Tunnel junction investigations of superflow in thin helium-4 films

The author reports preliminary investigations of superflow onset and third-sound velocity made using superconducting tunnel junction transducers in the temperature range 0.85-1.05k, a technique of comparable, if not superior, sensitivity to any used before. Several novel effects have been observed, notably a discontinuous variation of superfluid density wth film thickness, interpreted as reflecting a change of state in the helium film, and also finite resistance to dc film flow above the conventional onset point. The technique promises to be of significant importance in future investigations of superflow in the <1k range.