The diffusion of adsorbates on a square lattice which contained steps at every fourth row was investigated by means of Monte Carlo simulations. In the case where steps were attractive (that is, step sites bound more tightly than terrace sites), there was very little diffusional anisotropy at low coverages since the steps acted almost equally as traps for diffusion parallel and perpendicular to the steps. As the coverage increased, the steps became highly populated and then acted as barriers to diffusion across them. Therefore, the anisotropy increased; especially as the temperature was lowered. Diffusion parallel to the steps then became temperature-independent, and diffusion perpendicular to the steps remained activated. In the case of diffusion coefficients which were obtained by using the fluctuation auto-correlation method, the inability to leave terraces at high coverages and low temperatures introduced artefacts for measurements perpendicular to steps. In the case of repulsive steps, which represented diffusion barriers but did not serve as adsorption sites, the anisotropy was marked at all temperatures. Diffusion parallel to the steps was temperature-independent, and diffusion perpendicular to the steps was activated at all coverages.

C.Uebing, R.Gomer: Surface Science, 1994, 306[3], 419-26