MODEL FOR DIFFUSION ON DEFORMABLE LATTICES .1. COLLECTIVE DIFFUSION

We present results of theoretical calculations of collective diffusion in a lattice-gas model of a deformable lattice, which has been proposed to explain the anomalous-diffusion anisotropy of adatoms on a deformable lattice, such as H on W(110). This model contains a competition between intercell and intracell diffusion jumps. The latter occur through a barrier created by a local distortion of the underlying substrate. The central parameter of the model is the branching ratio r, which is defined as the ratio of intracell-to-intercell diffusion rates. We perform extensive Monte Carlo random-walk simulations of diffusion as a function of coverage for several values of r, for a model including only intracell hard-core interactions. Using the Green's-function method, we obtain an analytic mean-field solution for the diffusion tensor. We also present the derivation of a higher-order solution in the Green's-function expansion. We study the validity of the analytic solutions by comparison with the simulations. Finally, we remark on the relevance of our results to diffusion experiments for H/W(110).