Quasi-hydrodynamic simulation of the conductivity of heavily doped nanosize layered heterostructures in high electric fields

The electrical conductivity of nanosize-layered heterostructures in high electric fields is studied using the methods of mathematical simulation based on the equation of energy balance. It is shown that a characteristic feature of the corresponding current-voltage characteristics is a peak of the differential conductance, whose position and magnitude are determined by the heterobarrier height and steepness, as well as by the doping level of the structure. A physical model is suggested that accounts for the shape of the calculated characteristics by the combined effect of the electrostatic reduction in the heterobarrier height and the increase in the electronic temperature in the vicinity of the injecting heterointerfaces.     

Computation of the velocity profile for nonlinearly viscous fluid flow in spiral channels with low reynolds numbers

Two methods are presented for solving the problem of nonlinear viscous fluid flow in spiral channels of arbitrary transverse section in the Stokes approximation, variational and iterational.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 55, No. 4, pp. 581–589, October, 1988.     

Dynamics of viscoelastic spherical shells during internal explosive loading

The dynamics of a thick-walled closed spherical shell was investigated theoretically and experimentally for determining the dynamic viscoelastic characteristics of steel St. 3. A liquid explosive charge placed in a capsule discharge in the center of a flask was used for uniform loading. The value of the logarithmic decrement was obtained from the results of measuring the meridional deformation by means of a strain-gauge complex.Arzamas. Translated from Fizika Goreniya i Vzryva, Vol. 28, No. 4, pp. 91–95, July–August, 1992.     

Thiacalixarenes with Sulfur Functionalities at Lower Rim: Heavy Metal Ion Binding in Solution and 2D-Confined Space

Sulfur-containing groups preorganized on macrocyclic scaffolds are well suited for liquid-phase complexation of soft metal ions; however, their binding potential was not extensively studied at the air–water interface, and the effect of thioether topology on metal ion binding mechanisms under various conditions was not considered. Herein, we report the interface receptor characteristics of topologically varied thiacalixarene thioethers (linear bis-(methylthio)ethoxy derivative L², O₂S₂-thiacrown-ether L³, and O₂S₂-bridged thiacalixtube L⁴). The study was conducted in bulk liquid phase and Langmuir monolayers. For all compounds, the highest liquid-phase extraction selectivity was revealed for Ag⁺ and Hg²⁺ ions vs. other soft metal ions. In thioether L² and thiacalixtube L⁴, metal ion binding was evidenced by a blue shift of the band at 303 nm (for Ag⁺ species) and the appearance of ligand-to-metal charge transfer bands at 330–340 nm (for Hg²⁺ species). Theoretical calculations for thioether L² and its Ag and Hg complexes are consistent with experimental data of UV/Vis, nuclear magnetic resonance (NMR) spectroscopy, and single-crystal X-ray diffractometry of Ag–thioether L² complexes and Hg–thiacalixtube L⁴ complex for the case of coordination around the metal center involving two alkyl sulfide groups (Hg²⁺) or sulfur atoms on the lower rim and bridging unit (Ag⁺). In thiacrown L³, Ag and Hg binding by alkyl sulfide groups was suggested from changes in NMR spectra upon the addition of corresponding salts. In spite of the low ability of the thioethers to form stable Langmuir monolayers on deionized water, one might argue that the monolayers significantly expand in the presence of Hg salts in the water subphase. Hg²⁺ ion uptake by the Langmuir–Blodgett (LB) films of ligand L³ was proved by X-ray photoelectron spectroscopy (XPS). Together, these results demonstrate the potential of sulfide groups on the calixarene platform as receptor unit towards Hg²⁺ ions, which could be useful in the development of Hg²⁺-selective water purification systems or thin-film sensor devices.     

Substructural Approach for Assessing the Stability of Higher Fullerenes

This review describes the most significant published results devoted to the study of the nature of the higher fullerenes stability, revealing of correlations between the structural features of higher fullerene molecules and the possibility of their producing. A formalization of the substructure approach to assessing the stability of higher fullerenes is proposed, which is based on a detailed analysis of the main structural features of fullerene molecules. The developed substructure approach, together with the stability of the substructures constituting the fullerene molecule, helps to understand deeper the features of the electronic structure of fullerenes.     

Analytical model of the electrical instability mechanism in multibarrier heterostructures with tunnel-opaque barriers

Through mathematical modeling of the conductivity of multibarrier heterostructures, the steady-state current-voltage characteristics, the S-shaped behavior of which is indicative of electrical instability, are obtained. To study its dynamic parameters, an analytical model of instability is developed using known approximations of the physics of semiconductor devices. In the steady-state case, the model yields an S-shaped current-voltage characteristic close to numerical simulation results. This fact is considered as confirmation of the adequacy of the developed analytical model. In the small-signal case, the latter is generalized to the situation with harmonic electrical perturbation. The resulting formula for the frequency dependence of the small-signal impedance indicates the possibility of dynamic-resistance negativity up to terahertz frequencies. A clear physical interpretation of the instability in terms of positive feedback in the unit cell of the multibarrier heterostructures under study is proposed. The results of measurements of the quasi-stead-state current-voltage characteristics of fabricated test multibarrier GaAs/AlGaAs structures with a pronounced portion of negative differential resistance are also presented.