The Effect of Film Thickness and the Substrate on Superfluid ³He Film Flow

Abstract

The self-emptying beaker technique was used to study the superfluid properties of thin film liquid ³He. Superfluid film flow was observed only below the bulk transition temperature at 0.93 mK. In a separate experiment the minimum film thickness was determined as a function of ³He level in the beaker. Thus, the flow rate, or critical current density, was determined as a function of temperature and film thickness. Extrapolation allowed a determination of the critical temperature (T_c) and zero temperature critical current density J_c (0) as a function of film thickness. T_c agreed with the predicted superfluid-normal phase boundary at 2d/(T) ç a where d is the film thickness and ξ(T) is the coherence length. J_c (0) was an order of magnitude smaller than expected for dissipation by pair-breaking. When a ⁴He monolayer was adsorbed on the surface of the beaker, it suppressed the diffuse scattering of ³He quasiparticles at the substrate boundary, as also observed by Freeman et al. There was no measurable suppression of T_c even for films as thin as 100 nm. With or without the ⁴He monolayer teere was always an abrupt drop in the flow-rate when the film thinned to about 200 nm. This may be associated with the transition between the expected thick film B-like phase and thin film A-like phase.