Prewetting of helium on weak binding substrates

Recent theoretical and experimental work has shown that alkali metal surfaces have extremely weak potentials for physical adsorption, which leads to very unusual thermodynamic properties of adsorbed films. We have used a microbalance adsorption isotherm technique to investigate the phase diagram of He films on cesium. This experimental system has provided the first example of a surface which is not wet by superfluid helium, and the first experimental observation of the prewetting line. Near the wetting temperature Tw ∼ 2 K, we have observed large hysteresis effects associated with the nucleation of the thin phase. At higher temperatures, the prewetting line terminates at a prewetting critical point Tcpw ∼ 2.5 K. This system is particularly well suited to surface phase transition studies, since the interaction parameters can be tuned continuously by varying the thickness of the cesium substrate, or by doping the film with 3He. (orig.)