Formation Of Local Regions Research Articles

Stability of space-periodic flow with separation of a laminar boundary layer

An unstable laminar flow over a wavy region of a plate placed parallel to the oncoming air flow is studied. The results have been obtained by the hot-wire method in a low-turbulent wind tunnel at low subsonic velocities. Flow near a wall with transverse undulation, whose amplitude is sufficiently large and can contribute to the formation of local regions of boundary layer separation, is considered. The external harmonic (acoustic) forcing of the periodic flow leads to the suppression of perturbations that dominate in the spectrum of velocity disturbances near the model surface. The results of this work may be useful in developing methods for controlling spatially inhomogeneous flows.

Proton Irradiation of 4H-SiC Photodetectors with Schottky Barriers

For the first time, comprehensive comparative investigations of ultraviolet photodetectors with Cr Schottky barriers formed on 4H-SiC epitaxial layers are carried out by the X-ray and optical methods before and after irradiation with 15-MeV protons with fluences in the range of (1–4) × 1012 cm–2. When increasing the fluence of proton irradiation, the formation of localized regions with negative deformation is observed along with the unperturbed silicon-carbide matrix. Agreement between the X-ray and optical studies is obtained, which makes it possible to explain the features of the spectral changes in the photosensitivity of detectors in the range of 200–400 nm with an increase in the fluence of proton irradiation. The ultraviolet Cr/4H-SiC photodetectors withstand irradiation by 15-MeV protons with a fluence of 4 × 1012 cm–2 virtually without any changes in the photosensitivity due to the gettering of simple defects by cluster and amorphous formations, which lead to partial structural improvement of the irradiated material.

Облучение протонами 4H-SiC фотоприемников с барьером Шоттки

AbstractFor the first time, comprehensive comparative investigations of ultraviolet photodetectors with Cr Schottky barriers formed on 4 H -SiC epitaxial layers are carried out by the X-ray and optical methods before and after irradiation with 15-MeV protons with fluences in the range of (1–4) × 10^12 cm^–2. When increasing the fluence of proton irradiation, the formation of localized regions with negative deformation is observed along with the unperturbed silicon-carbide matrix. Agreement between the X-ray and optical studies is obtained, which makes it possible to explain the features of the spectral changes in the photosensitivity of detectors in the range of 200–400 nm with an increase in the fluence of proton irradiation. The ultraviolet Cr/4 H -SiC photodetectors withstand irradiation by 15-MeV protons with a fluence of 4 × 10^12 cm^–2 virtually without any changes in the photosensitivity due to the gettering of simple defects by cluster and amorphous formations, which lead to partial structural improvement of the irradiated material.

Specific features of proton interaction with transistor structures having a 2D AlGaN/GaN channel

It has been shown that the interaction of 1 MeV protons at doses of (0.5–2) × 1014 cm–2 with transistor structures having a 2D AlGaN/GaN channel (AlGaN/GaN HEMTs) is accompanied not only by the generation of point defects, but also by the formation of local regions with a disordered nanomaterial. The degree of disorder of the nanomaterial was evaluated by multifractal analysis methods. An increase in the degree of disorder of the nanomaterial, manifested the most clearly at a proton dose of 2 × 1014 cm–2, leads to several-fold changes in the mobility and electron density in the 2D channel of HEMT structures. In this case, the transistors show a decrease in the source–drain current and an order-of-magnitude increase in the gate leakage current. In HEMT structures having an enhanced disorder of the nanomaterial prior to exposure to protons, proton irradiation results in suppression of the 2D conductivity in the channel and failure of the transistors, even at a dose of 1 × 1014 cm–2.

Plasma effect of the Houston horse

The effect of a strong shock wave on a weakly ionized collisional plasma was studied. The structure of the ion-acoustic perturbation caused by the shock wave was numerically investigated. The effect of the nonlinearity, dispersion, and dissipation on the formation of an oscillating wave profile was shown. It is found that in some modes, an increase in the shock wave velocity leads to a sharp increase in the concentration of charged particles and a reduction in the number of perturbation maxima. This change of the flow structure can be preceded by the formation of localized regions with an increased degree of plasma ionization. It is shown that the presence of plasmoids can lead to a strong influence of charges on the neutral component.

Overgrowth of Douglas fir (Pseudotsuga menziesii Franco) stumps with regenerative tissue as an example of cell ordering and tissue reorganization.

Main conclusion Stump overgrowth may serve as a unique model for studying cellular reorganization and mechanisms responsible for cell polarity changes during the process of vascular tissue differentiation from initially unorganized parenchymatous cells. Cellular ordering and tissue reorganization during the overgrowth process of the transverse surfaces of Douglas fir stumps in forest stand was studied. At the beginning of stump overgrowth, the produced parenchymatous cells form an unorganized tissue. Particular parenchyma cells start arranging into more ordered structures which resemble rays. Application of digital image analysis software based on structure tensor was used. The analysis showed that at this stage of tissue development, cellular elements display a wide range of angular orientation values and attain very low coherency coefficients. The progress of the tissue differentiation process is associated with the formation of local regions with tracheids oriented circularly around the rays. This coincides with an increase in the range of angular orientations and greater values of coherency coefficients. At the most advanced stage of tissue development, with tracheids arranged parallelly in longitudinal strands, the degree of cell ordering is the highest what is manifested by the greatest values attained by coherency coefficients, and the narrow range of angular orientations. It is suggested that the ray-like structures could act as organizing centers in the morphogenetic field responsible for differentiation of the overgrowth tissue. The circular pattern of tracheids around rays in the initial phase of tissue development can be interpreted in terms of local rotation of the morphogenetic field which afterward is transformed into irrotational field. This transformation is noted by the presence of tracheids arranged parallelly in longitudinal strands. The possible involvement of a mechanism controlling cell polarity with respect to auxin transport is discussed.

Effect of mass transfer and segregation on the formation of superhard nanostructured Ti-Hf-N(Fe) catings

Superhard nanostructured Ti-Hf-N(Fe) coatings are prepared. The formation of local regions of (Ti, Hf)N, FeN, and Hf is detected using μ-PIXE (ion microbeam). It is revealed that the synthesized coatings have a nanohardness of 48 ± 1 GPa and are composed of nanograins with a size of 4.8–10.6 nm, which are enveloped by finer entities of other phases (Ti, Fe)N and FeN. There is a good correlation between the results derived by XRD, TEM, AFM, and SEM and microanalysis, which in turn are complemented by the analysis results obtained using an ion microbeam and PIXE.

Effect of diffusion and fluctuations in the nonequilibrium charge carrier density on the angular distribution of the output intensity for injection lasers based on an InAsSb/InAsSbP heterostructure

We have used numerical modeling to study the effect of diffusion and fluctuations in the nonequilibrium carrier density in the active layer of injection lasers based on an InAsSb/InAsSbP heterostructure on the angular distribution of the output intensity. We show that diffusion smoothes out the nonequilibrium carrier distribution in the active layer, and the fundamental lasing mode is stable over a much broader range of stripe contact widths. At the same time, diffusional processes can lead to formation of local regions with a jump in the density of nonequilibrium charge carriers, fluctuations in which can act as a source of instability for the fundamental lasing mode. Analysis of the numerical modeling results gives qualitative agreement with experimental data on the dependence of the angular distribution of the output radiation for different stripe contact widths.

Structure of twins in GaAs nanowires grown by the vapour–liquid–solid process

GaAs nanowires have been grown on single-crystal (001) Si and GaAs substrates using the vapour–liquid–solid (VLS) mechanism in a MOCVD chamber. These nanowires typically exhibited a circular cross-section with diameters in the range 10–40 nm. They contain a large density of growth twins that are here investigated in detail using transmission electron microscopy and high-resolution electron microscopy. Results indicate that the wires predominantly have the zinc blende diamond cubic structure with the growth axis parallel to ⟨111⟩ and twins perpendicular to the growth axis. In many cases, multiple twins tend to cluster in groups along the length of the nanowires, either in a near-periodic fashion or on consecutive {111} planes, resulting in the formation of local regions exhibiting the hexagonal wurtzite structure of GaAs. The small difference in energy between these two forms of GaAs is likely to be the reason for the coexistence of both zinc blende and wurtzite structures within the same nanowire.

Mechanism of the Unfolding of Transmembrane α-Helical Segment (1−36)-Bacteriorhodopsin Studied by Molecular Dynamics Simulations

The unfolding of the first transmembrane segment 1−36 bacteriorhodopsin (BR) was studied using 1.25 ns molecular dynamics (MD) simulation with an explicit representation of chloroform/methanol 1:1 mixture and a series of long (from 10 to 60 ns) Langevin dynamics (LD) simulations. Comparison of MD and LD simulations shows that the random and frictional forces of Langevin equation provides a good model for the weakly polar chloroform/methanol mixture. The dielectric permeability eo = 1−2 (e = eor) was found to be suitable for the modeling of the shielding effect of a weakly polar solvent in the LD simulations. The enhancement of short-range electrostatic interactions stabilizes the α-helix in LD simulations. The α-helix unfolding proceeds through the formation of local regions, stabilized by π-helical hydrogen bonds (π-bulges), in the central part of (1−36)-BR. Being formed, the π-bulges can propagate to metastable structures with partially solvated backbone intermediates on the way to an α-helix−random coi...