A Model for Third Sound Attenuation in Thick 4He Films

Third sound attenuation in thick 4He films has been observed to be much greater than predictions based on known mechanisms. We propose a possible mechanism for this observed high attenuation. Pinned vortices, possibly created when the superfluid transition is traversed, undergo driven oscillations in the third sound wave flow field. The dissipation is caused by two related effects. The first is due to the mutual friction between the vortex cores and the normal component. The second, larger contribution, is due to the drag experienced by a vortex-induced surface dimple. Variations in vortex density explain quite naturally the observed lack of reproducibility in attenuation measurements. A vortex density on the order of 1017m^-2 is required to account for dissipation reported in several experiments. We discuss the temperature, frequency and thickness dependence of the dissipation. The proposed model is also applicable to a vortex contribution to fourth sound attenuation. If third sound attenuation is indeed a signature of a very dense array of pinned vorticity, then our conception of a homogeneous superfluid film needs considerable alteration.