Slippage of nonsuperfluid 4He films adsorbed on Grafoil

We measured the decoupling of 4He films adsorbed on Grafoil using the quartz crystal microbalance (QCM) technique. It was found that nonsuperfluid 4He films decouple from the oscillating substrate, and that the degree of decoupling depends on the oscillating amplitude and the temperature. The amplitude dependence of the decoupling demonstrates clearly that the frictional force between Grafoil substrate and 4He film is not proportional to the sliding velocity. On the other hand, the temperature dependence of the decoupling shows that this force decreases below a certain temperature, i.e., the film sets in a low-friction state at low temperatures. In addition, it was found that this state remains metastable concerning the change in the oscillating amplitude.