Superfluid onset and prewetting of4Heon rubidium

Quartz-crystal microbalance isotherms of ${}^4\mathrm{He}$ on rubidium are presented. The isotherms are used to locate the prewetting transition and the onset of superfluidity in the adsorbed helium films. The phase diagram of He on Rb is distinctly different from helium on either conventional strong substrates or on cesium. In particular, the superfluid transition is hysteretic and does not conform to the Kosterlitz-Thouless paradigm.