Wednesday, March 13, 2013

1303.2674 (Scott A. Norris et al.)

Molecular Dynamics estimation of coefficients for irradiated binary
alloys: Testing the Bradley-Shipman theory

Scott A. Norris, Juha Samela, Matias Vestberg, Kai Nordlund, Michael J. Aziz
Coupled PDE models describing both morphological and chemical evolution, such as recently introduced by Bradley and Shipman, offer a promising potential explanation for the highly-ordered patterns sometimes observed during the irradiation of binary compounds with low-energy ions. However, as currently employed in several contexts, these models require specific conditions on a number of unknown parameters to explain this behavior. Specifically, they require that the effect on surface morphology of atoms sputtered away from the surface is larger than the effect of the atoms merely redistributed to new locations on the surface, a formerly-common assumption that has recently been overturned for several pure materials. Here, we develop a framework in which many of the parameters of the BS theory can be estimated using the results of molecular dynamics simulations. By adapting the recent theory of "crater functions" to the case of binary materials, we are able to establish relationships between the parameters in coupled PDE models and various moments of the statistically-averaged crater function associated with single ion impacts. Applying our framework to the commonly-studied GaSb system, we find that the requirement of erosive dominance is not fulfilled, motivating the consideration of other potential pattern-forming mechanisms.
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