Superfluid Onset and Prewetting of 4He on Rubidium

J. A. Phillips, D. Ross, P. Taborek and J. E. Rutledge 
Department of Physics and Astronomy 
University of California, Irvine 92697 

(1998)
 
 
Abstract 

Introduction 

Experimental Methods 

Data 

Discussion 

Conlcusions 

References 
 
 
 
 

Projects Page 
 
 
 
 

Conclusions

Although the substrate potential in Rb is only 5% stronger[10] than Cs, the behavior of helium films on the two substrates is surprisingly different. The difference is presumably due to differences in the topology of the underlying phase diagram and in the degree of substrate disorder. In Cs, the KT line terminates on a noncritical point, while in Rb, the KT line seems to meet the prewetting line near a critical region. Hysteretic prewetting on Rb was observed in all of our samples as well as by other investigators [11,14] and seems to be a robust feature of prewetting on Rb. Perhaps the most important result of the experiments reported here is that the hysteresis ends at the critical point. This, together with the fact that the thick/thin transition always seems to occur between films in the normal state implies that superfluidity and prewetting are strongly coupled on Rb. This interaction yields an unusual superfluid transition which is not of the Kosterlitz-Thouless type. This type of behavior may be typical of a range of “intermediate” strength substrates characterized by prewetting (or layering) transitions without accompanying wetting transitions. 
Another surprising feature of superfluid onset on Rb substrates is that helium films with an average coverage of more than two layers can remain normal on Rb even at very low temperatures. The normal film is not inert, however. Once superfluidity is established, less than 1 layer is viscously locked to the substrate. We have also observed unusual phenomena in the thin phase near 0.3K, but our results cannot be simply explained in terms of a superfluid/normal transition in the thin phase. The interpretation of our primary data depends somewhat on assumptions about the coupling of the superfluid fraction of the film. The fact that the super/normal and thick /thin transition are so tightly intertwined and that the bottom layers are not inert make it difficult to determine the total film coverage near the transition, since our oscillators couple to a combination of the normal and superfluid part of the film which in principle depends on the coverage.   To remove this ambiguity we are developing oscillators which have displacements normal to the surface and therefore directly determine the total coverage. 

This work was supported by NSF grant DMR 9623976