Nonmonotonic Velocity Dependence of Atomic Friction Induced by Multiple Slips

The transition from single to multiple atomic slips, theoretically expected and important in atomic-scale friction, has never been demonstrated experimentally as a function of velocity. Here we show by high-resolution friction force microscopy on monolayer MoS_{2}/Au(111) that multiple slips leave a unique footprint-a frictional velocity weakening. Specifically, in a wide velocity interval from 10 to 100 nm/s, friction surprisingly decreases. Model simulations show a similar nonmonotonic behavior at velocities in quantitative agreement with experiment. Results suggest a velocity-corrugation phase diagram, whose validity is proposed more generally.