We have deposited cesium with monolayer thickness resolution on to the surfaces of a lowtemperature quartz microbalance. By controlling the cesium thickness from 1 to 25 monolayers, we have been able to continuously tune the substrate adsorption potential and observe its effect on the 4 He prewetting phase diagram. The wetting temperature can be conveniently adjusted between 1 and 2 K. Features in both the frequency shift and the dissipation of the microbalance were used to map out the superfluid-normal boundary in the prewetting phase diagram. The transition from thin normal to thick superfluid occurs without the dissipation peak characteristic of a Kosterlitz-Thouless transition.PACS numbers: 68.45.Gd It has been known for over 15 years that the alkali metals, and particularly cesium, have very weak adsorption potentials [1]. Only recently, however, it has become clear [2-4] that the phases and phase transitions of adsorbates on these weak binding substrates can be qualitatively different from those observed on conventional strong binding substrates, and several recent experiments [5][6][7][8] have confirmed novel behavior in alkali metal adsorption systems. These results have been especially provocative in the case of 4 He adsorption because alkali metals are the only known substrates which are not completely wet by superfluid helium, and also because this system is a particularly clear example of a more general type of surface phase transition known as prewetting [9,10]. Prewetting is a first order phase transition between thick and thin states of the unsaturated film [11,12]. It is characterized thermodynamically by nonmonotonic behavior of ji(d), the film chemical potential as a function of the film thickness. In the case of 4 He on cesium, this nonmonotonic behavior can be traced to the fact that at short distances, the He-Cs potential is weaker than the He-He potential, while at larger distances, the reverse is true. In previous work [10], we have used adsorption isotherms measured using a quartz microbalance to map out the phase diagram of 4 He/Cs. The main features of the phase diagram are a wetting transition at liquid vapor coexistence at approximately 2 K, with the prewetting line emanating from it and terminating in a prewetting critical point. These experiments were performed on cesium film substrates which were so thick (approximately 300 A) that the adsorption potential is expected to be indistinguishable from bulk cesium.In the experiments reported here, we have studied 4 He adsorption on cesium substrates (deposited on gold electrodes) which are only a few monolayers thick. By varying the cesium substrate thickness from 1 to 25 monolayers, we can tune the adsorption potential from essentially goldlike to bulk cesiumlike. These variations in the adsorption potential generate corresponding changes to ixid) and the phase diagram. In particular, the wetting temperature at liquid-vapor coexistence is sensitively dependent on the substrate potential; by sequentially adding layers of cesium, we ha...
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