Surface diffusion in mixed overlayers with superlattice ordering:percolative transport around obstacles and along domain bounaries

摘要

To elucidate surface diffu~iQn in the presen~e Qf a coadsorbate with superlattice ordering, w' consider p!!rticJe hopping o~ a squa,J;~ lattice with some fraction, (JB' of quenched blocking site arranged with checkerbo￥d pr c ( 2 X 2) ~rdering. Behavior for low (JB corresponds to diffusioJ around isolated ob~tacles, and can be described 15) exact'density expansions. Behavior for high (J] ,corr~sponds to percolative d~ffusion along (or ~ometimes away from) domain boundaries. Th, connectivity of these domain bouridarit1s is clos~ly related to the existence of symmetry breakini , , [i.e., long.rapge , c<.2X2) orde,r] in ,the distribution of blocking sites. In some cases, symmetr: breaking induces critical behavior for diffusive ~ransport which is fundamentally different from tha for the conventional "ant in the labyrinth" problem. Our results apply to recently developed model for CO oxidation, where CO(ads) diffuses rapidly through coadsorbed relatively immobile c(: X 2)-O(ads). The characterizationof~ diffusion in these systems is key to describing spatia pattern formation.