Friction through reversible jumps of surface atoms

We propose a microscopic model that incorporates the effect of thermally activated motion of surface atoms on nanoscopic friction. Our calculations demonstrate that the stick-slip motion of the tip is governed by two competing processes: (i) jumps of the surface atoms to the tip which tend to inhibit sliding, and (ii) jumps back to the sample which give rise to sliding. The energy dissipated during the reversible jumps of the surface atoms between the sample and tip contributes significantly to the friction force, and leads to a nonmonotonic dependence of friction on temperature, which has been observed in recent friction force microscopy experiments for different material classes. The proposed model elucidates the physical origin of microscopic instabilities introduced in phenomenological models for the interpretation of the experimental results. (paper)