Observation of depinning phenomena in the nanofriction of cryogenic films

We have used the quartz crystal microbalance technique to study the sliding friction of atomically thin films of rare gases. In the case of Kr monolayers on gold at liquid nitrogen temperatures, we have observed sharp pinning transitions separating a low coverage region, characterized by slippage at the solid-fluid boundary, from a high coverage region where the film is locked to the oscillating electrodes. Furthermore, we have induced sliding of a film by slowly increasing the amplitude of the substrate oscillations. Such a depinning transition is characterized by hysteresis as the amplitude is decreased back to zero. At temperatures below 10 K, we do not see a systematically reproducible sliding of heavy rare gases. However, preliminary data of Ne monolayers deposited on good quality lead substrates seem to indicate structural depinning of the film.