On the $\textrm{Si(111)}5\times2\textrm{-Au}$ surface, Si adatom diffusion is defect-mediated

The $\mathrm{Si(111)}5\times2\textrm{-Au}$ surface is a member of a family of metal-induced chain reconstructions of Si. Studies of these reconstructions have led to new understanding of the physics of one-dimensional electronic states. The $5\times2\textrm{-Au}$ surface is speckled with Si adatoms, which are intimately linked with the surface electronic properties. At temperatures $>423\textrm{ K}$, the adatoms diffuse along the chains between adjacent $5\times2$ cells. We have measured scanning tunneling microscopy movies of the diffusing adatoms. Distinctive diffusion statistics, e. g. correlations between displacements, imply that the displacements are triggered by an interaction with a defect. By a statistical characterization of the diffusion, we show that the adatoms move by a defect-mediated mechanism similar to the vacancy-mediated diffusion observed on some metal surfaces. We use a Monte Carlo simulation to model the diffusion process, accurately reproducing the unique diffusion statistics over the temperature range ($145-215^o$C) of our experiments. We have also determined the diffusion activation barrier=$1.24\pm0.08$ eV. Sandia National Labs is operated by Sandia Corp, a Lockheed-Martin Company, for the DOE under Contract No. DE-AC04-94AL85000. FJH and SB acknowledge NSF support under DMR-0705145.