The influence of steps and non-equilibrium conditions on surface diffusion in a strongly interacting surface adsorbate system was considered. This problem was addressed through Monte Carlo simulations of a lattice-gas model of O/W(110), where steps were described by an additional binding energy EB at the lower step edge positions. Both equilibrium fluctuation and Boltzmann-Matano spreading studies indicated that the role of steps for diffusion across the steps was prominent in the ordered phases at intermediate coverages. The strongest effects were found in the p(2 x 1) phase, whose periodicity Lp was 2. The collective diffusion then depended on two competing factors: domain growth within the ordered phase, which on a flat surface had two degenerate orientations [p(2 x 1) and p(l x 2)], and the step-induced ordering due to the enhanced binding at the lower step edge position. The latter case favored the p(2 x 1) phase, in which all adsorption sites right below the step edge were occupied. When these two factors competed, two possible scenarios emerged. Firstly, when the terrace width L did not match the periodicity of the ordered adatom layer (L/Lp was non-integer), the mismatch gave rise to frustration, which eliminated the effect of steps provided that EB was not exceptionally large. Under these circumstances, the collective diffusion coefficient behaved largely as on a flat surface. Secondly, if the terrace width did match the periodicity of the ordered adatom layer (L/Lp was an integer), collective diffusion was strongly affected by steps. In this case, the influence of steps was manifested as a disappearance of the major peak associated with the ordered p(2 x l) and p(1 x 2) structures on a flat surface. This effect was particularly strong for narrow terraces, but persisted up to about L ≈ 25Lp for small EB and up to about L ≈ 500Lp for EB, which was of the same magnitude as the bare potential of the surface. On real surfaces, similar competition was expected, although the effects were likely to be smaller due to fluctuations in terrace widths. Finally, Boltzmann-Matano spreading simulations included that even slight deviations from equilibrium conditions could give rise to transient peaks in the collective diffusion coefficient. These transient structures were due to the interplay between steps and non-equilibrium conditions and emerged at coverages which did not correspond to the ideal ordered phases.

Interplay between Steps and Nonequilibrium Effects in Surface Diffusion for a Lattice-Gas Model of O/W(110). M.Mašín, I.Vattulainen, T.Ala-Nissila, Z.Chvoj: Journal of Chemical Physics, 2007, 126[11], 114705