| For years, helium films were studied only on strong binding substrates. In these systems, the behavior of the adsorbed films was well characterized with the Kosterlitz-Thouless (KT) superfluid model. Another phase transition, prewetting, is also observed in films adsorbed on weak substrates where the potential well of the surface is comparable to the helium-helium binding energy. The experiments described here use quartz crystal microbalances to study the behavior of helium films on lighter alkali metal substrates, which lie between the strong binding limit, gold, and the weak binding limit, cesium. The phase diagrams of helium films on this type of substrate have unique features due to the interaction of prewetting with the KT transition.; Both the He-Rb and He-K systems included tricritical points that were located at 1.95K and 1.75K, respectively. Below these temperatures, the prewetting and superfluid transitions were strongly coupled and hysteretic. Unlike the only slightly weaker cesium surfaces, rubidium and potassium were always wetted by helium indicating that no wetting transitions exist.; The effects of surface inhomogeneity can be seen in the irreproducibility and continuous prewetting transitions of several systems. Each sodium surface, even those evaporated and annealed in the same manner, produced different prewetting and superfluid phase diagrams. Phase diagrams for helium on atomically thin cesium show the locations of superfluid, but since the films were continuously thickening, no first-order prewetting transitions were observed.; The superfluid transitions in all systems studied deviated from the universal KT model. In those systems that exhibit hysteresis, the superfluid density at onset is dependent on the direction traversed in Dm -T space. Remarkably, all systems studied failed to produce superfluid density predicted by the Kosterlitz-Thouless model.; Another transition that occurs only in these systems is the solidification of the first layers of helium. The two-dimensional solid forms due to the surface van der Waals force. As this force is made weaker, with different substrates, it is possible to decrease these inert layers. Using gold substrates with varying amounts of cesium coating, the surface potential was "tuned" to alter the surface from a strong binding substrate to a weak one. |
