Slippage of nonsuperfluid helium films

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