Prewetting ofHe4on Rb: The Coexistence of Two Superfluid Films?

We have found the prewetting line and estimated the wetting temperature for ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ on Rb. $T_w\approx 309$ mK is much lower than that found for Cs. This low value of $T_w$ indicates that Rb is likely to be the only other element besides Cs that is not wetted by ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. The prewetting line is not consistent with theory nor with previous data. We argue that the thin state of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ on Rb appears to be superfluid for $\Delta \mu$ near coexistence; if this is so then on the prewetting line there are two superfluid films of disparate thicknesses.