Slippage of nonsuperfluid helium films

We measured the mechanical response of 3He and 4He films on an oscillating substrate using an ultrasonic technique. Up to the coverage at which the fluid state appears at absolute zero, a part of the nonsuperfluid 3He and 4He films underwent slipping relative to the substrate at low temperatures. From the temperature dependence of the slippage, it was found that a thermally activated process plays an important role in the frictional force of this system.