709 research outputs found
Island Size Selectivity and island-shape analysis during 2D Island Coarsening of Ag/Ag (111) Surface
In our earlier study of Ag island coarsening on Ag(111) surface using kinetic
Monte Carlo (KMC) simulations we found that during early stages coarsening
proceeds as a sequence of selected island sizes resulting in peaks and valleys
in the island-size distribution and that this selectivity is independent of
initial conditions and dictated instead by the relative energetics of edge-atom
diffusion and detachment/attachment processes and by the large activation
barrier for kink detachment. In this paper we present a detailed analysis of
the shapes of various island sizes observed during these KMC simulations and
show that selectivity is due to the formation of kinetically stable island
shapes which survive longer than non-selected sizes, which decay into nearby
selected sizes. The stable shapes have a closed-shell structure - one in which
every atom on the periphery having at least three nearest neighbors. Our KMC
simulations were carried out using a very large database of processes
identified by each atom's unique local environment, the activation barriers of
which were calculated using semi-empirical interaction potentials based on the
embedded-atom method.Comment: 17 pages, 11 figure
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Is the Post-AGB Star SAO 40039 Mildly Hydrogen-Deficient?
We have conducted an LTE abundance analysis for SAO 40039, a warm post-AGB star whose spectrum is known to show surprisingly strong He I lines for its effective temperature and has been suspected of being H-deficient and He-rich. High-resolution optical spectra are analyzed using a family of model atmospheres with different He/H ratios. Atmospheric parameters are estimated from the ionization equilibrium set by neutral and singly ionized species of Fe and Mg, the excitation of Fe I and Fe II lines, and the wings of the Paschen lines. On the assumption that the He I lines are of photospheric and not chromospheric origin, a He/H ratio of approximately unity is found by imposing the condition that the adopted He/H ratio of the model atmosphere must equal the ratio derived from the observed He I triplet lines at 5876, 4471, and 4713 angstrom, and singlet lines at 4922 and 5015 angstrom. Using the model with the best-fitting atmospheric parameters for this He/H ratio, SAO 40039 is confirmed to exhibit mild dust-gas depletion, I. e., the star has an atmosphere deficient in elements of high condensation temperature. The star appears to be moderately metal-deficient with [Fe/H] = -0.4 dex. But the star's intrinsic metallicity as estimated from Na, S, and Zn, elements of a low condensation temperature, is [Fe/H](o) similar or equal to -0.2 ([Fe/H](o) refers to the star's intrinsic metallicity). The star is enriched in N and perhaps O as well, changes reflecting the star's AGB past and the event that led to He enrichment.National Science FoundationRobert A. Welch Foundation of Houston, Texas F-634McDonald Observator
SLKMC-II study of self-diffusion of small Ni clusters on Ni (111) surface
We studied self-diffusion of small 2D Ni islands (consisting of up to 10
atoms) on Ni (111) surface using a self-learning kinetic Monte Carlo (SLKMC-II)
method with an improved pattern-recognition scheme that allows inclusion of
both fcc and hcp sites in the simulations. In an SLKMC simulation, a database
holds information about the local neighborhood of an atom and associated
processes that is accumulated on-the-fly as the simulation proceeds. In this
study, these diffusion processes were identified using the drag method, and
their activation barriers calculated using a semi-empirical interaction
potential based on the embedded-atom method. Although a variety of concerted,
multi-atom and single-atom processes were automatically revealed in our
simulations, we found that these small islands diffuse primarily via concerted
diffusion processes. We report diffusion coefficients for each island size at
various tepmratures, the effective energy barrier for islands of each size and
the processes most responsible for diffusion of islands of various sizes,
including concerted and multi-atom processes that are not accessible under
SLKMC-I or in short time-scale MD simulations
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