Third sound study of the wetting of rubidium by helium
Investigation of helium on rubidium, sodium and potassium surfaces followed the cesium studies. My own involvement in wetting experiments was triggered by the results of an experiment conducted by Wyatt and Klier. In their experiment the helium-rubidium system seemed to exhibit nonwet behavior below 300 mK. The very thin film of adsorbed helium on nonwet rubidium seemed to behave like a submonolayer superfluid. From it was born the idea that we could explore superfluidity in the thin film of helium atop rubidium by conducting an experiment which attempts to propagate a tidal-like wave (third sound) along the film.
The basic idea behind the experiment is that third sound can only propagate in films which are superfluid. Therefore, if the thin film of helium on rubidium is superfluid, we can expect to propagate third sound through it. We designed an experiment to examine third sound that propagates across a rubidium substrate. We were able to use the third sound reflection off of the rubidium boundary as a diagnostic to determine when the rubidium was wet (or prewet) by helium and when it was not wet by helium. We produced a rubidium substrate that could be caused to make a transition from being nonwet by helium to being wet by helium. Third sound propagated across the rubidium. We therefore have obtained strong evidence to support the idea that helium in the thin film state on rubidium can behave like a superfluid. We also observed strong evidence that, very close to saturation, the rubidium was always wet by helium to temperatures as low as 150 mK. On a second rubidium surface, the rubidium was always in the wet state, evidence that it is extremely difficult to fabricate rubidium surfaces of reproducible quality.
In this document, we present the results of our third sound experiments on rubidium and explain how we have drawn conclusions about the wetting behavior of helium on rubidium. In addition, a chapter is devoted to some of the low temperature third sound data on glass. (Abstract shortened by UMI.)