Spreading of step-like density profiles in interacting lattice gas on a hexagonal lattice

Diffusion in a single-layer adsorbate on a crystalline surface with a hexagonal lattice site symmetry is studied by Monte Carlo simulations. A step-like density profile of the adsorbate is allowed to spread. From the evolution of this density profile the diffusion coefficient is obtained by use of standard Boltzmann-Matano analysis. Results are compared with those obtained from the harmonic profile evolution studies undertaken by us previously. Agreement between both approaches is found at high and low temperatures. We show results for systems with two different activated state energies. For the first model the diffusion coefficient increases smoothly with the density, and for the second one the diffusion coefficient reaches a maximum for a half-occupied lattice. For this density the step profile forms a long flat terrace. No increase of the diffusion constant for densities where perfect ordering happens was observed. (C) 2000 Published by Elsevier Science B.V. All rights reserved.