Diffusion studies in nonequilibrium systems with attractive interactions

Diffraction experiments can be used easily to measure the time evolution of a system under nonequilibrium conditions to attain a new equilibrium state and deduce "nonequilibrium'' surface diffusion coefficients. It is not clear how the "nonequilibrium" diffusion coefficients extracted from such diffraction experiments should be interpreted. We study with Monte Carlo simulations the behavior of the "nonequilibrium" tracer and collective diffusion coefficients in a lattice-gas model with attractive nearest-neighbor interactions, as the system evolves in time from an initial random state to attain the (1x1) ordered state for temperatures T/T-c <1. We calculate the dependence of the mean-square displacement [R-2] on time and the collective diffusion from the relaxation of the nonequilibrium structure factor S((q) over right arrow,t) within time sub-intervals under the assumption that quasiequilibrium holds. We determine the time-dependent "nonequilibrium" diffusion coefficients and extract the time-dependent activation energies. For both diffusion coefficients the "nonequilibrium" values obtained at late times are compared to the corresponding values obtained at equilibrium.