Friction coefficient of atoms near a metal surface

Abstract

The friction coefficient α of an atom moving near a metal surface and that of two interacting adatoms is calculated by using a Newns-Anderson hamiltonian. In the absence of Coulomb interaction, it is found convenient to decompose α into (∇ηv)2 and sin2 (η v - ηv') terms where the η v's are the generalized phase shifts at Fermi level. For one atom, α is evaluated numerically in a simple model. The origin of the sharp maxima reported by other authors is clarified. The Coulomb interaction is then taken into account, first within the Hartree-Fock approximation. The latter appears to be little valuable, especially near the onset of the localized magnetism. In the case of one atom, treated in a simple model, an exact expression of α in terms of the density of states at the atom is derived, leading to a more precise picture of the behaviour of α. Finally, the respective role of electronic excitations and phonons in friction is discussed