Surface dynamics and friction of K/Cu(001) characterized by helium-3 spin-echo and density functional theory

Abstract

Helium-3 spin-echo measurements of K/Cu(001) are presented, the diffusional surface dynamics of the system at low coverages and on picosecond time scales. Langevin molecular-dynamics simulations are used, together with a potential-energy surface derived from density functional theory calculations, to provide further understanding of the experimental data. An anisotropic potential with a corrugation of 35--64 meV and a friction parameter of $1/4.5\text{ }{\text{ps}}^{\ensuremath{-}1}$ are found to give a good fit to the data for the lower coverages with the adsorbate interactions modeled with a dipole-dipole repulsion. Additionally, at the highest coverage, $\ensuremath{\theta}=0.084$, a component of motion is observed perpendicular to the surface, analogous to that recently found for Na/Cu(001).