Sandia National Laboratories

About: Sandia National Laboratories is a facility organization based out in Livermore, California, United States. It is known for research contribution in the topics: Laser & Combustion. The organization has 21501 authors who have published 46724 publications receiving 1484388 citations. The organization is also known as: SNL & Sandia National Labs.

In situ atomic-scale observation of twinning-dominated deformation in nanoscale body-centred cubic tungsten

Abstract: Little is known about the micromechanisms by which deformation twinning occurs in body-centred cubic crystals. An atomic-scale microscopy study now provides new insight, by the in situ testing of tungsten nanowires.

Entropy Stable Spectral Collocation Schemes for the Navier--Stokes Equations: Discontinuous Interfaces

Abstract: Nonlinear entropy stability and a summation-by-parts framework are used to derive provably stable, polynomial-based spectral collocation element methods of arbitrary order for the compressible Navier--Stokes equations. The new methods are similar to strong form, nodal discontinuous Galerkin spectral elements but conserve entropy for the Euler equations and are entropy stable for the Navier--Stokes equations. Shock capturing follows immediately by combining them with a dissipative companion operator via a comparison approach. Smooth and discontinuous test cases are presented that demonstrate their efficacy.

Model of metallic cohesion: The embedded-atom method.

Abstract: The embedded-atom method (EAM) [Phys. Rev. B 29, 6443 (1984)] has proven to be a significant improvement in simplified total-energy calculations for metallic systems. In the current work, the ansatz used in the EAM is derived from the local-density functional for the energy. The expression demonstrated here is most appropriate for simple metals and for transition metals with nearly empty or nearly full d bands. An embedding energy is defined as a function of an optimal constant background density, and an equation for that optimal background density is obtained. The cohesive energy is then related to the embedding energy and an electrostatic two-body interaction. It is shown that lowest-order electronic relaxations can be absorbed into the same ansatz. Model calculations are presented for fcc nickel within the Thomas--Fermi--Dirac--von Weizs\"acker model for the kinetic energy, with local exchange and correlation and frozen-electron distributions. The model is shown to provide a good description of the ground-state properties of nickel (e.g., energetics and structure of vacancies and surfaces) and also a good framework for evaluating the approximations used in justifying the EAM form. In particular, the model exhibits a simple relationship between the optimal constant background density and the background density at the atomic site. Corrections involving the gradient of the background density are shown to be important in the calculation of the surface energy. This work then provides a basis for the use of the EAM in semiempirical applications.

Linearized theory of peridynamic states.

Abstract: A state-based peridynamic material model describes internal forces acting on a point in terms of the collective deformation of all the material within a neighborhood of the point. In this paper, the response of a state-based peridynamic material is investigated for a small deformation superposed on a large deformation. The appropriate notion of a small deformation restricts the relative displacement between points, but it does not involve the deformation gradient (which would be undefined on a crack). The material properties that govern the linearized material response are expressed in terms of a new quantity called the modulus state. This determines the force in each bond resulting from an incremental deformation of itself or of other bonds. Conditions are derived for a linearized material model to be elastic, objective, and to satisfy balance of angular momentum. If the material is elastic, then the modulus state is obtainable from the second Frechet derivative of the strain energy density function. The equation of equilibrium with a linearized material model is a linear Fredholm integral equation of the second kind. An analogue of Poincare’s theorem is proved that applies to the infinite dimensional space of all peridynamic vector states, providing a condition similar to irrotationality in vector calculus.

Gain-dependent polarization properties of vertical-cavity lasers

Abstract: We show that the partitioning of power into the two orthogonal eigen polarizations of infra-red gain-guided vertical cavity lasers depends upon the relative spectral overlap of the nondegenerate polarization cavity resonances with the laser gain spectrum. Furthermore, at the condition where the polarization resonances and the peak laser gain are aligned, abrupt switching of power between the eigen polarizations is observed as the gain sweeps through the polarization resonances. The gain-dependence of the polarization requires spectral splitting between the eigen polarizations, which is found to be strongly influenced by local strain. The polarization of the fundamental and higher-order spatial modes can be selected and maintained for all InGaAs vertical-cavity lasers in a wafer simply by employing a 20 nm or greater blue-shift offset of the peak laser gain relative to the cavity resonances. >