## The energy spectrum of PbBi4Te7 on evidence derived from light reflection and absorption coefficients

Published in category «Condensed matter physics»

Spectral dependencies of light reflection and absorption coefficients on photon energy for layered tetradymit-like n-PbBi4Te7 semiconductor doped with silver (or cadmium) and intrinsic defects have been investigated at T = 300 K. The compound under study was established to exhibit a narrow direct forbidden gap. It was found that its optical band gap Eg opt = 0.31 eV, an energy gap parameter Eg0 between absolute extremums of the valence and conduction bands in the sample with the Hall electron concentration of 5.45·1020 cm–3 was equal to 0.08 – 0.12 eV, and a mdn/mdp ratio (effective masses of electron and hole states densities) was equal to about unity. No other subbands located near the absolute extremum of conduction band was revealed in PbBi4Te7. The electron dispersion law was also found to be non-parabolic in this matter. The law involved can be described in the context of Kane non-parabolisity model taking into account an exchange interaction of free electrons. It was shown that the most probable value of the Eg0 parameter fell within the range from 0.16 to 0.24 eV for PbBi4Te7 with nondegenerate free electron gas.

## Effect of barium titanate admixture on the stability of potassium nitrate ferroelectric phase in (1 – x)KNO3 + (x)BaTiO3 composites

Published in category «Condensed matter physics»

The study of temperature evolution of KNO3 structure in ferroelectric (1–x)KNO3 + (x)BaTiO3 composites at х = 0.25 and 0.50 has been carried out on cooling and on heating using X-ray diffraction. It was shown that on cooling the phase transition temperature (Tc) from the high-temperature paraelectric phase into the ferroelectric one did not depend on barium titanate concentration and coincided practically with Tc for the pure KNO3. Simultaneously the admixture of BaTiO3 enlarged essentially the temperature interval of the KNO3 ferroelectric phase stability in these composites. The structure refinement did not confirm the suppression of the ferroelectric phase of potassium nitrate proposed formerly for (0.5)KNO3 + (0.5)BaTiO3 sample on a basis of dielectric spectroscopy data. The transition from the ferroelectric phase into the lowtemperature paraelectric α-phase was not observed in this composite on cooling down to 348 K.

## Peculiarities of diffuse synchrotron radiation scattering in the SBN-60 single crystal at room temperature

Published in category «Condensed matter physics»

The study of Sr0.6Ba0.4Nb2O6 (SBN-60) crystal structure at room temperature and at the zero applied electric field has been carried out by synchrotron radiation scattering. The wavelength of incident X-rays was λ = 0.7749 Å, the Δλ/λ value was ≈ 2۰10-4. The two-dimensional distributions of diffuse and elastic intensities were obtained and analyzed in the several cross-sections: (H K 0), (H K 1/2), (H K 1), (H K 2), (H 0 L)and (H H L). As a result, it was shown the coexistence of two different types of ordering with different correlation lengths and various correlation functions: the first type is described by squared Lorentzian and the second one – by Lorentzian. The first component is characterized by the correlation lengths ξ(1) ab ≈ 10 nm and ξ(1)c ≈ 15 nm, the second one is with ξ(2)ab ≈ 3 nm and ξ(2)c ≈ 25 nm.

## Dielectric properties of potassium nitrate – ammonium nitrate system

Published in category «Condensed matter physics»

Potassium nitrate has a rectangular hysteresis loop and is thought to be a promising material for nonvolatile ferroelectric memory. However, its polar phase is observed in a narrow temperature range. This paper deals with an effect of ammonium nitrate NH4NO3 on the dielectric properties of potassium nitrate. Thermal dependencies of the linear dielectric permittivity ε and the third-harmonic coefficient g3 for potassium nitrate and polycrystalline binary (KNO3)1–x(NH4NO3)x system (x = 0.025, 0.035, 0.050, 0.100) in the temperature range between 300 and 460 K have been investigated. A temperature range extension of the ferroelectric phase with increasing the x value was revealed. The ferroelectric phase was retained in the (KNO3)1–x(NH4NO3)x composites (x = 0.050, 0.100) down to room temperature. A thermodynamic model for the ammonium-nitrate effect on the temperature of reconstructive phase transition II → I in potassium nitrate was suggested.

## Field emitters made of the contacted ytterbium and carbon nanolayers

Published in category «Physical electronics»

The operation of field emitters of a new type prepared from contacted nanolayers of ytterbium and carbon has been investigated. The performed calculations and experiments allowed to optimize the emission characteristics of the emitters. The calculations took into account the existence of a transition zone between the layers of Yb and C. Emission characteristics of the cathodes including up to 40 pairs of layers of carbon and ytterbium with optimum thicknesses of 5 and 2 nm respectively were measured. The created multilayered emitters provide the average emission current density over the surface of the emitter up to 10 - 20 A/cm2 and show promise for use in miniature electronic devices.

## Liquefied-petroleum-gas sensing performance of CuO–Ag2O bimetallic oxide nanoparticles

Published in category «Physical materials technology»

In the present work, we have synthesized CuO–Ag2O bimetallic oxide nanoparticles using microwave-assisted and solid state diffusion routes. The structural, morphological, optical and thermal studies of the synthesized materials were done with an X-ray diffractometer (XRD), a scanning electron microscope (SEM), Fourier transform infrared (FTIR), ultraviolet-visible (UV-VIS) and thermogravimetric analysis (TGA), respectively. Comparatively different sensing parameters such as sensing response at room temperature, operating temperature, response and recovery time and stability characteristics were investigated and discussed for liquefied petroleum gas (LPG).

## The new method for studying neuronal activity: optogenetics

Published in category «Biophysics and medical physics»

The article is devoted to problems of realization and application of optogenetic methods used to identify reasons of various diseases, to monitor the biochemical processes of cell activity and to study various organisms. The problems of delivery, embedding and monitoring the expression of opsin genes into the cell genome of interest have been considered. In the article, the parameters and properties of various opsins and also the main ways of achievement of precise optical control over cell using opsins were presented. The rules for choosing the parameters of a light beam and the features of its putting were pointed out. The characteristic properties of the different measurement technique and recording the experimental quantities were analyzed and given.

## Implanable devices for optogenetic studies and stimulation of excitable tissue

Published in category «Biophysics and medical physics»

The article deals with currently available implants used in optogenetic experiments on laboratory animals in vivo. We present a brief description of the optogenetic investigation stages. Various types of the implantable devices generating and recording signals in excitable tissues have been considered. The features of the control signal transduction inside living tissues were analyzed. We discussed the possibility of medical and biological use of optical fibers to excitable tissues stimulation. Then we proposed a device of an implantable optical electrode system for scanning and controlling the bioelectric parameters. The device can be used in medical diagnostics, prosthetics, myostimulation, neurostimulation and cardioacceleration, for instance, at the neurological and rehabilitation medical institutions. With this in mind an attempt will be made to make special combined microelectrode arrays to implant them into living tissue. The arrays should be able to change their profile according to the implantation-area contour and biophysical features of the substrate surface. It is necessary to provide a point generation and layer-by-layer scanning of excitation pulse through integration of individual microelectrode arrays into a single test-system.

## Colloidal CdSe and ZnSe/Mn quantum dots: their cytotoxicity and effect on cell morphology

Published in category «Biophysics and medical physics»

The CdSe and ZnSe:Mn colloidal quantum dots (QDs) have been synthesized in order to use them as a contrast agent for bioimaging. The synthesis of QDs was made in the aqueous solution. These compounds are fluorescent semiconductor nanoparticles and are held to be promising fluorophores which can be used asan important research tool in biology and medicine. They can be exploited to allocate the problematic biological tissues and individual cells. Their applicability to human examination was studied. For this purpose we investigated the morphological changes in the cells by reacting with the CdSe / L-Cys and ZnSe: Mn / MPA quantum dots. The cytotoxicity of CdSe / L-Cys in the line of breast carcinoma was examined using confocal microscopy. The results can be seen as encouraging.

## Stress singularity in a top of composite wedge with internal functionally graded material

Published in category «Mechanics»

The antiplane problem of the composite wedge consisting of two homogeneous external wedge-shaped areas and an intermediate zone of the interphase is studied. The interphase material is assumed functionally graded. It is shown that the problem in each area is harmonic within the quadratic law of inhomogeneity of the material in the transverse direction. The influence of the interphase on the stress state at the top of the wedge is analyzed. As compared to the ideal contact of external materials, the presence of the interphase leads both to decrease and increase in the singularity exponent. Moreover, the stress asymptotic may have two singular terms for some values of the composite parameters.

## The exact solution of the problem on a concentrated- force action on the isotropic half-space with the boundary fixed elastically

Published in category «Mechanics»

We present the analytical solution of the axisymmetric mixed problem for the isotropic half-space withthe surface fixed elastically outside the circular area of the application of a distributed load. In the solution of the problem, the transition procedure from a distributed load to the concentrated force has been justified. A compact form of the exact analytical solution of the problem on the concentrated force applied to the half-space with the surface fixed elastically was obtained. In the specific case when the proportionality factor of normal stresses and displacements vanishing under the condition of the elastic fixing of the boundary, the constructed analytical solution was shown to coincide with the well-known Boussinesq formulae.

## The development of a stratified flow following over a sphere inside the viscous fluid in the presence of internal or surface waves

Published in category «Mechanics»

This study has used the Large Eddy Simulation (LES) for numerical simulation of internal or surface waves. The viscous stratified flow over a sphere was investigated at the Reynolds and the Froude numbers Re = 2۰105 , Fr = 1.3 for simulation of the flow over the sphere in the presence of the internal waves, and at the internal Froude number Fri = 25 for that in the presence of the surface ones. The presence of background internal waves was found to result in an increase in the turbulent viscosity in the flow behind the sphere and in the vertical shift of the turbulent viscosity’s maximum value. Moreover, their presence in the linearly stratified flow leads to a change in the density distribution of the near-surface layers of liquid. In this case the internal-wave breaking and wave mixing occur. The last one is caused by interaction between the internal waves generated by the surface waves and the sphere.

## A comparative analysis of wind pressure on flat and stair-step constructions of solar plant trackers

Published in category «Mechanics»

The article describes a comparative experimental research of aerodynamic processes and forces occurring due to airflow moving past by concentrator photovoltaic (CPV) modules assembled on a flat and stair-step frame. The subsequent analysis of aerodynamic properties of these design schemes has revealed significant advantage of stair-step arrangement of CPV modules over the flat ones concerning smaller wind loads affecting the platform. In order to calculate the value of the forces operating on full-size solar installations, values for aerodynamic resistance for different schemes of an arrangement of the modules have been obtained. The detailed research of various solar installation models utilizing a wind tunnel and aerodynamic scales is carried out for the first time.

## The description of deformation and destruction of materials containing hydrogen by means of rheological model

Published in category «Mechanics»

The two-continuum rheological model taking account of a change in the hydrogen-binding energy has been proposed in this paper. As in the case of conventional approach our model makes it possible to describe the hydrogen transfer and its accumulation in the metals and to explain changes in the mechanical properties of metals that are caused by that accumulation. The proposed rheological model describes the hydrogen transition from a mobile state to the bound one, depending on the stress-strain state. Concurrent with this achievement, our model describes the changes in the material matrix taking place as a result of the hydrogen addition to the matrix atoms. These processes lead to weakening and destruction of the material.

## Imaging the dielectric objects by microwave tomography method

Published in category «Radiophysics»

The paper deals with the development of a physical method for tomographic imaging the dielectric objects in the microwave region, specifically, in upper X and lower Ku ranges. To test the proposed method an experimental setup has been made. The principal elements of the setup are 6 receive antennas and 2 masses of transmitters including 256 transmitting elements which operate at 32 frequencies of the microwave range between 8 and 18 GHz. Tested dielectric samples were made of wax and polyvinylchloride with known dimensions and permittivity values. The performed experiments demonstrated both the possibility of tomographic imaging and of permittivity determination in principle, and some disadvantages related to imperfections in the experimental setup. To improve the experimental setup we plan to rig some additional receive antennas and to optimize the arrangement of the principal elements. dielectric object, TOMOGRAPHIC IMAGE, microwave region, receive antenna, permittivity.

## Canonical decomposition of fluctuation interferences using the delta-function formalism

Published in category «Radiophysics»

The paper deals with the discrete spectral-orthogonal decompositions of centered Gaussian random processes for two cases. In the first case, the process implementations are a sequence of pulses that are short in comparison with the observation time. The process decomposition was obtained as a generalized Fourier series on the basis of the delta-function formalism, and the variances of the coefficients (random values) of this series were found as well. The resulting expressions complement Kotelnikov’s formula because they cover both the highfrequency and the low-frequency regions of the canonical-decomposition spectrum. In the second case, a random process is a superposition of narrow-band Gaussian random processes, and its implementations are characterized by oscillations. For such a process the canonical decomposition in terms of Walsh functions was obtained on the basis of the generalized function formalism. Then this decomposition was re-decomposed in terms of trigonometric functions; it follows from the resulting series that the canonical decomposition spectrum is not uniform since a pedestal is formed in the constant component region.

## Analyzing the acoustic spectra of sound velocity and absorption in the amphiphilic liquids

Published in category «Radiophysics»

The paper analyzes the theoretical approaches to the study of the acoustic spectrum and the speed of sound absorption in the frequency range up to 10 GHz in liquid systems. For example oxyethylated derivatives of normal decyl alcohol ODSn, belonging to nonionic surfactants (SAW) showed that at room temperature and low degrees of ethoxylation n acoustic spectra can be described in terms of the relaxation theory. It is shown that within the experimental error of the ODSn acoustic spectra, in the studied range of frequencies and temperature, are composed of two prime areas of acoustic dispersion. The results of calculations of relaxation and thermodynamic parameters of fast and ultrafast processes restructuring ODSn, which can be used in the development of combined technologies of enhanced oil recovery using surfactant solutions and various physical fields and factors.

## Electron and positron propagation in straight and periodically bent axial and planar silicon channels

Published in category «Theoretical physics»

In this paper the results of simulations of axial and planar channeling of electrons and positrons in straight and periodically bent Si crystals are presented. Simulations with direct calculation of trajectories of projectiles accounting for all-atom interactions were carried out using the MBN Explorer software package. The full atomistic approach for particle trajectories simulation allows to quantitatively compare axial and planar channeling processes. The results of the simulations show significantly lower dechanneling length and number of channeling projectiles in the axial channeling case. For this case the dependence of characteristics of the channeling process on the choice of an axis direction and on a direction of the crystal bending have been investigated.

## A small-amplitude crystalline undulator based on 20 GeV electrons and positrons: simulations

Published in category «Theoretical physics»

This paper presents the results of numerical simulations of a crystalline undulator based on the channeling of 20 GeV electrons and positrons. The device considered is characterized by a small amplitude and a short period of periodic bending. Calculations have been performed accounting for all-atom interactions using the MBN Explorer software package. The effect of low crystal thickness (less than a channeling oscillations period) on radiation spectrum was studied. A new scheme to product a high-energy radiation was proposed. It is based on the short-period small-amplitude crystalline undulator and allows decreasing the intensity of the non-undulator part of the spectrum.