1,139 research outputs found
Vacancy-Vacancy Interaction in Copper
The binding energy of two vacancies in a static lattice as a function of their separation and the positions of their displaced neighboring atoms has been calculated using a Morse potential function model for copper. It was found that two vacancies attract one another at separation less than about 7 A. At separations greater than 7 A the vacancies do not interact appreciably. The most stable separation was found to be the first nearest- neighbor separation or the divacancy configuration, for which the binding energy was found to be 0.64 ev. Based on these calculations, it is shown that third-stage annealing in irradiated copper may be accounted for by divacancy migration. The role of the divacancy in copper self-diffusion is also explained
The relation between solar cell flight performance data and materials and manufacturing data Final report
Flight performance data for solar cell power systems in satellites and correlation with manufacturing methods and material
Van der Waals interactions of parallel and concentric nanotubes
For sparse materials like graphitic systems and carbon nanotubes the standard
density functional theory (DFT) faces significant problems because it cannot
accurately describe the van der Waals interactions that are essential to the
carbon-nanostructure materials behavior. While standard implementations of DFT
can describe the strong chemical binding within an isolated, single-walled
carbon nanotube, a new and extended DFT implementation is needed to describe
the binding between nanotubes. We here provide the first steps to such an
extension for parallel and concentric nanotubes through an electron-density
based description of the materials coupling to the electrodynamical field. We
thus find a consistent description of the (fully screened) van der Waals
interactions that bind the nanotubes across the low-electron-density voids
between the nanotubes, in bundles and as multiwalled tubes.Comment: 6 pages, 4 figures (5 figure files
Effect of Static Strains on Diffusion
A theory is developed that gives the diffusion coefficient in strained systems as an exponential function of the strain. This theory starts with the statistical theory of the atomic jump frequency as developed by Vineyard. The parameter determining the effect of strain on diffusion is related to the changes in the inter-atomic forces with strain. Comparison of the theory with published experimental results for the effect of pressure on diffusion shows that the experiments agree with the form of the theoretical equation in all cases within experimental error
Energy of Cohesion, Compressibility, and the Potential Energy Functions of the Graphite System
The lattice summations of the potential energy of importance in the graphite system have been computed by direct summation assuming a Lennard-Jones 6-12 potential between carbon atoms. From these summations, potential energy curves were constructed for interactions between a carbon atom and a graphite monolayer, between a carbon atom and a graphite surface, between a graphite monolayer and a semi-infinite graphite crystal and between two graphite semi-infinite crystals. Using these curves, the equilibrium distance between two isolated physically interacting carbon atoms was found to be 2.70 a, where a is the carbon-carbon distance in a graphite sheet. The distance between a surface plane and the rest of the crystal was found to be 1.7% greater than the interlayer spacing. Theoretical values of the energy of cohesion and the compressibility were calculated from the potential curve for the interaction between two semi-infinite crystals. They were (delta)E(sub c) = -330 ergs/sq cm and beta =3.18x10(exp -12)sq cm/dyne, respectively. These compared favorably with the experimental values of (delta)E(sub c) = -260 ergs/sq cm and beta = 2.97 X 10(exp -2) sq cm/dyne
Vacancy Relaxation in Cubic Crystals
The configuration of the atoms surrounding a vacancy in four face-centered cubic and three body-centered cubic metals has been computed, using a pairwise, central-force model in which the energy of interaction between two atoms was taken to have the form of a Morse function. Only radial relaxations were considered. The first and second nearest-neighbor relaxations for the face-centered systems were found to be: Pb (1.42,0.43), Ni (2.14,0.39), Cu(2.24,0.40) and Ca (2.73,0.41, expressed in percentages of normal distances. For the body-centered systems the relaxations out to the fourth nearest neighbors to the vacancy were: Fe (6.07,2.12, 0.25, ), Ba (7.85, 2.70, 0.70, 0.33) and Na (10.80, 3.14, 3.43, 0.20). The positive signs indicate relaxation toward the vacancy and the negative signs indicate relaxation away from the vacancy. The energies of relaxation (eV) are: Pb (0.162), Ni (0.626), Cu (0.560), Ca (0.400), Fe (1.410), Ba (0.950) and Na (0.172)
Dependence of band structures on stacking and field in layered graphene
Novel systems of layered graphene are attracting interest for theories and
applications. The stability, band structures of few-layer graphite films, and
their dependence on electric field applied along the c-axis are examined within
the density functional theory. We predict that those of Bernal type and also
rhombohedral type tri- and tetra-layer graphite films exhibit stability. The
rhombohedral-type systems including AB-bilayer, show variable band gap induced
by perpendicular electric field, whereas the other systems such as the
Bernal-type films stay semi-metallic.Comment: 5 pages, 5 figures, accepted for publication in Solid State
Communication
An analytic model of the Gruneisen parameter at all densities
We model the density dependence of the Gruneisen parameter as gamma(rho) =
1/2 + gamma_1/rho^{1/3} + gamma_2/rho^{q}, where gamma_1, gamma_2, and q>1 are
constants. This form is based on the assumption that gamma is an analytic
function of V^{1/3}, and was designed to accurately represent the
experimentally determined low-pressure behavior of gamma. The numerical values
of the constants are obtained for 20 elemental solids. Using the Lindemann
criterion with our model for gamma, we calculate the melting curves for Al, Ar,
Ni, Pd, and Pt and compare them to available experimental melt data. We also
determine the Z (atomic number) dependence of gamma_1. The high-compression
limit of the model is shown to follow from a generalization of the Slater,
Dugdale-MacDonald, and Vashchenko-Zubarev forms for the dependence of the
Gruneisen parameter.Comment: 14 Pages, LaTeX, 5 eps figues; changes in the tex
Infinite compressibility states in the Hierarchical Reference Theory of fluids. II. Numerical evidence
Continuing our investigation into the Hierarchical Reference Theory of fluids
for thermodynamic states of infinite isothermal compressibility kappa[T] we now
turn to the available numerical evidence to elucidate the character of the
partial differential equation: Of the three scenarios identified previously,
only the assumption of the equations turning stiff when building up the
divergence of kappa[T] allows for a satisfactory interpretation of the data. In
addition to the asymptotic regime where the arguments of part I
(cond-mat/0308467) directly apply, a similar mechanism is identified that gives
rise to transient stiffness at intermediate cutoff for low enough temperature.
Heuristic arguments point to a connection between the form of the Fourier
transform of the perturbational part of the interaction potential and the
cutoff where finite difference approximations of the differential equation
cease to be applicable, and they highlight the rather special standing of the
hard-core Yukawa potential as regards the severity of the computational
difficulties.Comment: J. Stat. Phys., in press. Minor changes to match published versio
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