[en] As modern, technologically important materials have become more complex, element specific techniques have become invaluable in studying the electronic structure of individual components from the system. Soft x-ray fluorescence (SXF) and absorption (SXA) spectroscopies provide a unique means of measuring element and angular momentum density of electron states, respectively, for the valence and conducting bands in complex materials. X-ray absorption and the decay through x-ray emission are generally assumed to be two independent one-photon processes. Recent studies, however have demonstrated that SXF excited near the absorption threshold generate an array of spectral features that depend on nature of materials, particularly on the localization of excited states in s and d-band solids and that these two processes can no be longer treated as independent. Resonant SXF offers thus the new way to study the dynamics of the distribution of electronic valence states in the presence of a hole which is bound to the electron low lying in the conduction band. This process can simulate the interaction between hole-electron pair in wide gap semiconductors. Therefore such studies can help in understanding of transport and optics phenomena in the wide gap semiconductors. The authors report the result of Mn and S L-resonant emission in Zn_1-xMn_xS (with x=0.2 and 0.3) and MnS as the energy of exciting radiation is tuned across the Mn and S L_3,2 absorption edge, along with the resonant excited spectra from elemental Mn as a reference

[en] Multicrystalline silicon is a very interesting material for terrestrial solar cells. Its low cost and respectable energy conversion efficiency (12-15%) makes it arguably the most cost competitive material for large-volume solar power generation. However, the solar cell efficiency of this material is severely degraded by regions of high minority carrier recombination which have been shown to possess both dislocations and microdefects. These structural defects are known to increase in recombination activity with transition metal decoration. Therefore, gettering of metal impurities from the material would be expected to greatly enhance solar cell performance. Contrary to this rationale, experiments using frontside phosphorus and/or backside aluminum treatments have been found to improve regions with low recombination activity while having little or no effect on the high recombination regions and in turn only slightly improving the overall cell performance. The goal of this research is to determine the mechanism by which gettering is ineffectual on these high recombination regions. The authors have performed studies on integrated circuit (IC) quality single crystal and multicrystalline solar cell silicon (mc-silicon) in the as-grown state and after a variety of processing/gettering steps. With Surface Photovoltage measurements of the minority carrier diffusion length which is inversely proportional to carrier recombination, they have seen that aluminum gettering is effective for improving IC quality material but ineffective for improving the regions of initially low diffusion lengths (high recombination rates) in mc-silicon. Of particular interest is the great increase in diffusion length for IC material as compared to the mc-silicon. Clearly the IC material has benefited to a greater extent from the gettering procedure than the mc-silicon

[en] Magnetization processes in soft ferromagnetic materials consist of complex domain wall movements and domain structure transitions. A micromagnetic model is developed to numerically simulate such dynamic behaviors in order to achieve fundamental understanding of the switching mechanisms involved in magnetic recording sensors, such as thin film inductive heads and magnetoresistive heads. Various energy minimization schemes have been utilized to study the quasi-static equilibrium properties of these micro-structures, whereas the phenomenological Landau-Lifshitz equation is solved to follow the dynamic magnetization response to external driving fields. It is observed that the intrinsic gyromagnetic damping effect introduces dissipation loss in domain wall motion, and induces dynamic transitions of all structures at high amplitude and frequency driving fields. The dynamics of the flux-closure asymmetric vortex wall in permalloy thin films with thicknesses ranging from 500 angstrom to several microns has been studied in detail. The effectiveness of eddy current damping on thick film wall motions is discussed within the micromagnetic context. The magnetic reversals of very thin permalloy platelets have also been investigated. The interactions of magnetostatic and exchange effects introduce formations of transient vortices and multi-domain states for these small (micron-scale) devices, which in turn act as the sources of hysteretic and noisy response to external fields

[en] We investigate the validity of the Fourier transformations that have long been used to transform a given Hamiltonian written in configuration space into momentum space. We show that although these transformations are valid for ideal (perfect) lattices, their use for lattices with vacancies poses mathematical problems and renders inaccuracies in the form of the Hamiltonian by not conserving the number of degrees of freedom in the Hilbert space. Consequently, transformations that avoid this problem are found. We also investigate another solution to this problem that was proposed in the literature. We study the difference between these three methods by investigating the ground-state energy of an electron system in the context of the s-band tight-binding model due to removing (a) one site and (b) two sites. The three methods give similar expressions for the ground-state energy for both of these cases, but they predict different binding energies of electrons at the vacancies

[en] Recent experiments to measure electrical conductivities and temperatures of liquid hydrogen shock-compressed up to Mbar pressures are described and contrasted with results for liquid nitrogen. Some thoughts about future possibilities for high-pressure shock experiments are discussed. copyright 1994 American Institute of Physics

[en] Incoherent exciton dynamics in one-dimensional perfect lattices with traps at sites arranged according to aperiodic deterministic sequences is studied. We focus our attention on Thue-Morse and Fibonacci systems as canonical examples of self-similar aperiodic systems. Solving numerically the corresponding master equation we evaluate the survival probability and the mean-square displacement of an exciton initially created at a single site. Results are compared to systems of the same size with the same concentration of traps randomly as well as periodically distributed over the whole lattice. Excitons progressively extend over the lattice on increasing time and, in this sense, they act as a probe of the particular arrangements of traps in each system considered. The analysis of the characteristic features of their time decay indicates that exciton dynamics in self-similar aperiodic arrangements of traps is quite close to that observed in periodic ones, but differs significantly from that corresponding to random lattices. We also report on characteristic features of exciton motion suggesting that Fibonacci and Thue-Morse orderings might be clearly observed by appropriate experimental measurements. In the conclusions we comment on the implications of our work on the way towards a unified theory of the ordering of matter

[en] The chemical interactions of ultra-thin aluminum films with clean and chemically modified stoichiometric TiO₂ (110) surfaces have been studied by several electron spectroscopies (ultraviolet photoelectron, x-ray photoelectron, and Auger electron) and low-energy electron diffraction. The chemical properties of the Al overlayers on clean TiO₂ surfaces were compared to those on surfaces pre-dosed with submonolayer amounts of carbon or potassium. The thermal stability of each system was also examined. On the clean surface, the first monolayer of aluminum completely wets the surface and becomes oxidized, while the Ti 4⁺ cations in the near surface region are reduced. Submonolayer amounts of potassium interact with the stoichiometric TiO₂ surface in a similar manner and form a potassium oxide overlayer. Aluminum deposited onto a potassium-dosed TiO₂ (110) surface competes with K for oxygen and reduces more Ti 4⁺ cations in the interface region. Carbon species adsorbed on the TiO₂ (110) surface by electron beam decomposition of ethylene prior to the aluminum deposition interact only weakly with the oxide surface, but retard the oxidation rate of aluminum and decrease the temperature stability of the aluminum oxide overlayer

[en] Holographic grating are used in many important scientific experiments as well as an industrial applications. We give the profiles and main characteristics of grating used in space experiments (Lyman FUSE and Meris), on synchrotron, and in a standard biochemical spectrophotometer. (authors). 12 refs., 8 figs

[en] In thie field of solid mechanics, certain problems give rise to nonlinear algebraic, transcendental, differential, or integral equations containing a parameter. This book discusses using the continuation method to solve m algebraic or transcendental equations, with m unknowns and also an explicit parameter

[en] Dipole radiation in a two-dimensional periodic dielectric structure is studied both theoretically and experimentally in this article. For the two-dimensional square lattice structure with dielectric cylinders, the emission power of an electric dipole in the photonic band structure is computed numerically by a combination of the plane wave method, the dyadic Green close-quote s function, the Poynting theorem, and the triangular integration method. The theoretical results are compared with experimental measurements in a microwave frequency region. The computed radiation spectra in the photonic band structure show good agreement with the experimentally measured results. copyright 1996 American Institute of Physics