Ba²⁺(x) content leads to a reduction in the phase transition temperature (from 418 K at x = 0 to 256 K at x = 0.15), to the transition from the normal ferroelectric state to the relaxor ferroelectric one (at x ≥ 0.025), and to the disappearance of temperature hysteresis of dependences of the relative dielectric permittivity. It has been hypothesized that a tricritical point is present near x ∼ 0.125 in the x–T phase diagram of the studied solid solutions.

Elastic properties of NaClO₃, KClO₃, LiClO₄, NaClO₄, and KClO₄ have been investigated from first principles by the method of linear combination of atomic orbitals in the gradient approximation of the density functional theory using CRYSTAL software. The elastic constants and moduli, hardness, Poisson’s ratio, and the anisotropy parameters have been calculated. The velocities of sound, the Debye temperature, the thermal conductivity, and the Grüneisen parameter have been estimated. It has been found that these compounds are mechanically stable, anisotropic, and ductile materials. The dependences of their elastic parameters on the atomic number of the cation have been calculated. The obtained results are in good agreement with the available experimental data.

A theoretical model that effectively describes the nucleation of cracks in stress fields of dislocation pile-ups near the free surface in metallic nanomaterials with a bimodal structure has been developed. The dependences of the critical shear stress τ_c (for the formation of a crack with an equilibrium length of 10 nm on a dislocation pile-up near the surface) on the size d of a grain containing the dislocation pile-up have been calculated for copper with a bimodal structure. Theoretically, it has been found that the critical shear stress τ_c for the nucleation of a crack near the free surface in a nanomaterial with a bimodal structure is approximately 30% higher than that for the crack nucleation within the nanomaterial at a distance from the free surface.

The paper presents the results of measurements of shock-wave compression profiles of VT1-0 titanium samples after rolling and in the annealed state. In the experiments, the pressure of shock compression and distance passed by the wave before emerging to the sample surface were varied. From measurements of the elastic precursor decay and compression rate in a plastic shock wave of different amplitudes, the plastic strain and the corresponding shear stresses in the initial and subsequent stages of high-rate deformation in an elastoplastic shock wave are determined. It is found that the reduction in the dislocation density as a result of annealing reduces the hardness of the material but significantly increases its dynamic yield strengh, corresponding to the strain rate above 10⁴ s^–1. With a reduction in the strain rate, this anomalous difference in the flow stresses is leveled off.

The crystal structure and phonon spectrum of PrFe₃(BO₃)₄ are ab initio calculated in the context of the density functional theory. The ion coordinates in the unit cell of a crystal and the lattice parameters are evaluated from the calculations. The types and frequencies of the fundamental vibrations, as well as the line intensities of the IR spectrum, are determined. The elastic constants of the crystal are calculated. A “seed” frequency of the vibration strongly interacting with the electron excitation on the praseodymium ion is obtained for low-frequency A₂ mode. The calculated results are in agreement with the known experimental data.

A method of surface curvature of carbon nanotubes has been proposed for quantitative estimation of the longitudinal conductivity of nanotubes. A dispersion relation for the electron spectrum of single-walled carbon nanotubes has been obtained analytically. The change in the zone structure of nanotubes of various types and diameters caused by taking into account the surface curvature has been analyzed. The temperature dependence of the longitudinal component of conductivity with allowance for the surface curvature for a series of nanotubes has been calculated. The comparison with the conductivity of a plane graphene has been performed. It has been shown that, in zig-zag tubes, the correction of the conductivity for the surface curvature decreases with an increase in temperature as well as with an increase in the radius of curvature.

A method has been proposed for the formation of three-dimensional arrays of isolated magnetic clusters NiO, Co₃O₄, and NiCo₂O₄ in the sublattice of pores in the matrix of bulk synthetic opals through a single impregnation of the pores with melts of nickel and cobalt nitrate crystal hydrates and their thermal degradation. The method makes it possible to controllably vary the degree of filling of pores in the matrix with oxides within 10–70 vol %. The composition and structure of the synthesized materials, as well as the dependences of their static magnetic susceptibility on the magnetic field strength, have been investigated.

Sodium nanoparticles embedded in porous glass have been studied by NMR. The measurements have been carried out on pulse spectrometers in magnetic fields of 9.4 and 17.6 T in a wide temperature range. Changes in the magnitude and temperature dependence of the ²³Na Knight shift with respect to those in bulk sodium have been discovered. An additional component of the NMR line shifted to high frequencies has been observed in the temperature range from 240 to 100 K. Investigation of the specific heat has revealed a considerable decrease in the melting and crystallization temperatures of sodium under nanoconfinement, which were not accompanied by abrupt changes in the Knight shift.

The conditions (regimes of deposition and thermal treatment) for gas bubble formation in ferroelectric Pb(Ti_1–yZr_y)O₃ films have been determined by thermal desorption and electron and optical micros-copy. A mechanism of bubble formation has been proposed. This mechanism rests upon the notion that lead can form oxides of the PbO₂ type with a high oxygen content at relatively low temperatures and that these oxides break down with the release of oxygen to lower oxides of the PbO type upon subsequent heating. These ideas have been taken as the basis of a technique for the fabrication of Pb(Ti_1–yZr_y)O₃ films with a reduced (by an order of magnitude) density of through defects.

The results of the investigation of the electronic structure of the conduction band in the energy range 5–25 eV above the Fermi level E_F and the interfacial potential barrier upon deposition of aziridinylphenylpyrrolofullerene (APP-C₆₀) and fullerene (C₆₀) films on the surface of the real germanium oxide ((GeO₂)Ge) have been presented. The content of the oxide on the (GeO₂)Ge surface has been determined using X-ray photoelectron spectroscopy. The electronic properties have been measured using the very low energy electron diffraction (VLEED) technique in the total current spectroscopy (TCS) mode. The regularities of the change in the fine structure of total current spectra (FSTCS) with an increase in the thickness of the APP-C₆₀ and C₆₀ coatings to 7 nm have been investigated. A comparison of the structures of the FSTCS maxima for the C₆₀ and APP-C₆₀ films has made it possible to reveal the energy range (6–10 eV above the Fermi level E_F) in which the energy states are determined by both the π* and σ* states and the FSTCS spectra have different structures of the maxima for the APP-C₆₀ and unsubstituted C₆₀ films. The formation of the interfacial potential barrier upon deposition of APP-C₆₀ and C₆₀ on the (GeO₂)Ge surface is accompanied by an increase in the work function of the surface E_vac–E_F by the value of 0.2–0.3 eV, which corresponds to the transfer of the electron density from the substrate to the organic films under investigation. The largest changes occur with an increase in the coating thickness to 3 nm, and with further deposition of APP-C₆₀ and C₆₀, the work function of the surface changes only slightly.

The emission spectra of thermally stimulated surface plasmon polaritons (thermally stimulated surface plasmon polaritons) and a blackbody have been analyzed and compared, and the temperature dependence of these spectra has been studied. It has been found that the total energy of the entire ensemble of surface plasmons is proportional to the cube of temperature and their spectrum is red-shifted from the blackbody spectrum. It has been shown that the spectrum of thermally stimulated surface plasmon polaritons obeys the Wien’s displacement law, yet with another constant. The fraction of the photon energy of the conducting layer transferred to the surface plasmons has been estimated. It has been demonstrated numerically by the example of a gold layer that this fraction can exceed 10% for a layer thickness of less than 1 mm.

The components of the dielectric constant of a terbium-based liquid-crystalline complex have been measured in the frequency range of 350–5 × 10⁶ Hz. The magnitude and sign of the dielectric anisotropy of the complex have been determined. Dispersion of the dielectric constants in the liquid-crystalline and isotropic phases has been found. The mechanisms responsible for the relaxation phenomena that appear in the studied sample have been determined. The time of dielectric relaxation, the activation energy, and the dipole moment of the complex have been obtained.

Scrolled packings of single-layer and multilayer graphene can be used for the creation of supercapacitors, nanopumps, nanofilters, and other nanodevices. The full atomistic simulation of graphene scrolls is restricted to consideration of relatively small systems in small time intervals. To overcome this difficulty, a two-dimensional chain model making possible an efficient calculation of static and dynamic characteristics of nanoribbon scrolls with allowance for the longitudinal and bending stiffness of nanoribbons is proposed. The model is extended to the case of scrolls of multilayer graphene. Possible equilibrium states of symmetric scrolls of multilayer carbon nanotribbons rolled up so that all nanoribbons in the scroll are equivalent are found. Dependences of the number of coils, the inner and outer radii, lowest vibrational eigenfrequencies of rolled packages on the length L of nanoribbons are obtained. It is shown that the lowest vibrational eigenfrequency of a symmetric scroll decreases with a nanoribbon length proportionally to L^–1. It is energetically unfavorable for too short nanoribbons to roll up, and their ground state is a stack of plane nanoribbons. With an increasing number k of layers, the nanoribbon length L necessary for creation of symmetric scrolls increases. For a sufficiently small number of layers k and a sufficiently large nanoribbon length L, the scrolled packing has the lowest energy as compared to that of stack of plane nanoribbons and folded structures. The results can be used for development of nanomaterials and nanodevices on the basis of graphene scrolled packings.

The thermal diffusion, heat capacity, and thermal conductivity of BiFeO₃, Bi_0.91Nd_0.09FeO₃, and BiFe_0.91Mn_0.09O₃ multiferroics have been studied at high temperatures (300–1120 K). The dominant mechanisms of phonon transfer in the regions of the antiferromagnetic and ferroelectric phase transitions have been determined. The temperature dependence of the mean free path of phonons has been found.