Nonequilibrium effects in diffusion of interacting particles on vicinal surfaces.

We study the influence of nonequilibrium conditions on the collective diffusion of interacting particles on vicinal surfaces. To this end, we perform Monte Carlo simulations of a lattice-gas model of an ideal stepped surface, where adatoms have nearest-neighbor attractive or repulsive interactions. Applying the Boltzmann-Matano method to spreading density profiles of the adatoms allows the definition of an effective, time-dependent collective diffusion coefficient D(C) (t)(theta) for all coverages theta. In the case of diffusion across the steps and strong binding at lower step edges we observe three stages in the behavior of the corresponding D(xx,C) (t)(theta). At early times when the adatoms have not yet crossed the steps, D(xx,C) (t)(theta) is influenced by the presence of steps only weakly. At intermediate times, where the adatoms have crossed several steps, there are sharp peaks at coverages theta<1L and theta>1-1L, where L is the terrace width. These peaks are due to different rates of relaxation of the density at successive terraces. At late stages of spreading, these peaks vanish and D(xx,C) (t)(theta) crosses over to its equilibrium value, where for strong step edge binding there is a maximum at theta=1L. In the case of diffusion in direction along the steps the nonequilibrium effects in D(yy,C) (t)(theta) are much weaker, and are apparent only when diffusion along ledges is strongly suppressed or enhanced.