Volume 44, Nº 9 (2018)

A radiophysical model of two delay-coupled FitzHugh–Nagumo self-sustained generators coupling is constructed. The possibility of introducing both a constant and adaptive coupling between the generators is experimentally implemented in the model. It is shown that the use of an adaptive coupling makes it possible to shift from the antiphase synchronization of nonidentical generators to their in-phase synchronization.

A study of the influence exerted by extremely high frequencies (54 GHz) on conditioning in rats subjected to a current stress in the absence and in the presence of electromagnetic radiation is reported. The biophysical processes underlying the mechanism of this influence are discussed.

High-efficiency photovoltaic converters (PVCs) have been developed and fabricated by liquidphase epitaxy in the AlGaAs–GaAs system with laser light (λ = 850 nm) introduced through the edge surface in parallel to the plane of the p–n junction of the device structure. To raise the efficiency of light “capture” by the p–n junction, an Al_xGa_1–xAs waveguide layer is formed, in which the content of aluminum gradually varies from x = 0.55 to 0.15 so that the refractive index gradient is created in this layer and light beams are diverted toward the p–n junction. When a PVC (having no antireflection coating) is exposed to 0.1- to 0.2-W laser light, an efficiency of 41.5% is obtained. Depositing an antireflection coating on the edge surface of a PVC raises its efficiency to 55%.

Results of physical modeling of how arrays of large model hydrometeors exert an influence on processes forming subsequent negative lightning strikes are presented. The modeling involved a system of artificial thunderstorm cells. It has been shown for the first time that the introduction of an array of large model hydrometeors to the space between negatively charged cells significantly increases the probability of the formation of a subsequent discharge, total length of the discharge system, and magnitude of the neutralized charge. The influence of hydrometeors on parameters of current pulses of the first and subsequent discharges has been established. The results obtained can be used in developing methods of artificial stimulation of multistrike lightning and directed discharging of a thunderstorm cloud.

Phase transitions (PTs) in a mixture of normal alkanes—tricosane (C₂₃H₄₈) and pentacosane (C₂₅H₅₂)—have been investigated by differential scanning calorimetry. Elimination of methodological errors makes it possible to obtain true values of the thermodynamic PT parameters and reveal their nature. Jumps of the specific heat have been comparatively analyzed based on the theory of diffuse first-order PTs.

The results of the study of vitreous Se films obtained by a method of vacuum-thermal evaporation after their laser modification are presented. It is shown for the first time that a low-power laser irradiation of the film at a wavelength of 632.8 nm and room temperature results in a nucleation and growth of nanocrystallites of monoclinic and rhombohedral Se in an amorphous matrix of the film. The formation of the stable hexagonal Se phase is not observed. The appearance of nanocrystallites with an average size of ~20–30 nm is accompanied by a growth of a reflection coefficient of the film. The established new optical and structural properties of the formed films are stable in time at room temperature.

A simple phenomenological model for estimating the upsurge in drift velocity of electrons in transistor heterostructures is proposed. This model is based on a self-consistent solution of Schrödinger and Poisson equations and the hydrodynamic system of equations of energy and momentum conservation. It is demonstrated that the conditions in the layer channel of DA-pHEMT structures with additional potential barriers, which are produced by donor–acceptor doping and enhance the localization of hot electrons, are even more conducive to reducing the time of flight of electrons under the gate than those established in heterostructures with deeper quantum wells produced by increasing the conduction band offset at the heterojunction interface.

We have studied the influence of the surface layer on the parameters of the multiply repeated shape memory effect, developing stresses, characteristic temperatures, and intervals of martensitic transformations in thin (1 mm diameter) wires made of TiNi-based alloys. Examination of the surface layer structure showed that, in 1-mm-diameter TN-1V grade alloy wire, the oxide layer is about 15 μm thick and consists mostly of titanium, nickel, oxygen, and carbon. Removal of this surface layer leads to an increase in the maximum accumulated deformation, shift of the temperature interval of formation toward higher temperatures, and increase in the strength and plasticity characteristics.

Structural defects formed in epitaxial GaAs layers as a result of 250-keV N⁺ ion implantation to doses within 5 × 10¹⁴–5 × 10¹⁶ cm^–2 have been studied by the X-ray diffraction (XRD) and transmission electron microscopy techniques. No amorphization of the ion-implanted layer took place in the entire dose range studied. The implantation to doses of 5 × 10¹⁴ and 5 × 10¹⁵ cm^–2 led to the appearance of an additional peak on XRD curves, which was related to the formation of a stressed GaAs layer with positive deformation arising due to the formation of point-defect clusters. The implantation to a dose of 5 × 10¹⁶ cm^–2 led to the formation of a dense structure of extended defects in the implanted layer, which was accompanied by the relaxation of macrostresses to the initial state.

Full wave profiles were monitored by the laser interferometry method by means of a VISAR laser Doppler velocimeter under shock-wave loading of samples of AMg6 aluminum alloy. Analysis of these profiles was used to study the laws of elastic precursor formation and its amplitude variation during elastic–plastic transition front propagation in samples loaded by a shock wave of variable intensity. Critical stresses leading to the spall fracture of samples were determined as dependent on the strain rate under unloading.

We present results of a theoretical and experimental investigation of the output characteristics of transparent (containing no local absorbers) traveling-wave tubes (TWTs) operating on multivelocity (turbulent) electron beams. It is established that higher gain (up to 20 dB) and higher efficiency (40–50%) can be achieved in this case in comparison to the output characteristics of transparent TWTs employing monovelocity (laminar) electron beams. This effect is explained by the formation of space-charge bunches of much higher density in multivelocity electron beams than in monovelocity beams.

It has been shown that the fundamental results obtained in the works by Levine, Polevoi, and Rytov (1980) and Polevoi (1990) adequately describe the rate of radiative heat exchange and dissipative frictional force in a system of two parallel plates in relative lateral motion. Within the framework of the Levine–Polevoi–Rytov theory, a new expression has been obtained for the force of fluctuation-electromagnetic friction of two plates in the terms of local dielectric permittivities and magnetic permeabilities of the plate materials.

Characteristic features of the phenomenon of longitudinal photocurrent generation in Ag/Pd nanocomposite films under the obliquely incident nanosecond pulsed laser irradiation with wavelengths in the 1350–4000 nm range have been studied for the first time. Films of 20-μm thickness consisting of nanocrystals of Ag–Pd solid solution and palladium oxide (PdO) were obtained using a thick-film technology. The influence of radiation wavelength on photocurrent pulses has been studied for s- and p-polarized laser pumping. It is established that the photocurrent pulses generated by s-polarized pumping in the 1350–4000 nm wavelength range are monopolar, while p-polarized pumping can generate both mono- and bipolar pulses depending on the wavelength. The obtained results are explained by simultaneous generation of photocurrent due to the dragging effect and the surface photogalvanic effect.

Two-dimensional (2D) periodic gratings of rectangular graphene nanoribbon elements on substrates, comprising dielectric and graphene layers have been studied to learn their absorption properties in the terahertz range. It is established that, at the surface plasmon polariton resonance frequencies, these graphene structures represent electrically controlled absorbers of almost 100% incident energy in a broad THz range.