Interfacial friction of thin 3He slabs in the Knudsen limit.

A high precision torsional oscillator has been used to study 3He films of thickness from 100 to 350 nm, in the temperature range 5<T<200 mK. This thickness is much smaller than the viscous penetration depth, and also much smaller than the low temperature inelastic mean-free path of 3He quasiparticles. Such films are observed to decouple from the motion of the oscillator below 60 mK. We interpret the frequency shift and dissipation data in terms of an interfacial friction model.