Surface diffusion in mixed overlayers with superlattice ordering: Percolative transport around obstacles and along domain boundaries

To elucidate surface diffusion in the presence of a coadsorbate with superl attice ordering, we consider particle hopping on a square lattice with some fraction, theta (B), of quenched blocking sites arranged with checkerboard or c(2x2) ordering. Behavior for low theta (B) corresponds to diffusion ar ound isolated obstacles, and can be described by exact density expansions. Behavior for high theta (B) corresponds to percolative diffusion along (or sometimes away from) domain boundaries. The connectivity of these domain bo undaries is closely related to the existence of symmetry breaking [i.e., lo ng-range c(2x2) order] in the distribution of blocking sites. In some cases , symmetry breaking induces critical behavior for diffusive transport which is fundamentally different from that for the conventional "ant in the laby rinth" problem. Our results apply to recently developed models for CO oxida tion, where CO(ads) diffuses rapidly through coadsorbed relatively immobile c(2x2)-O(ads). The characterization of CO diffusion in these systems is ke y to describing spatial pattern formation. (C) 2000 American Institute of P hysics. [S0021-9606(00)70146-2].