Localization of vibrational states in crystalline structures containing distortions of the dynamic matrix

The problem of vibrations of a harmonic crystalline lattice containing distortions has been considered. Distortions are described by smooth functions which decay with distance from the nucleus of the distortion. Features of the spectrum of localized states have been determined and their number has been established.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 75–78, January, 1984.     

Numerical simulation of dynamic processes in gyrotrons with low-Q cavities

We study theoretically models of gyrotrons having fixed and self-consistent axial structures of the high-frequency field in the cavity with an actual profile. The limits of applicability of the model with a given field structure are determined during the study of competition of the operating and parasitic modes. It is shown that for the regimes which are optimal in terms of efficiency, the influence of the nonfixed nature of the high-frequency field structure on the starting current, the stability of stationary generation of the operating mode, the efficiency, the output power, and the thermal ohmic load of the cavity is insignificant. As the beam current and the mismatch between the cutoff frequencies of the operating and parasitic modes increase, significant qualitative differences between the models with given and self-consistent axial structures of the high-frequency field are observed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 4, pp. 307–320, April 2006.     

Crystal-binary collision simulation of atomic collisions and dynamic damage buildup in crystalline silicon

Abstract

Channeling profiles of 5–200 keVB⁺, 100–500 keV P⁺ and 40–300 keV As⁺ implanted near <100> and <110> axes in crystal silicon are calculated using the binary collision approximation code CRYSTAL. Calculated profiles are compared to carefully chosen experimental ones. Also, high dose phosphorus profiles with damage accumulation are compared to published experimental data.     

On the modeling of hydrogen diffusion processes and complex formation in p-type crystalline silicon

Deuterium diffusion profiles in p-type silicon doped with boron (10¹⁷–10¹⁹ cm^-3) and aluminum (10¹⁸ cm^-3) are simulated with an improved version of a previously reported model. The new approach, based on the observation of experimental profiles, excludes H₂ molecule formation and leads to a reduced fit parameters model. The different diffusion coefficients and activation energies of H⁰ and H⁺ species are determined and discussed in the light of available data. The dissociation energies of BH and AlH complexes are also calculated and found to be in good agreement with the corresponding reported values in the literature.     

The structure and thermodynamics of binary microclusters: A Monte Carlo simulation

The Monte Carlo computer simulation technique of classical statistical mechanics is employed to determine the structure and thermodynamics of binary microclusters of Lennard-Jones atoms as a function of cluster size, composition and temperature. Further, amorphous microclusters are prepared by a Monte Carlo quench, and their structural properties are examined. The properties of interest include the internal energy, instantaneous “snapshot” pictures of the microcluster's atomic configuration, and the single-particle and pair distribution functions. The Lennard-Jones potential parameters are chosen to model Ar₁₃, Ar₇Kr₆, Ar₃₆Kr₁₉ and Ar₁₉Kr₃₆, as well as to crudely model the bimetallic clusters of Cu₁₉Ni₃₆, Cu₁₉Ru₃₆ and Cu₁₉Os₃₆. A large variety of interesting features associated with these systems are described.     

Radiative processes in bulk crystalline silicon

For the luminescence from optical centres in bulk silicon to be of use in opto-electronic devices the silicon should emit at room temperature at a well-defined wavelength, with a minimum of heat generation, and with a high quantum efficiency. These criteria are not satisfied. The reasons are reviewed in terms of the intrinsic properties of typical centres and of effects which are extrinsic to the centres.     

CdTe aggregates in KBr crystalline matrix

In this work, we report the experimental results on the fabrication and optical characterization of Czochralski (Cz) grown KBr single crystals doped with CdTe crystallites. The results of the optical absorption have shown two bands, the first one located at 250 nm demonstrates the incorporation of cadmium atoms in the KBr host followed by a partial chemical decomposition of CdTe, the second band located at 585 nm shows an optical response of CdTe aggregates. Photoluminescence spectra at room temperature before annealing showed a band located at 520 nm (2.38 eV), with a blue shift from the bulk gap of 0.82 eV (E_g (CdTe)=1.56 eV). While the photoluminescence spectra after annealing at 600 °C showed a band situated at 640 nm (1.93 eV), these bands are due to band-to-band transitions of CdTe nanocrystals with a blue shift from the bulk gap at 0.38 eV. Blue shift in optical absorption and photoluminescence spectra confirm nanometric size of dopant. X-ray diffraction (XRD) spectra have shown the incorporation of CdTe aggregates in KBr.     

Three-dimensional simulation of primary break-up in a close-coupled atomizer

Until now much of the modelling activity around close-coupled gas atomization has been mainly focused on gas-only flow with discrete phase interaction using Lagrangian-based models. However, this approach is unable to supply valuable information regarding the primary break-up mechanism of the melt being injected. Furthermore, much of existing numerical work is based on two-dimensional axisymmetric geometries, and therefore suffers the absence of three-dimensional flow features. In order to overcome these aspects the authors have carried out an analysis using a three-dimensional geometry by means of an Eulerian, Volume of Fluid, model to accurately present the early stages of melt stream behaviour at the atomizer’s melt inlet. The study investigates the mechanisms associated with primary break-up, and the results obtained highlight three modes under which a close-coupled atomizer may operate.     

Three-dimensional non-stationary processes in a vertically falling film of a viscous liquid

Three-dimensional wave processes in vertically falling films of viscous liquid are considered. The 3D localized perturbations, which are studied insufficiently, are of a particular interest. The numerical method for watching evolution of initial perturbations was developed. The final stage of this evolution is formation of the 3D localized structures: solitons. The boundaries of 3D soliton stability were determined.     

Electron-hole recombination confinement in self-organized AgBr nanocrystals in a crystalline KBr matrix

The phenomenon of spatial confinement of the electron-hole recombination in exchange-coupled donor-acceptor pairs was observed by optically detected magnetic resonance in AgBr nanocrystals formed as a result of the self-organized growth in an ionic KBr crystal matrix. The effect is manifested by the maximum distance between recombining donors and acceptors being restricted to the nanocrystal size and by a change in the g value of shallow electron donor centers. Based on an analysis of the exchange interactions, the distribution of distances in the donor-acceptor pairs is determined and the dimensions of nanocrystals are estimated.     

Molecular simulation and theory of a liquid crystalline disclination core

Molecular simulations of a nematic liquid crystal confined in cylinder geometry with homeotropic anchoring have been carried out. The core structure of a disclination line defect of strength +1 has been examined, and comparison made with various theoretical treatments, which are presented in a unified way. It is found that excellent fits to the cylindrically symmetrized order tensor profiles may be obtained with appropriate parameter choices; notwithstanding this, on the time scales of the simulation, the cylindrical symmetry of the core is broken and two defects of strength +1 / 2 may be resolved.     

Three-dimensional simulation of a positive streamer in air near curved anode

We report the results of the first fully three-dimensional numerical simulation of a positive streamer in air in a 5 cm gap between a curved anode and a plane cathode. The formation of a negative radial field ahead of the streamer is found. This field leads to the formation of a current-free region in the streamer channel. The origin of the effect is discussed and the characteristic radius of this region is obtained.     

Field dependence of CIDNP in photoreactions in plastic crystalline matrix

Photo-CIDNP (chemically induced dynamic nuclear polarization) of three ketones (cycloheptanone, cyclodecanone, and acetone) in cyclohexane-d₁₂ and its dependence on the external magnetic field (0-7 T) is measured in liquid, plastic crystalline and rigid crystalline phases by temperature variation between 185 and 300 K. Spin polarization is seen only in liquid and plastic crystalline phases, but not in the rigid crystal giving evidence that molecular mobility is a requisite for the formation of CIDNP. From the characteristics of the field dependence the mechanisms of polarization are identified and properties of paramagnetic reaction intermediates determined. The factors causing the observed differences in polarization between the liquid and the solid phase are discussed in detail. *** DIRECT SUPPORT *** A04RK026 00003     

Manifestation of dynamic chaos in spin-spin relaxation processes in a solid

The results of several experiments on the dynamics of paramagnetic spin systems are analyzed on the basis of the theory developed here. It is shown that the asymptotic expressions for time correlation functions of various (high) orders are mutually similar and have a characteristic form [18], which probably indicates the “forgetting” of the initial conditions due to the development of Lyapunov instability in the dynamic system. The existence of exponential functions with close exponents, which describe the damping of correlations in the experimental observation of signals from free NMR precession and their buildup in experiments on multiquantum NMR, are manifestations of this instability because the Lyapunov exponents exist in pairs. The results suggest the development of “deterministic chaos” in the systems under investigation.