SURFACE-DIFFUSION AND CONTINUOUS-PHASE TRANSITIONS IN ADSORBED OVERLAYERS

Singularities in the coverage dependence of the chemical diffusion coe fficient near the critical points corresponding to transitions from on e phase to another are studied by employing two different lattice-gas models. (i) For the hard hexagon model, the chemical diffusion coeffic ient is shown to be exactly equal to zero at the critical coverage and its behavior near this point is in agreement with the predictions of the scaling theory provided that one takes into account the Fisher ren ormalization of the specific-heat exponent. (ii) For a square lattice with repulsive interactions between nearest-neighbor particles, the de tailed transfer-matrix and Monte Carlo data indicate that the chemical diffusion coefficient has a minimum at critical coverages and its val ue at these points decreases with increasing lattice size (there is ev en evidence that D-->0 at L-->infinity).