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

Cubic boron nitride (c-BN) is an ultrawide band gap, superhard material with significant potential for applications under extreme temperatures and pressures. Nevertheless, two major challenges hinder its practical utilization in technology: (i) the difficulty in producing high-quality c-BN films, and (ii) the challenge of effectively n- and p-doping its matrix. This theoretical study focuses on investigating the solubility limits of silicon (Si) and carbon (C) in the c-BN and wurtzite BN (WZ–BN) on the basis of the strictly regular solution approximation. These elements are key candidates as n-type dopants in BN, addressing a critical concern in the realization of c-BN based electronics. The calculated Gibbs free energies of binary mixtures within the temperature range of 800–3000 K indicate that the presence of unstable regions, and consequently, miscibility gaps, are a prevalent characteristic of this system. The analysis reveals that the immiscibility gap is influenced not only by temperature, but also by the crystallographic structure. The calculated results of Si solubility limit in BN–Si binary system are in a good agreement with the latest experimental results. The findings presented here are applicable to the fabrication of multicomponent bulk crystals, epitaxial thin films, and nanostructures based on BN–Si–C solid solutions.

In this study, we have performed investigations on the structural stability, electronic band structures, magnetic exchange couplings, and half-metallic performance of Cr_xCa_{1– x}Se materials using computational methods of density functional theory via GGA-WC, GGA-PBE and, TB-mBJ exchange-correlation potentials. The Cr_xCa_1–xSe compounds are thermodynamically stable and synthesizable owing to their negative formation energies. The structural parameters of Cr_xCa_1–xSe with GGA-WC and GGA-PBE approximations appear to be in excellent concordance compared to the experimental data and recent theoretical calculations. According to GGA-PBE and TB-mBJ calculations, the Cr_xCa_1–xSe compounds revealed integral magnetic moments and a half-metallic behavior with better half-metallic gaps, and spin-polarization of 100%. The Cr_xCa_1–xSe alloys appear to be better materials for use in spintronic devices.

The Er³⁺/Yb³⁺ co-doped Y₄GeO₈ crystal powders were successfully synthesized using a high-temperature solid-phase method. The crystal structure of the obtained phosphors was confirmed to be pure Y₄GeO₈ through X-ray diffraction (XRD) analysis. A regression equation correlating Er³⁺/Yb³⁺ doping concentrations with luminescent intensity was established based on the optimized theoretical model derived from experimental design. The optimal concentrations of Er³⁺ and Yb³⁺ under 980 nm laser excitation were determined as 7.41 and 21.34%, respectively, while under 1550 nm laser excitation, the concentrations were 2.66 and 17.42%, respectively. The fluorescence emission spectra of the up-conversion samples were measured, revealing intense green and red emissions with peaks at 542, 546, and 654 nm under 980 nm excitation, and peaks at 546, 557, and 663 nm under 1550 nm excitation. These peaks correspond to transitions from ²H_11/2 to ⁴I_15/2, ⁴S_3/2 to ⁴I_15/2, and ⁴F_9/2 to ⁴I_15/2 energy levels. The relationship between up-conversion luminescence and laser operating current for the optimal samples under 980 nm and 1550 nm was investigated, uncovering that up-conversion luminescence occurs through both two-photon and three-photon processes. A detailed analysis and discussion of the up-conversion luminescence mechanisms were conducted. Furthermore, the relationship between up-conversion fluorescence and temperature for the optimal samples was studied, revealing excellent temperature-sensing characteristics under 980 nm and 1550 nm laser excitations. The calculated illumination region coordinates for the optimal samples under 980 nm and 1550 nm wavelength excitations were (0.5558, 0.4362) and (0.5256, 0.4687), respectively. The research highlights the potential of rare-earth ion-doped up-conversion luminescent materials for diverse anti-counterfeiting applications. In particular, the Y₄GeO₈: Yb³⁺/Er³⁺ phosphors, incorporating a dual-excitation mechanism, enhance the security of anti-counterfeiting strategies in multifaceted scenarios. The study underscores the promising developments in this field.

A simple quantum chain with a topological phase is constructed. It is formed by parafermions (quasiparticles obeying the fractional statistics) with angular parameter (theta = {{2pi } mathord{left/ {vphantom {{2pi } 3}} right. kern-0em} 3}). The model is derived from the one-dimensional spin model with topological order protected by the ({{mathcal{Z}}_{3}} times {{mathcal{Z}}_{3}}~) symmetry (generalized cluster model). Parafermionic zero modes arising at the chain boundaries, which remain stable because of the vacuum gap and topological order, are constructed and studied.

Samples of natural zeolite from the Nor-Kokhb deposit (Armenia), activated with zinc oxide (30% ZnO) and silver (30% ZnO + 5% Ag), were studied during chemical and thermal treatment by microwave and traditional methods. It was previously shown that microwave processing is 3 times faster than the traditional method. The X-ray diffraction analysis shows the presence of zeolite in the form of clinoptilolite and zinc oxide in the studied samples. Based on the results of optical and EPR spectroscopy, the presence of Fe³⁺ ions in the framework and extra-framework positions of the zeolite of the studied samples was revealed. The study of diffuse reflectance spectra showed that in zeolite, which is a wide-gap material (with a band gap of 4.14 eV), the activation of 30% ZnO decreases the band gap to 3.38 eV, and with additional activation with a silver (30% ZnO + 5% Ag) the bandgap decreases to 3.31 eV. The photocatalytic activity of natural zeolite activated with zinc oxide (30% ZnO) and silver (30% ZnO + 5% Ag) was studied according to the decomposition reaction of methylene blue under UV irradiation.

In the presence of the axial magnetic field, the behavior of the electron in the colloidal CdSe nanoplatelet containing multi-impurity centers has been investigated. The two-dimensional Schrodinger equation with the effective potential of the interaction of electron and impurity centers, localized in the central plane of the nanoplatelet has been obtained. It has been shown that the magnetic field influence on the electron energy levels weakens with increasing the number of impurities.

The article presents the results of studies of the characteristics of lead tungstate crystals (PbWO₄). Measurements of light transmission and light output from the passage of cosmic muons were carried out. The average light transmittance of crystals in the transverse direction is 62.82, 68.38, and 75.68% at wavelengths λ = 360, 420, and 620 nm, and the light output is ~16 pe/MeV. A prototype of an electromagnetic calorimeter was designed and built from crystals arranged in a 4 × 4 matrix which has been tested by cosmic muons. The results obtained confirm the conclusions of other groups in the Electron-Ion Collider collaboration that the quality of the crystals produced by CRYTUR meets the requirements for an electromagnetic calorimeter, and that they can be the basis for its creation.

The results of the studies on the production of ¹⁵⁵Tb isotope, which is used in theranostic applications for oncological diseases, utilizing the proton beam of the medical cyclotron C18/18 are presented. The measurements of the induced activity were carried out using the activation method, followed by spectrometric analysis. Natural gadolinium was used as the main target. Excitation functions were calculated for the ^natGd(p, xn)¹⁵⁵Tb reactions using TALYS 1.96 and EMPIRE 3.2 theoretical nuclear codes. The measured cross-section values were compared with the results of the theoretical calculations as well as with the experimental data from other works. The experimental value of the integral activity of the radionuclide ¹⁵⁵Tb was compared to the results obtained with the TALYS 1.96 code and with early published data.

The wettability of black silicon (b-Si) layers formed by reactive ion etching, metal-assisted chemical etching, and laser-induced etching has been studied. The wetting contact angles of the prepared samples with deionized water, glycerol, diiodomethane, and ethylene glycol were determined. It has been shown that the silicon oxide surface film and the enlargement area factor of b-Si layers have a significant influence on their wettability, varying from hydrophilic to hydrophobic properties.

Locally inclined targets are developed to produce medical radioisotopes which can be used for high-intensity proton beam irradiation. The results of a numerical analysis of thermal processes in locally inclined targets made of pressed molybdenum powder with a niobium target holder for the Nirta Solid Compact Model TS06 target module are presented. Computations were carried out using the Fluid Flow (Fluent) engineering package of the ANSYS Workbench 18.2 software platform for variants of targets with linear and axially symmetric grooves in the proton irradiation zone. It has been shown that using various models of locally inclined targets with a cooling area increased by 2.37 times, it is possible to increase the irradiation current by 1.34–1.46 times compared to a standard flat target. An analysis of the distribution of heat flows and temperatures in the targets was also carried out.