Vol 165, No 3 (2024)

Nonlinear absorption features of CdSe nanoplatelets colloidal solutions with a thickness of 2.5 and 3.5 monolayers were experimentally studied depending on the concentration in the case of resonant stationary excitation by nanosecond laser pulses. An increase in the amplitude of differential transmission and absorption saturation intensity at the wavelengths of excitonic transitions associated with heavy holes was detected for two series of samples with increasing concentration of nanoplatelets in the colloidal solution and explained by the process of excitons phase space filling. For colloidal solutions of high-concentration nanoplatelets, a region of negative differential transmission values was revealed at a sufficiently high pump intensity and explained by the transition from the absorption saturation regime to the optical amplification regime.

An analytical solution to the canonical problem of diffraction radiation of a uniformly moving point charge at the open end of a circular waveguide with a uniform dielectric filling is presented. The case of motion along the axis is considered. A modified mode-matching method was used in the solution, leading to the Wiener–Hopf–Fock equation, and after its formal solution, to an infinite linear system of equations for the excitation coefficients of waveguide modes. This system is solved numerically by the reduction method with any given accuracy. Numerical results are obtained for the case of charge exit from the waveguide.

Transition metal dichalcogenides (TMDs) are attracting continuously growing attention due to a number of their unique properties. Possibilities of their application are significantly defined by improvement of obtaining methods. In this work we study formation of TMD (MoS₂, WS₂) mesoporous films during chemical vapor deposition with the use of gaseous H₂S and thermally evaporated transition metals (Mo or W). Morphology, Raman spectra, photoluminescent properties and electrical conductivity of TMD films are investigated at different precursors concentrations and deposition duration times. The analysis revealed main stages of TMD films growth: isolated 2D monocrystalline islands formation (i), partial overlapping of these crystallites with their gradual growth in the plane of the substrate (ii), formation and growth of plate-like crystallites oriented perpendicular to the substrate surface (iv). Qualitative changes of morphology, electrical conductivity and PL properties of TMD films are explained with taking into account interaction of TMD electronic sub-system with the substrate and neighboring crystallites.

Such phases of silver sulfide as body-centered cubic argentite and monoclinic acanthite are widely known. Traditionally, acanthite is considered as the only low-temperature phase of silver sulfide. Low-temperature monoclinic acanthite can be considered as a result of the ordering of sulfur atoms in a non-metallic volume-centered cubic sublattice of argentite, accompanied by a redistribution of silver atoms. However, the possible existence of other low-temperature phases of silver sulfide cannot be excluded. The search for the model phases of the silver sulfide was performed using an evolutionary
algorithm. The possibility of the formation of Ag₂S phases with cubic, tetragonal, orthorhombic, trigonal, monoclinic and triclinic symmetries is considered. The calculation of the cohesion energy and enthalpy of formation showed that the formation of low-symmetry phases of Ag₂S is energetically most favorable. The elastic stiffness constants cij of all predicted phases of Ag₂S are calculated and their mechanical stability is determined. The electron state densities of the predicted Ag₂S phases are calculated. Channels of disorder-order transitions associated with the formation of low-temperature unrelaxed monoclinic acanthite ɑ-Ag₂S and cubic (space group Pn3m) silver sulfide Ag₂S from disordered argentite have been found. The spatial distributions of Young’s modulus and comprehensive compression of cubic (space group Pn3m) silver sulfide Ag₂S are determined and a weak anisotropy of its elastic properties is established.

The first experimental investigation of the magnetic and magnetistriction properties of the alfa-manganese monosulphide (α-MnS) with the cubic (NaCl-type) structure and antiferromagnetic transition at T=150 K are presented in the temperature range of 4.2–300 K at applied magnetic field up to 90 kOe. It is found, that the field dependences of the magnetization and longitudinal magnitostriction of the α-MnS single crystal have an anomalies, which are correlated with the anomalies of its dielectric permittivity. The spin-flop (SF) transition with Hsf ~ 50–70 kOe governed by the magnetic easy-plane anisotropy was observed in the temperature range below 130 K. Isotermal investigations show that the longitudinal magnetostriction and dielectric permittivity in external magnetic fields reveal its value change of 10^-3 at Hsf.

In the framework of the density functional theory, the alloying effect of Si on the magnetic and elastic properties, as well as the thermodynamic stability at T = 0 K of ferromagnetic Fe-Cr solid solutions in the BCC structure was studied. Calculations of lattice parameters, mixing enthalpy, elastic constants, bulk moduli, Young’s and shear moduli of disordered binary Fe-Cr and triple Fe-Cr-Si alloys containing 2.3 at. % and 4.7 at. % Si were performed using PAW-SQS and EMTO-CPA methods. Effective chemical interactions of the configuration Hamiltonian, magnetic characteristics and exchange interactions of the Heisenberg Hamiltonian are obtained. A comparative analysis of all obtained properties for ternary Fe-Cr-Si alloys with respect to binary Fe-Cr alloys is carried out. It was found that the addition of 2.3 at. % Si increases the thermodynamic stability of Fe-Cr alloys; this effect is enhanced with an increase in the silicon concentration to 4.7 at.%. The result is due to the Fe-Si and Cr-Si chemical interactions in addition to the magnetic Fe-Cr interactions that determine the stability of the diluted binary alloys. It is shown that with Si addition an increase in the elastic constant C₄₄ is observed, the values of the constants C₁₁, C₁₂ and elastic moduli are close to the corresponding values of binary FeCr alloys. Analysis of the concentration dependence of the ductility parameter G/B and charge density difference maps allowed to establish correlations between the changes in interatomic bonding and the properties of the alloys.

Fractal properties in the formation of the branching structure of deciduous trees have been studied by numerical Fourier analysis. It is shown that the lower levels of branching of adult trees are formed obeying the law of the logarithmic fractal in two-dimensional space, according to which the surface area of the lower branch is equal to the sum of the surface areas of the branches after its branching, i.e. the law of conservation of area when scaling is fulfilled. The structure of branches at the upper levels of branching obeys the law of the logarithmic fractal in three-dimensional space, i.e. the law of volume conservation during scaling, which is natural, since living tissue occupies completely an young branch, and not only its surface. A mathematical model is proposed that generalizes the concepts of a logarithmic fractal on the surface for adult branches and a logarithmic fractal in volume for young branches. Thus, an integral fractal concept of the growth and branching structure of deciduous trees is constructed.