THEORY OF CLASSICAL SURFACE-DIFFUSION

We present a review of recent theoretical developments in classical surface diffusion. We start with a brief survey of theoretical and experimental works in surface diffusion to date, and point out some of the open problems in the field that have not received widespread attention. We then proceed to a detailed exposition of an analytic, microscope theory of classical diffusion of adatoms on surfaces. This is based on a Mori projection operator formalism in which the various dynamic correlation functions of the adatom are evaluated. The zero frequency limit of the velocity autocorrelation function yields the diffusion tensor. Subsequently, we use the microscopic theory to examine the qualitative, universal features of surface diffusion. These include the validity and emergence of the Arrehnius form of the activated diffusion at low temperatures, the random walk theory and the diffusion anisotropy on surfaces with different symmetries. We then discuss the surface diffusion on substrates with special properties. The first of these is a substrate with large adatom induced local distortions. This is motivated by the observed anomalous diffusion anisotropy of the H/W(110) system. We introduce a two-step lattice gas model for this problem and justify the model by a parallel description of the deformable lattice within the microscopic theory. Detailed Monte Carlo simulation studies are then presented for the two-step lattice gas model. Another special case is a substrate which undergoes an intrinsic structural transition. We show that the adatom diffusion coefficient would vanish anomalously as the transition is approached due to the coupling with critical fluctuations. We conclude with a summary of the results and a discussion on the directions for future research.