ANISOTROPY OF DIFFUSION ALONG STEPS ON THE (111)FACES OF GOLD AND SILVER

We present a molecular-dynamics simulation of adatom diffusion along t he two close-packed steps on the (111) surfaces of gold and silver. Bo th metals are modeled by employing many-body potentials derived within the second-moment approximation to the tight-binding model. The simul ation predicts very different behaviors for the two metals. For Au, th e diffusion is much faster along the step with (111) microfacets (step B), whereas for Ag the diffusion is faster along the step with (100) microfacets (step A). The difference between the diffusion coefficient s along the steps is more marked in gold and, for both metals, the Arr henius plots show a dynamical lowering of the activation barriers with respect to the static potential barriers; no evidence of an inversion of the anisotropy of diffusion is obtained. As the diffusion along st eps is quasi-one-dimensional, the results of the simulations have been compared to those based on the Fokker-Planck equation in a one-dimens ional periodic potential. The agreement between the model and the simu lations is remarkable for B steps both in gold and silver; the model p redicts the temperature dependence of the rate, the correct proportion of long jumps, and the details of the behavior of the mean-square dis placement. In A steps, the agreement is satisfactory for Ag and qualit ative for Au; in the latter case, the diffusion path is rather differe nt from a straight line and the application of a one-dimensional model may be questionable.