Unexpected temperature and velocity dependencies of atomic-scale stick-slip friction

We report experiments of atomic stick-slip friction on NaCl that were performed over a wide range of surface temperatures and scanning velocities. As in previous experiments, we found a nonmonotonic relation between the time-averaged friction and temperature. In contrast to the previous works, here atomic-scale stick-slip friction was resolved for all measured temperatures and velocities. We further introduce a model that explicitly includes a periodic structure of a multi-asperity contact, and we demonstrate that the simulations reproduce the experimental observations. The presented results show that analysis of mean friction force only is not enough for an unambiguous understanding of the friction mechanisms, and measurements of force traces at different temperatures and velocities provide important additional information.