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Surface Diffusion of Pb on (100) Cu: Coverage Dependence and Influence of Ordered-Phase Formation

The spread of two-dimensional submonolayer Pb deposits on (100) Cu has been measured as a function of coverage θ and temperature T by Rutherford backscattering in ultra-high vacuum, providing the chemical-diffusion coefficient D(θ, T). An overall increase of D with θ is observed (more than 3 orders of magnitude) with a lowering in the θ and T domain where ordered Pb structures exist. These effects are qualitatively predicted by Monte Carlo simulations in the framework of a simple square lattice gas model with nearest-neighbour repulsive interactions.