Noncontact Friction in Electric Force Microscopy over a Conductor with Nonlocal Dielectric Response

Electric force microscopy, in which a charged probe moves above a surface, can measure thermally generated electric field fluctuations from the sample. Noncontact friction measurements of energy loss from the probe have been performed over insulators, semiconductors, and conductors and have been interpreted in terms of the dielectric response of the sample. Noncontact friction over metal surfaces has recently been ascribed to dielectric relaxation of adsorbed molecules, motivating the examination of the baseline noncontact friction over a bare metal surface. The noncontact friction for a thin conducting film is calculated for a wavevector-dependent dielectric function, complementing previous calculations for insulators and semiconductors employing a dielectric continuum representation. Inclusion of the wavevector dependence in the dielectric response of a conductor enhances the friction and alters its dependence on tip-sample separation, relative to the continuum treatment.