Journal of Contemporary Physics (Armenian Academy of Sciences)最新文献

We describe the non-barotropic variational principle in both Lagrangian and Eulerian forms. Thus, we must consider a non-barotropic equation of state in which the internal energy is a function of both density and specific entropy. This implies an extension of our previous work on barotropic flows to the more general non barotropic flows case and may serve in the future for the study of the implications of variational analysis on the redefinition of conserved quantities of topological significance such as circulation and helicity.

The modal properties of a terahertz (THz) pulse propagating in a parallel-plate waveguide partially filled with a nonlinear crystal – either lithium niobate (LiNbO₃) or 4N, N dimethylamino 4N methylstilbazolium tosylate (DAST), are investigated. THz pulses are generated remotely in the nonlinear crystal via optical rectification of a femtosecond Ti:sapphire laser pulse. A numerical analysis of the mode composition of the THz pulse in the structure of a “metallic parallel-plate waveguide partially filled with a nonlinear crystal” was performed using MATCAD and COMSOL software. The study demonstrates the possibility of achieving single-mode coupling between the input linearly polarized optical pulse and the THz pulse with an ultra-broad spectral bandwidth of 0.1–2.6 THz. By numerically determining the width and height of the nonlinear crystal for given waveguide dimensions and known dielectric permittivity (ε) of the crystal, conditions for single-mode propagation of the THz pulse were achieved. It is also shown that as the THz frequency increases, a greater portion of the pulse energy is confined within the nonlinear crystal. At the same time, the remainder propagates in the surrounding air-filled region.

A method of optical transmission imaging of strongly scattering objects is proposed, based on point-by-point spatial scanning of a beam of pulsed-periodic laser radiation over the surface of the object under study and recording the number of transmitted light pulses at each scanning point using a patented photoreceiver that selectively registers short light pulses regardless of the level of background illumination. The conditions for obtaining “proportional” and “contour” images of model and biological objects are determined. The features of the method are a low level of required average radiation power, high image recording speed, and a wide operational spectral range, which makes it promising for use in biomedical diagnostics, materials quality control, security, and other areas.

An acoustoplasma discharge is experimentally investigated when low-intensity acoustic oscillations are far from the intensity region where shock waves exist. The frequency range of 20–50 kHz is considered, when non-axial modes, rather than a plane longitudinal wave, propagate in the discharge tube. The discharge trajectory in the tube is shown to take the form of a planar sinusoid. The calculated and experimentally measured spectra of acoustic oscillations arising during discharge current modulation and sound dispersion for non-axial modes are presented. The dispersion is found to be significant. The spectrum of non-axial modes has a band structure. In acoustoplasma, the transition from one frequency band to the next is characterized by a phase transition. Within each band, dispersion can be either normal or anomalous, depending on the frequency. When moving from one band to the next, dispersion becomes anomalous.

Computational methods of quantum chemistry were used to study the energy pathways of possible reactions of ammonia with hydrogen and oxygen atoms, as well as with the hydroxyl radical. The B3LYP, M062X, MP2 and CBS-QB3 methods were used to localize stationary points on the potential energy surface of the system containing the above reagents. A number of intermediate compounds formed during the interaction of ammonia with atoms and radicals were identified, and information on the structure and thermochemical parameters of the intermediates formed was obtained. Based on the calculation results, diagrams of the energy levels of the systems under consideration were constructed. The results obtained demonstrate the important role of the NH₃ + H, NH₃ + O and NH₃ + OH reactions in the processes of ammonia combustion.

The Van der Waals effect (VW) in ³⁹K atomic vapours was experimentally studied using a nanocell made of technical sapphire. At distances of less than 100 nm from the sapphire surface, a strong broadening of atomic transitions and their frequency shifts to the low-frequency region of the spectrum occur. The method of selective reflection (SR) of laser radiation from the sapphire window-atomic vapours boundary was used as a sub-Doppler method, which made it possible to measure the VW interaction coefficient C₃ for the potassium atoms. It is shown that a change in the nanocell thickness from 100 to 50 nm results in a decrease in C₃, as calculated from the “red” VW shift, i.e., the so-called “retardation” of the VW effect, which was predicted in theoretical works and experimentally observed. The obtained results are important for the development of miniature submicron devices containing atomic vapours.

In the present work, an attempt is made to carry out a comprehensive theoretical analysis and interpretation of a large body of industrial and laboratory experimental data accumulated in the field of the synthesis of polycrystalline diamond of the carbonado type. The study covers the physicochemical mechanisms of phase transitions in carbon systems, barodiffusion processes, and the role of catalytic components, as well as the kinetic aspects of crystal structure growth. The analysis is based both on experimental data obtained from industrial high-pressure high-temperature (HPHT) installations and on results of targeted laboratory series in which key parameters—pressure, temperature, composition, and dwell time—were varied. Particular attention is paid to correlating industrial and laboratory results with theoretical models of phase equilibrium and growth dynamics, which not only makes it possible to explain the observed phenomena but also to propose approaches for the targeted optimization of the process.

The structure, morphology, and dielectric properties of polycrystalline samples of the borate compound BaZr(BO₃)₂ synthesized by the method of solid-phase reactions were studied. The effect of thermal annealing in the temperature range of 1000–1250°C on the grain morphology, porosity, bulk density, and dielectric properties of the BaZr(BO₃)₂ samples was investigated. The dielectric properties were investigated in the radio frequency range of 1–10 MHz. The frequency dependences of the dielectric constant and dielectric loss, and the effect of annealing temperature on these dependences were analyzed. For the sample with the highest bulk density (annealed at 1250°C for 2 hours), the dielectric constant and dielectric loss at 1 MHz were found to be 22.36 and 1.05 × 10^–2, respectively. The synthesized ceramics BaZr(BO₃)₂ can be considered as a promising material for the preparation of substrates for integrated circuits and ceramic capacitors.

The Stark effect in the generalized MIK–Kepler problem is considered. For a discrete energy spectrum, the wave function of the generalized MIK–Kepler problem is presented in parabolic coordinates, and integrals of motion whose eigenfunctions are the parabolic basis. It is shown that in the generalized MIK–Kepler problem, there is a linear Stark effect, completely removing the degeneracy of energy levels in the azimuthal quantum number, and its dipole moment is calculated. An explicit expression for the additional integral of motion for the generalized MIK–Kepler problem in the presence of a constant uniform electric field is obtained.

It has been established for the first time that an aqueous solution of potassium iodide releases molecular iodine under the action of ultrasound radiation when cavitation occurs. The process of iodine release occurs in an oscillatory mode. It has been shown that the rate of iodine release and its maximum concentration depend on the ultrasound intensity, the concentration of the potassium iodide solution and the ultrasound frequency. At the same time, an increase in the concentration of potassium iodide in the initial solution above 5–6% does not lead to a further increase in the concentration of the iodine formed. The dependence of the maximum iodine concentration on the oscillation amplitude of the waveguide end is nonlinear.