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Vol 106, No 1 (2017)
- Year: 2017
- Articles: 12
- URL: https://journals.rcsi.science/0021-3640/issue/view/9736
Fields, Particles, and Nuclei
Low-energy positron scattering upon endohedrals
Abstract
We investigate positron scattering upon endohedrals and compare it with electron-endohedral scattering. We show that the polarization of the fullerene shell considerably alters the polarization potential of an atom, stuffed inside a fullerene. This essentially affects both the positron and electron elastic scattering phases as well as corresponding cross sections. Of great importance is also the interaction between the incoming positron and the target electrons that leads to formation of the virtual positronium P̃s. We illustrate the general trend by concrete examples of positron and electron scattering upon endohedrals He@C60 and Ar@C60, and compare it to scattering upon fullerene C60. To obtain the presented results, we have employed new simplified approaches that permit to incorporate the effect of fullerenes polarizability into the He@C60 and Ar@C60 polarization potential and to take into account the virtual positronium formation. Using these approaches, we obtained numeric results that show strong variations in shape and magnitudes of scattering phases and cross sections due to effect of endohedral polarization and P̃s formation.
1-6
Charge-exchange pigmy resonances of tin isotopes
Abstract
Charge-exchange states, the so-called “pigmy” resonances, which are below the giant Gamow–Teller resonance, have been studied in the self-consistent theory of finite Fermi systems. Microscopic numerical calculations and semiclassical calculations are presented for nine tin isotopes with the mass numbers A =112, 114, 116, 117, 118, 119, 120, 122, and 124, for which experimental data exist. These data have been obtained in the Sn(3He,t)Sb charge-exchange reaction at the energy E(3He) = 200 MeV. The comparison of calculations with experimental data on the energies of charge-exchange resonances gives the standard deviation δE < 0.40 MeV for microscopic numerical calculations and δE < 0.55 MeV for calculations by semiclassical formulas, which are comparable with experimental errors. The strength function for the 118Sn isotope has been calculated. It has been shown that the calculated resonance energies are close to the experimental values; the calculated and experimental relations between heights of pygmy resonance peaks are also close to each other.
7-11
Condensed Matter
Band gap tuning of Ge/SiC bilayers under an electric field: a density functional study
Abstract
The structure and electronic properties of Ge/SiC van der Waals (vdW) bilayer under the influence of an electric field have been investigated by the first-principles method. Without an electric field, the system shows a small band gap of 126 meV at the equilibrium state. Interestingly, by applying a vertical external electric field, the results present a parabola-like relationship between the band gap and the strength. As the negative E-field changes from 0.0 to −0.40 V/Å, the band gap first increases to a maximum of about 378 meV and then decreases to zero. A similar trend is exhibited for the positive E-field, ranging from 0.0 to +0.40 V/Å. The band gap reaches a maximum of about 315 meV at +0.10 V/Å. The significant variations of band gap are owing to different states of Ge, Si, and C atoms in conduction band and valence band. The predicted electric field tunable band gap of the Ge/SiC vdW heterostructures is very promising for its potential use in nanodevices.
46-50
Observation of the Talbot effect for ultrasonic waves
Abstract
The diffraction of ultrasonic radiation on an amplitude diffraction grating in the near-field area (Fresnel diffraction) has been studied. The effect of self-imaging of the grating (Talbot effect) has been detected for ultrasonic radiation at distances from the grating in the range from z = 0 to z = 2LT, where LT is the Talbot length. The fractional Talbot effect, i.e., the ultrasonic image of the grating with the period d/2, has been observed.
23-25
Optical detection of magnetoplasma resonances in indirect-gap AlAs/AlGaAs quantum wells
Abstract
Magnetoplasma excitations in AlAs/AlGaAs quantum wells, characterized by strong anisotropy in the effective mass of two-dimensional electrons, are investigated using the optical detection of resonance microwave absorption. This technique is used for the first time for an indirect-gap semiconductor. It is found that the magnetic dispersion of the cyclotron magnetoplasma mode deviates significantly from the theoretically expected behavior. This may be related to the considerably more pronounced manifestation of retardation effects in two-dimensional systems with an anisotropic energy spectrum.
26-29
Phase diagram of the ground state of a classical anisotropic frustrated ferromagnet
Abstract
The phase diagram of the ground state is obtained for the one-dimensional easy-axis model of classical spins coupled by ferromagnetic and antiferromagnetic exchanges between nearest and next-nearest neighbors, respectively. The parameters of the incommensurate magnetic structure with a variable step (soliton lattice) are calculated in the mean field approximation from the condition of the collinearity of spins to the effective exchange fields in the continuous approximation. The ground state of the soliton lattice and interfaces between soliton and collinear (ferromagnetic and “up–up–down–down”) phases are determined by the numerical minimization of the average energy over the initial angular velocity of spins.
30-34
Premelting generation of interstitial defects in polycrystalline indium
Abstract
High-precision measurements of the shear modulus of polycrystalline indium from room temperature to the melting temperature Tm and molecular statics computer simulation have been performed. It has been found that interstitial defects are intensely generated near Tm in a dumbbell (split) configuration and their concentration can exceed the concentration of vacancies.
35-39
Intensification of acoustic beams in crystals at mode conversion near total internal reflection
Abstract
Geometrical conditions under which mode conversion occurs in a crystal near total internal reflection of an acoustic beam are found. In this case, the entire energy of an incident pump wave is spent on the excitation of a narrow intense reflected beam close in structure to an eigenmode. A consistent choice of orientations of the sagittal plane and surface that excludes the reflection of a parasitic wave of leakage is found.
40-45
Scientific Summaries
Superconducting spin-valve and triplet superconductivity
Abstract
Recent experimental results on the superconducting spin-valve effect and generation of the long-range triplet superconductivity in a F1/F2/S structure are reviewed (here, F1 and F2 are uncoupled ferromagnetic layers, and S is the superconducting layer). The main results are the following: (i) the maximum of the magnitude of the superconducting spin-valve effect increases with decreasing the exchange field h in the ferromagnetic layer; (ii) a full switching between the normal and superconducting states may be realized with the aid of the triplet contribution to the spin-valve effect.
57-67
Optics and Laser Physics
Atomic condensate in an optical trap formed by a cavity mode
Abstract
It has been shown that an optical trap for an atomic condensate formed by a mode of a ring cavity possesses specific properties. These properties are not featured by a trap formed by free beams and arise owing to quantum correlations (entanglement) between the localized atoms and the optical mode. In particular, there is an effect similar to an optomechanical phenomenon known as an “optical spring” (S. Martellucci et al., Bose–Einstein Condensates and Atom Lasers (Kluwer, Dordrecht, 2002)) and manifested by the emergence of an effective correction to the interaction between the localized atoms. The magnitude and sign of this correction can be controlled by varying the frequency of the source forming the trap.
12-17
Nature of luminescence of PbS quantum dots synthesized in a Langmuir–Blodgett matrix
Abstract
The luminescence and structural properties of PbS quantum dots in a Langmuir–Blodgett matrix and after the removal of this matrix have been studied. It has been found that the photoluminescence peak of quantum dots in the matrix is near 1.6 eV, and luminescence disappears after the removal of the matrix. Beyond the matrix, quantum dots begin to luminesce at an energy of 0.8 eV only after aging in air lasting many days, which can be attributed to the passivation of surface defects because of oxidation. The size distribution of quantum dots in the initial Langmuir–Blodgett matrix and after its removal has been determined by transmission microscopy. Using the size distribution of quantum dots, the photoluminescence spectra have been calculated for quantum dots with different shells—Langmuir–Blodgett matrix/alkanes or oxide. It has been established that the shift of the photoluminescence spectrum of quantum dots toward lower energies is due to a decrease in the height of barriers and to an increase in the sizes of quantum dots.
18-22
Miscellaneous
Multiferroic based on nanoparticles consisting of a silica nucleus and a shell of spin-variable iron complexes
Abstract
A new class of high-temperature multiferroics has been created. They are polymer composites based on a polystyrene matrix and nucleus–shell nanoparticles, where the nucleus consists of silica nanoparticles and the shell consists of Fe(III) spin-variable ions.
51-56