264 research outputs found
Probing the Structure and Energetics of Dislocation Cores in SiGe Alloys through Monte Carlo Simulations
We present a methodology for the investigation of dislocation energetics in
segregated alloys based on Monte Carlo simulations which equilibrate the
topology and composition of the dislocation core and its surroundings. An
environment-dependent partitioning of the system total energy into atomic
contributions allows us to link the atomistic picture to continuum elasticity
theory. The method is applied to extract core energies and radii of 60 degrees
glide dislocations in segregated SiGe alloys which are inaccessible by other
methods.Comment: 5 pages, to be published in Physical Review Letter
Dinitrosyl formation as an intermediate stage of the reduction of NO in the presence of MoO_3
We present first-principles calculations in the framework of
density-functional theory and the pseudopotential approach, aiming to model the
intermediate stages of the reduction of NO in the presence of MoO(010). In
particular, we study the formation of dinitrosyl, which proves to be an
important intermediate stage in the catalytic reduction. We find that the
replacement of an oxygen of MoO by NO is energetically favorable, and that
the system lowers further its energy by the formation of (NO). Moreover,
the geometry and charge distribution for the adsorbed dinitrosyl indicates a
metal-oxide mediated coupling between the two nitrogen and the two oxygen
atoms. We discuss the mechanisms for the dinitrosyl formation and the role of
the oxide in the reaction.Comment: 6 pages, 4 figs, RevTeX. To be published in J. Chem. Phy
Softening of ultra-nanocrystalline diamond at low grain sizes
Ultra-nanocrystalline diamond is a polycrystalline material, having
crystalline diamond grains of sizes in the nanometer regime. We study the
structure and mechanical properties of this material as a function of the
average grain size, employing atomistic simulations. From the calculated
elastic constants and the estimated hardness, we observe softening of the
material as the size of its grains decreases. We attribute the observed
softening to the enhanced fraction of interfacial atoms as the average grain
size becomes smaller. We provide a fitting formula for the scaling of the
cohesive energy and bulk modulus with respect to the average grain size. We
find that they both scale as quadratic polynomials of the inverse grain size.
Our formulae yield correct values for bulk diamond in the limit of large grain
sizes.Comment: 5 pages, 3 figures, to be published in Acta Materiali
Insights into the fracture mechanisms and strength of amorphous and nanocomposite carbon
Tight-binding molecular dynamics simulations shed light into the fracture
mechanisms and the ideal strength of tetrahedral amorphous carbon and of
nanocomposite carbon containing diamond crystallites, two of the hardest
materials. It is found that fracture in the nanocomposites, under tensile or
shear load, occurs inter-grain and so their ideal strength is similar to the
pure amorphous phase. The onset of fracture takes place at weakly bonded sp^3
sites in the amorphous matrix. On the other hand, the nanodiamond inclusions
significantly enhance the elastic moduli, which approach those of diamond.Comment: 6 pages, 4 figure
A new generalized Kohn-Sham method for fundamental band-gaps in solids
We developed a method for calculating solid-state ground-state properties and
fundamental band-gaps using a generalized Kohn-Sham approach combining a local
density approximation (LDA) functional with a long-range explicit exchange
orbital functional. We found that when the range parameter is selected
according to the formula \gamma=A/(\eps_\inf - \eps_\tilde) where \eps_\inf is
the optical dielectric constant of the solid and \eps_\tilde= 0.84 and A=
0.216a0^(-1), predictions of the fundamental band-gap close to the experimental
values are obtained for a variety of solids of different types. For most solids
the range parameter \gamma is small (i.e. explicit exchange is needed only at
long distances) so the predicted values for lattice constants and bulk modulii
are similar to those based on conventional LDA calculations
Thermodynamics of C incorporation on Si(100) from ab initio calculations
We study the thermodynamics of C incorporation on Si(100), a system where
strain and chemical effects are both important. Our analysis is based on
first-principles atomistic calculations to obtain the important lowest energy
structures, and a classical effective Hamiltonian which is employed to
represent the long-range strain effects and incorporate the thermodynamic
aspects. We determine the equilibrium phase diagram in temperature and C
chemical potential, which allows us to predict the mesoscopic structure of the
system that should be observed under experimentally relevant conditions.Comment: 5 pages, 3 figure
Energetics and stability of nanostructured amorphous carbon
Monte Carlo simulations, supplemented by ab initio calculations, shed light
into the energetics and thermodynamic stability of nanostructured amorphous
carbon. The interaction of the embedded nanocrystals with the host amorphous
matrix is shown to determine in a large degree the stability and the relative
energy differences among carbon phases. Diamonds are stable structures in
matrices with sp^3 fraction over 60%. Schwarzites are stable in low-coordinated
networks. Other sp^2-bonded structures are metastable.Comment: 11 pages, 7 figure
Band structure and optical properties of germanium sheet polymers
The band structure of H-terminated Ge sheet polymers is calculated using density-functional theory in the local density approximation and compared to the optical properties of epitaxial polygermyne layers as determined from reflection, photoluminescence, and photoluminescence excitation measurements. A direct band gap of 1.7 eV is predicted and a near resonant excitation of the photoluminescence is observed experimentally close to this energy
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