Competition between ordering and heterogeneity effects on surface diffusion

Tracer and collective diffusion of repulsively interacting particles adsorbed on a bivariate trap surface are studied by means of Monte Carlo modelling. Adsorption isotherms are obtained via simulations in the grand-canonical ensemble. Simulations in the canonical ensemble are used to mimick surface diffusion and to calculate the tracer, jump and chemical diffusion coefficients. The latter is determined utilizing the fluctuation and Kubo–Green methods. Both methods are in good agreement at high temperatures, but produce significantly different results at low temperatures where the repulsive nearest-neighbor interaction favors c(2×2) ordering of the adatoms. These discrepancies are discussed and explained. The effects of both the heterogeneities and the ad–ad interactions on surface diffusion are prominent at low temperatures. The existence of heterogeneities disturbs c(2×2) ordering at θ=0.5. It is shown that this breakdown of the order substantially affects surface diffusion.