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

The dipole matrix elements for optical transitions 1S –(2{{P}^{ + }}) and 2S –(2{{P}^{ + }}) of a shallow donor impurity in a graphene monolayer with an opened energy gap are calculated in a perpendicular magnetic field using a variational approach. In the next step, population transfer between the two metastable states 1S and 2S by a bichromatic phase modulated (frequency chirped) laser pulse is analyzed taking into account relaxation processes in the system. It is demonstrated that such population transfer may be performed in a robust way without a considerable population of the excited state (2{{P}^{ + }}) if the duration of the bichromatic pulse is much shorter than the relaxation times of the system. Spontaneous decay of the excited state and dephasing relaxation processes result in not complete transfer of the population between the metastable states.

The applicability of the stepwise approximation of the Gaussian current density distribution is justified for calculating the thermal properties of flat and locally inclined targets (LIT) in the ANSYS program. Within the framework of a six-zone model, numerical simulation of the temperature of flat and axially symmetric LIT was carried out for uniform and several variations of Gaussian distributions of the irradiating current density. The analysis of the obtained temperature dependencies for the flat target variant reveals that they represent the sum of two contributions: a constant one, independent of the coordinate, and a variable, Gaussian one. It has been shown that transitioning from a uniform current density distribution to a Gaussian one results in similar changes in the temperature regimes of both targets. This prevents additional risks of LIT overheating due to the combined influence of the target’s corrugated surface and the inhomogeneity of the irradiating current in its Gaussian distribution. A comparative analysis of heat fluxes from the target holder into the water was carried out for various Gaussian distributions of the irradiating current density. It has been demonstrated that the maximum gain in critical heat flux values into the water is achieved when choosing LIT, provided that a uniform current density distribution is maintained through the target. In this case, the efficiency coefficient, i.e., the ratio of the maximum allowed currents, is 1.45. For Gaussian current density distributions, the gain monotonically decreases when the standard deviation parameter Sigma decreases but remains practically acceptable, with an efficiency coefficient of 1.29, down to the minimum considered value of Sigma = 3.14 mm.

The thermodynamic characteristics of a non-interacting electron gas in a strongly oblate asymmetric ellipsoidal quantum dot of InAs have been investigated. The analytical expressions for the system’s energy spectrum and partition function in the Boltzmann approximation are obtained. Analytical expressions for the system’s average energy, entropy, and heat capacity are presented. The dependences of the listed thermodynamic characteristics on the temperature and geometric parameters of the quantum dot have been studied.

Solar concentrator photovoltaics (CPV) technology is one of the promising methods for increasing the efficiency of solar energy conversion. In CPV systems, the luminous flux absorbed by the photodetector (photovoltaic module or solar cell) depends on the reflectivity of the concentrator surface, which is a function of the angle of incidence of light. The absorbed luminous flux also depends on the relative angular transmission of light into the photovoltaic module. In this paper, we propose a refined method for determining the absorbed luminous flux by the photodetector using the concept of effective light concentration. The proposed method considers the angular dependence of the specular reflection coefficient of light, as well as the relative angular transmission of light into the photovoltaic module. As an example, two specularly reflecting CPV systems are considered – planar and V-shaped.

Some problems related to electron mobility fluctuations in polar semiconductors are discussed. The theory of electron-polar optical phonon field-induced tunnel scattering has been developed. Based on the obtained relation for the scattering rate in the case of weak fields, it was shown that in the presence of an electric field, the electron mobility variance takes on finite values, in contrast to the absence of a field, when it diverges. Using GaAs as an example, the dependence of the electron mobility variance on the electric field is computed, which is given by the decreasing function according to the logarithmic law. The frequency dependence of the spectral density of mobility fluctuations in polar semiconductors has the same qualitative form as in non-polar semiconductors: in a fairly wide frequency range it is described by the 1/f law, and in the presence of an electric field in the low-frequency region a saturation region appears. The temperature dependence of the electron mobility noise parameter in GaAs is computed, which qualitatively and quantitatively agrees with some experimental data on the temperature dependence of the Hooge parameter of low-frequency current noise in n-GaAs.

A new method of using bispectral analysis for detecting moving targets is presented. The cepstrum method is used to improve the detection of harmonic frequencies. The method involves reading a file and using an algorithm that finds the desired harmonics in the bispectrum, reducing noise. This targeted approach facilitates tracking and understanding moving targets. The method has been tested in simulations and found to significantly improve target detection and frequency resolution compared to traditional methods of bispectral analysis. This new methodology helps to track moving targets more accurately and efficiently, especially in radar and sonar systems where precise tracking is crucial.

The propagation characteristics of q-Gaussian laser beam in Cold Collisionless plasma are explored in current research work. In Cold Collisionless plasma, the carriers get redistributed away from axial portion due to ponderomotive force. This carrier redistribution leads to production of density gradients thereby causing self-focusing of beam. The Wentzel–Kramers–Brillouin and paraxial approximations are utilized for deriving nonlinear 2nd order differential equation for beam width of the beam. Since, this differential equation can’t be solved analytically. So, numerical solution has been obtained for exploring the variation of beam width of beam against normalized propagation distance. Impact of variation in established parameters of laser and plasma such as beam intensity, beam radius, density of plasma electrons, plasma temperature and q-parameter on behavior of laser beam is also examined.

The energies of the ground 1S and the first three excited levels of 2S, 2P^– and (2{{P}^{ + }})of a shallow impurity in graphene monolayer with an opened energy gap in a perpendicular magnetic field, are studied by a variational approach. The dependences of the energy of impurity levels on magnitude of the magnetic field are obtained for various values of the “thickness” parameter b of “softened” Coulomb potential. The ability to control the position of impurity energies and distance between them in graphene by changing the magnetic field opens possibilities for implementation of new schemes of coherent interaction of laser pulses with graphene systems.

An analysis of adsorption isotherms was carried out based on experimentally obtained data on the binding of the antitumor compound mitoxantrone (MTX) and ethidium bromide (EB) to A-form nucleic acids (NA). The obtained results show that there is selectivity in binding with base pairs of NA. Calculations show that the enthalpy changes due to the binding of MTX and EB to A-form NA (within the experimental error) are almost the same, but the entropy changes upon binding are significantly different. Generalizing the obtained experimental data and comparing them with the literature, it can be concluded that the binding of these compounds to the B-form is stronger compared to the A-form, leading to a more pronounced change in the structure of the double helix of NA.

New relationships that express the relationships between some optical and acoustic quantities and the stoichiometry of the lithium niobate single crystal suitable for rapid estimation have been obtained. It is shown that by determining the damping coefficients of transverse and longitudinal hypersonic waves, it is possible to estimate the refractive indices in the terahertz range with great accuracy and determine the stoichiometric composition of the lithium niobate crystal.