Polarization characteristics of adatoms self-diffusing on metal surfaces under high electric fields
Although atomic diffusion on metal surfaces under high electric fields has been studied theoretically and experimentally since the 1970s, its accurate and quantitative theoretical description remains a significant challenge. In our previous work, we developed a theoretical framework that describes the atomic dynamics on metal surfaces in the presence of an electric field in terms of the local polarization characteristics of the surface at the vicinity of a moving atom. Here, we give a deeper analysis of the physics underlying this framework, introducing and rigorously defining the concept of the effective polarization characteristics (permanent dipole moment $μ$ and polarizability $α$) of a moving atom on a metal surface, which are shown to be the relevant atomic quantities determining the dynamics of a moving atom via a compact equation. We use density functional theory (DFT) to calculate $μ$ and $α$ of a W adatom moving on a W {110} surface, where additional adatoms are present in its vicinity. We analyze the dependence of $μ$ and $α$ and hence the migration barriers under electric fields on the local atomic environments (LAE) of an adatom. We find that the LAE significantly affects $μ$ and $α$ of a moving atom in the limited cases we studied, which implies that further systematic DFT calculations are needed to fully parameterize surface diffusion in terms of energy barriers for long-term large scale simulations, such as our recently developed Kinetic Monte Carlo model for surface diffusion under electric field.