MONTE-CARLO STUDY OF DIFFUSION ON STEPPED SURFACES .1. STEP ATTRACTION OR REPULSION ONLY

Diffusion of adsorbates on a square lattice containing steps every fou rth row was investigated by Monte Carlo simulations. For the case wher e steps are attractive, i.e. step sites bind more tightly than terrace sites, there is very little diffusion anisotropy at low coverage, sin ce steps act as traps about equally for diffusion parallel and perpend icular to steps. As coverage increases steps become highly populated a nd then act as barriers for diffusion across them so that anisotropy i ncreases, particularly as temperature is lowered. Diffusion parallel t o steps then becomes temperature independent and diffusion perpendicul ar to steps remains activated. For diffusion coefficients obtained by the fluctuation autocorrelation method the inability to leave terraces at high theta and low T introduces artifacts for measurements perpend icular to steps. For repulsive steps, constituting diffusion barriers but not serving as adsorption sites anisotropy is pronounced at all te mperatures; diffusion parallel to steps is temperature independent, an d diffusion perpendicular to steps is activated at all coverages.