Tuning the wetting transition: Prewetting and superfluidity ofHe4on thin cesium substrates

Abstract

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...

Nonwetting of Cesium by Neon near Its Critical Point

“…In 1991 the first observation in a physisorption system of a wetting transition not tied to a bulk phase transition [2], and the associated prewetting transition [3,4], was made in the system 4 He on Cs, following the explicit prediction of Cheng et al [5]. Taborek and Rutledge [4] determined the wetting temperature T w of 4 He on Cs to be 1.95 K. When the cesium layer deposited on the gold electrode of their quartz crystal microbalance was reduced to a few atomic layers, these authors found a significant reduction of T w [6]. The calculated potential well depth of such a composite substrate is intermediate between that of Cs and Au [7].…”

Wetting behavior of liquidHe4on rough Cs films: Pinning, memory effect, and micropuddles

“…This happens, for example, in quartz microbalance experiments. 4,10 To study the wetting at T Ͻ T w , this thick helium film has to be removed by heating the Cs. The Cs surface is then covered with the thermodynamically stable thin-film state of helium, perhaps as thick as one or two monolayers at T Շ T w , 4 but going rapidly to very low coverage as T is lowered.…”

“…5 The prewetting line has been seen for the first time. 10 The thermodynamic thin-film state 11 which occurs in the nonwet regime ͑see Fig. 1͒ has been shown to be very thin when T Ӷ T w and ⌬ Ϸ 0, with a statistical thickness which is orders of magnitude less than one monolayer ͑ML͒.…”

Helium on planar and nanostructured alkali-metal surfaces

“…[8][9][10][11], locate T w between 1.75 and 2.179 K, 8 and it has become common practice to assign a value of T w to each specific Cs substrate, 8,12 in view of the fact that varying the width of deposited Cs is a convenient strategy to tune the 4 He-Cs interaction and thus the wetting diagram of the system. 7 Some uncertainties may also obscure the established value of T pw around 2.5 K, 13 as the experimental isotherms above that temperature still exhibit a somewhat visible jump, as shown for instance in Ref. 6 for the isotherm at 2.7 K.…”