Large-scale quantum mechanical simulations of high- Z metals

High-Z metals constitute a particular challenge for large-scale ab initio electronic-structure calculations, as they require high resolution due to the presence of strongly localized states and require many eigenstates to be computed due to the large number of electrons and need to accurately resolve the Fermi surface. Here, we report recent findings on high-Z metals, using an efficient massively parallel planewave implementation on some of the largest computational architectures currently available. We discuss the particular architectures employed and methodological advances required to harness them effectively. We present a pair-correlation function for U, calculated using quantum molecular dynamics, and discuss relaxations of Pu atoms in the vicinity of defects in aged and alloyed Pu. We find that the self-irradiation associated with aging has a negligible effect on the compressibility of Pu relative to other factors such as alloying. The US Goverment’s right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.     

Momentum in nonlinear optics

Abstract

Although it is well known that light carries momentum and exerts a pressure on objects, a conservation of momentum principle is apparently rarely used in optics. In nonlinear optics light waves interact and may exchange both energy and momentum. We demonstrate that a conservation of momentum principle holds in these cases and in fact its use is widespread but generally unrecognized in the standard mathematical methods. In both the cases of linear basis waves interacting nonlinearly, e.g. coupled-wave theory and frequency mixing, and fully nonlinear waves, we demonstrate that a governing Hamiltonian is related to momentum. Action principles are used to discuss the generality of these results.     

Variational approach to nonlinear waveguides-Gaussian approximations

Gaussian and equivalent step index approximations have been very useful for characterising linear waveguides. A variational approach can be used to derive similar, very accurate approximations for nonlinear propagation in both planar and circular waveguides.<>     

Bonding of Dental Glass to Nickel-Chromium Alloys

Two 80Ni · 20Cr-type alloys, one 99.99% pure and one with 98% purity, formed a multilayer oxide scale with an outer layer of NiO and a single-layer scale with outer portion richer in Cr₂O₃, respectively. An applied K-aluminosilicate glass at 1020°C penetrated the latter oxide scale. Adherence was present at all interfaces due to the existence of chemical equilibrium. Fracture occurred in the multilayer oxide scale and in the glass close to the oxide glass interface, respectively.     

Nonlinear polarisation behaviour on circular core bimodal optical fibres

Examines how the natural polarisation rotation phenomenon that is inherent on circular-core bimodal optical fibres is modified when nonlinear effects become important. For large power in the fundamental mode the proper second-order modes become stable linearly polarised modes.<>     

Ytterbium-doped silica fiber lasers: versatile sources for the1-1.2 μm region

Ytterbium-doped silica fibers exhibit very broad absorption and emission bands, from ~800 nm to ~1064 nm for absorption and ~970 nm to ~1200 nm for emission. The simplicity of the level structure provides freedom from unwanted processes such as excited state absorption, multiphonon nonradiative decay, and concentration quenching. These fiber lasers therefore offer a very efficient and convenient means of wavelength conversion from a wide variety of pump lasers, including AlGaAs and InGaAs diodes and Nd:YAG lasers. Efficient operation with narrow linewidth at any wavelength in the emission range can be conveniently achieved using fiber gratings. A wide range of application for these sources can be anticipated. In this paper, the capabilities of this versatile source are reviewed. Analytical procedures and numerical data are presented to enable design choices to be made for the wide range of operating conditions     

Formation of Mullite from Sol-Gel Mixtures and Kaolinite

Critical analyses of published reaction studies on kaolinite, and sol-gel and powder mixtures equivalent to 3Al₂O₃·2SiO₂ mullite, were made. Supplementary experiments on reactions of kaolinite at different rates of heating were analyzed. Complete atomic homogeneity of precursors results in formation of tetragonal mullite. Colloidal precursors initially form spinel which reacts with the remaining silica by a diffusion-nucleation mechanism to form orthorhombic mullite. Precursors with intermediate atomic homogeneity follow both reaction paths. Kaolinite with a layer lattice structure has two-dimensional atomic homogeneity. Both reaction routes are followed and their degree of interaction is dependent on the crystallinity and impurities of the kaolinite.     

Properties of modes on perturbed fibres

The terms in the Fourier series for the radius of a perturbed step-index optical fibre dictate which particular modes become truly LP and determine their optical axes; these axes need not coincide for different modes. Nonlinear optics experiments may be used to measure the perturbations.     

Sintering of α-Al2O3—Amorphous silica compacts

Sintering of α-Al₂O₃—fused SiO₂ compacts at temperatures of 1350–1500 °C is affected by metastable and stable phase reactions. Shrinkage maxima in the range 10 to 30 mole % Al₂O₃ are the result of cristobalite in the fused SiO₂ and its reaction with α-Al₂O₃ to form the eutectic of the metastable phase equilibrium diagram for SiO₂ (cristobalite)—α-Al₂O₃ without mullite. The retardation of densification of α-Al₂O₃ compacts with additions of SiO₂ up to about 50 mole % at temperatures about 1400°C and above is associated with the appearance of mullite.     

Slab waveguides characterized by moments of the index profile

The fundamental mode of a symmetric slab waveguide is described to a high degree of accuracy using only two moments of the refractive index profile rather than its exact form. The two moments can be used to define an equivalent step index slab waveguide, and the fundamental mode propagation constant of an arbitrary profile guide is given by the step index value multiplied by a simple correction factor. The limit to single-mode operation, the second-mode cutoff point, is also given by a simple formula requiring only profile moments. Numerical confirmation of the method is given.