We measured the mechanical response of ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$ 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 ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$ 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.

• Received 30 June 2000

DOI:https://doi.org/10.1103/PhysRevLett.85.5142