The ab-initio Schwoebel barrier calculated for downward self-diffusion across A-type steps on Pt(111) is E@super S@(A) ~ 0.02 eV. This is too small to be consistent with the notion that lowering E@super S@(A) is how O promotes layer-by-layer epitaxy on Pt(111). Geometric arguments explain why E@super S@(B), the Schwoebel barrier at B-type steps, is more than an order of magnitude larger than E@super S@(A), a result in conflict with the observation of three-dimensional Pt islands on Pt(111) bounded by A-type steps. The first-principles downward-transport barriers are vindicated by new observationsof Pt growth morphology, in which the CO background has been greatly reduced.
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@footnote 1@ Work supported by the U. S. Department of Energy under Contract DE-AC04-94AL85000. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed-Martin Company for the U. S. Department of Energy.