


卷 165, 编号 3 (2024)
Articles
EFFECT OF THE CdSe NANOPLATELETS CONCENTRATION IN A COLLOIDAL SOLUTION ON THE NONLINEAR CHANGE IN ABSORPTION
摘要
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.



THE EFFECT OF THE PHASE OF AN IONIZING ULTRASHORT LASER PULSE ON THE FORMATION OF QUANTUM VORTICES IN THE DENSITY DISTRIBUTION OF A PHOTOELECTRON
摘要
Quantum vortices formed by a photoelectron obtained as a result of over-barrier ionization of a two-dimensional hydrogen atom by an extremely short laser pulse are theoretically investigated. The sensitivity of quantum vortices to the initial phase of the ionizing field is analyzed. The interference effects responsible for the appearance of vortices are being clarified. For the model under consideration, the use of various gauges in describing the interaction of an electron with a field is discussed.



DIFFRACTION RADIATION AT THE OPEN END OF A CIRCULAR WAVEGUIDE WITH DIELECTRIC FILLING
摘要
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.



POLARIZATION CHARACTERISTICS OF ELECTRODYNAMIC STARK EFFECT SPECTRA
摘要
The detailed study of physical processes responsible for the hydrogen atom spectra under its motion transverse to the strong magnetic field – due to electrodynamic Stark effect, known also as MSE (Motional Stark Effect) – is performed. The formation mechanisms of excited hydrogen levels population due to collisions with protons of plasma are investigated. The experimental and theoretical data on the total and partial excitation cross sections along with parabolic quantum numbers in the laboratory
frame of moving atom are confronted. The universal approach for the calculations of cross sections in the basis of the parabolic wave functions with an account of their adiabatic suppression in the low energy range of collisions and selective in terms of the parabolic quantum numbers is proposed. The method developed is applied for the construction of the collisional-radiative kinetic model for the partial populations of the excited Stark sublevels calculations taking into account the ionization due to collisions with protons. The sources of the thermodynamically nonequilibrium origin of the Stark sublevel populations in the electrodynamic Stark effect are revealed in the wide diapason of the plasma density variation. The intensities of π- and σ Stark components of Hα line versus beam energy, magnetic field and plasma density are calculated. The polarization characteristics of MSE spectra of Hα line are calculated in the magnetically confined thermonuclear plasma. The obtained results are in the reasonable agreement with the literature data. The developed method is of interest as from the general physical point of view, as for the MSE-spectroscopy of tokamaks and in the other experimental conditions.



CHANGES IN TRANSITION METAL DICHALCOGENIDE FILMS PROPERTIES ON VARIOUS STAGES OF CHEMICAL VAPOR DEPOSITION
摘要
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.



CHARACTERISTICS OF DEFECTS AND ENTROPY OF MIXING IN HIGH-ENTROPY ALLOYS OF THE FeNiCrCoCu SYSTEM
摘要
Classical molecular dynamics simulation for a number of single crystals ofFeNiCrCoCu system showed that with increasing entropy of mixing the average formation enthalpy of interstitial defects and their shear susceptibility decreases monotonically. For interstitial defects in crystals and defect subsystems of glasses of the same composition, has been established that the average deviator components of dipole tensors decrease with increasing entropy of mixing, and the decrease occurs more strongly in the high-entropy region. All this may indicate the presence of a correlation between mixing entropy and properties of the defect subsystem of crystalline and glassy states.



PREDICTION OF LOW-TEMPERATURE SILVER SULFIDE PHASES, DERIVATIVE FROM ARGENTITE
摘要
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.



FERROMAGNETIC ORDER IN VAN-DER WAALS COMPOUND Fe3GeTe2
摘要
The phase transition from the paramagnetic to the ferromagnetic phase in a van der Waals volume Fe3GeTe2 compound was studied. A renormalization group approach was used, the action for which was constructed using group theoretical analysis to determine the irreducible representation of the spatial group responsible for this transition, in the case of magnetic moments localized on iron. It is shown that such a representation exists, which allows the orientation of magnetic moments along the c axis of the crystal. The influence of vacancies in one of the iron positions on this transition was considered using replica method by analogy with the description of frozen impurities. Power law of change magnetization was found near the transition taking into account the presence of vacancies. A condition has been determined when vacancies are pressure this transition. Possible influence of strong electron correlations and free electrons on the stability of the ferromagnetic phase was analyzed using the t–J model for non-degenerate electrons. In the generalized random phase approximation, the additional contribution of free electrons to the formation of long-range ferromagnetic order occurs through Pauli susceptibility gas of free electrons. The condition for the stability of the ferromagnetic state in this case was written out.



SPIN-FLOP TRANSITION, INDUCING THE MAGNITOSTRICTION AND DIELECTRIC ANOMALIES IN α-MnS SINGLE CRYSTAL
摘要
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.



STUDY OF PHASE TRANSITIONS AND THERMODYNAMIC PROPERTIES OF THE POTTS MODEL WITH FRUSTRATIONS ON THE KAGOME LATTICE
摘要
The Monte Carlo method was used to study phase transitions and thermodynamic properties of the two-dimensional antiferromagnetic Potts model with the number of spin states q = 4 on the kagome lattice with interactions of the first J1 and second J2 neighbors. The studies were carried out for the magnitude of the interaction of second neighbors in the interval . It was found that at r = 0 the system exhibits disorder and strong degeneracy of the ground state. It is shown that taking into account ferromagnetic interactions of second neighbors will remove the degeneracy of the ground state. An analysis of the nature of phase transitions in the considered interval r was carried out. It is shown that in the range 0.2 ≤ r ≤ 1 a second-order phase transition is observed.



THE ALLOYING EFFECT OF Si ON THERMODYNAMIC, MAGNETIC, AND ELASTIC PROPERTIES OF BCC Fe-Cr ALLOYS
摘要
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.



REGIMES OF ELECTRONIC TRANSPORT IN DOPED InAs NANOWIRE
摘要
We report on the low temperature measurements of the magnetotransport in Si-doped InAs quantum wire in the presence of a charged tip of an atomic force microscope serving as a mobile gate, i.e. scanning gate microscopy (SGM). By altering the carrier concentration with back gate voltage, we transfer the wire through several transport regimes: from residual Coulomb blockade to nonlinear resonance regime, followed by linear resonance regime and, finally, to almost homogeneous diffusion regime. We demonstrate direct relations between patterns measured with scanning gate microscopy and spectra of universal conductance fluctuations in the dependence of conductance on magnetic field (R-1(B)). Additionally, a clear sign of fractal behavior of R-1(B) curve is observed for non-linear and linear resonance transport regimes.



TWO STAGES IN THE FORMATION OF THE BRANCHING STRUCTURE OF A DECIDUOUS TREE
摘要
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.


