Quantum Monte Carlo study of surface diffusion

Abstract

We have computed the diffusion constant for hydrogen and deuterium on Ni(001) using the quantum version of transition-state theory formulated in terms of the transition rate for the path centroid. The embedded-atom method was used for the potential. We find a transition beween thermally activated diffusion at high temperatures and almost temperature-independent diffusion at lower temperatures. We interpret the transition as being from semiclassical motion over the barrier to quantum tunneling through the barrier. One-dimensional model calculations suggest that the distinct transition is due to the shape of the potential barrier along the surface. The calculated values for the diffusion constant at high temperatures agree with the experimental results; however, the transition is located at a too low temperature (40 K) compared with experiments (100 K). For deuterium the transition occurs at approximately 25 K, which is in disagreement with the weak isotope effect found in the experiments.