WETTING TRANSITIONS OF HELIUM ON WEAK BINDING SUBSTRATES

We review recent experiments on the behavior of helium films adsorbed on alkali metal substrates, which bind He-4 more than an order of magn itude more weakly than conventional substrates such as graphite or gol d. The phase diagram of He-4 on cesium has several unique features whi ch have not been previously observed in any other system. At low tempe rature, He-4 does not wet cesium. At the wetting temperature T(w) cong ruent-to 2 K, a first order phase transition occurs, and it becomes po ssible to form thick films; there are large hysteresis effects near T( w) associated with the nucleation of the thin phase. For T < T(w), fil ms grow in a discontinuous way, with a first order thin-thick transiti on, which is known as prewetting. Prewetting can interfere with the us ual Kosterlitz-Thouless superfluid transition. The position of the pre wetting line can be tuned by varying the thickness of the cesium subst rate, or by doping the film with He-3.