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

Multichannel quantum tunneling of an electron through a potential barrier of a thin impurity introduced into a nanotube in the presence of a longitudinal magnetic field of arbitrary intensity is investigated. Tunneling and polarization characteristics of the system are studied in detail and illustrated as a function of the electron energy, nanotube radius, and magnetic field induction. It is indicated that in some electron energy ranges the system can operate as an ideal polarizer of the electron flow.

By the analysis of the low-temperature absorption and emission spectra of the crystal 20% Yb³⁺:Lu₃Al₅O₁₂, the energies of the Stark levels of Yb³⁺ ion in dodecahedral positions are determined. Within the framework of the crystal field (CF) theory of point charges (D₂ point symmetry group), the CF parameters are determined, and the wave functions of the Stark states are constructed. The line strengths of indirect electric dipole and magnetic dipole inter-Stark transitions are calculated and the Judd–Ofelt parameters are determined. Low temperature absorption and emission spectra show a complex fine structure that cannot be explained on the basis of the standard level structure of Yb³⁺ occupying sites with dodecahedral crystal field symmetry.

The application of an elliptical horn element for focusing radiation from relatively hot objects in the environment onto a thermal infrared sensor is presented. Theoretical calculations were used to determine the radiation concentration values in the horn element with the normal incidence of rays relative to the entrance section and a single reflection from the inner surface. The device with the horn element and thermal infrared sensor enables the detection of a person at a long distance (16 m) with a wide field of view in the horizontal direction (100°). The setup can operate both independently and in conjunction with other devices that monitor internal and external adjacent spaces.

The excitation functions of proton-induced reactions on natural tin ^natSn(p, x)^{118m;120m;122;124}Sb and ^113;117mSn, were investigated using statistical nuclear codes TALYS 1.96 and EMPIRE 3.2. Various models were implemented in these codes for the proton energy range from reaction thresholds to 70 MeV. A comparison of theoretical calculations, utilizing different models of the TALYS 1.96 and EMPIRE 3.2 codes, with experimental data from the EXFOR database was performed. The experimental data were compared with theoretical results through statistical analysis. Weighted statistical parameters of relative deviation were used to assess the differences between experimental and theoretical cross-section values and to evaluate the reliability of the codes. The importance of this study is determined by the practical applications of the ¹²⁴Sb, ¹²²Sb, and ^117mSn isotopes.

In a stochastic approximation, taking into account the conclusions of the theory of elasticity for micro nonhomogeneous media, a model of diamond nucleation in a high-pressure chamber (HPC) is considered as a result of the direct graphite–diamond phase transition. Considerations are presented regarding the distribution of the number of nuclei N as a function of the intensive parameters pressure P and temperature T. The assumption that the number of grown diamond crystals equals the number of formed diamond nuclei made it possible to use the normal distribution (Gaussian distribution) for analyzing the diamond nucleation processes. As a result, a probabilistic analytical expression for the number of nuclei was obtained, allowing for sufficiently accurate determination of N depending on the measured pressure (P_meas) and temperature (T_meas) values. The rate of diamond nucleation was estimated within the framework of a martensitic-type phase transition model.

The process of electromagnetically induced transparency (EIT) was investigated using two different probe radiations in Cs atomic vapor: in the first case probe radiation– transmission through a nanocell (NC) with the formation of EIT_T; in the second–selective reflection (SR) of laser radiation from the boundary of the atom-dielectric vapor (dielectric is nanocell window) with the formation of EIT_SR. To form EIT resonances, we used two continuous narrow-band lasers with λ = 852 nm and a nanocell with an atomic vapor column thickness in the range of 150–1500 nm. A comparison of the formation of EIT resonance in the first and second cases was carried out. Probe radiation containing EIT_T and EIT_SR-resonances propagates in opposite directions. It is noted that in a few cases, the formation of EIT resonance is more effective using SR radiation. The splitting of the EIT_SR-resonance in the longitudinal magnetic field into seven equidistant components was recorded, which allows remote monitoring of the magnetic field.

The problem of laser-induced hydrodynamic reorientation (LIHR) of the director of a hybrid-oriented nematic liquid crystal (NLC) with arbitrary boundary conditions has been numerically solved. LIHR is studied at different laser powers and for two opposite directions of the temperature gradients. The hydrodynamic flow velocity gradient results in a small increase in curvature when light creates a temperature gradient in the out-of-curvature side of the hybrid orientation. The curvature changes sign when light creates a temperature gradient from outside to inside the curvature of the hybrid orientation. The dependence of reorientation on the anchoring energy of molecules to the boundaries is also investigated.

The article considers the features of heat propagation released by a single photon in a thermoelectric sensor with a surface area of 0.25 µm² consisting of a dielectric substrate (Al₂O₃), a heat sink (Mo), a thermoelectric layer (La_0.99Ce_0.01B₆), and an absorber (W) sequentially located on each other. The results of heat propagation simulation in the sensor with an operating temperature of 0.5, 0.8, 1, 1.2, and 1.5 K upon absorption of photons with energies of 0.8 and 1.65 eV are presented. The equivalent power of Johnson and phonon noise is calculated. The power of the signal arising on the sensor and the signal-to-noise ratio are determined. A comparison is made with the characteristics of a sensor of the same design with a surface area of 1 μm². The calculations were performed using the three-dimensional matrix method based on the equation of heat propagation from a limited volume. It is shown that decreasing the surface area of the sensor leads to an increase in the signal/noise ratio, and therefore to an increase in the efficiency of recording the already absorbed photon. This result is especially important for single-photon detection in the near infrared region.

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.