J. E. Rutledge

A low drift high resolution cryogenic null ellipsometer

A cryogenic ellipsometer capable of measuring submonolayers of liquid helium has been developed. Methods for controlling drift and noise created by the cryostat windows and ellipsometer optics are discussed in detail. The cryostat vacuum windows were made from SF-57 glass due to its low stress-optic coefficient. Custom low-stress vacuum window mounts were designed and all windows were temperature controlled. Placing the compensator wave plate inside the cryostat was necessary to avoid room temperature fluctuation induced noise. These steps produced 12 h drifts in the measured polarization of less than 0.002°. A helium adsorption isotherm taken at 1.5 K on gold is presented to show the high resolution obtained once noise and drift associated with window birefringence is minimized.

Experimental Survey of Wetting and Superfluid Onset of 4He on Alkali Metal Surfaces

We present a survey of the wetting behavior of4He on evaporated films of rubidium, potassium and sodium.4He wets these surfaces at all temperatures. Off coexistence on rubidium and potassium there is a prewetting transition that is closely coupled with superfluid onset and possibly a surface tricritical point where the prewetting transition and superfluid onset intersect. Sodium has a prewetting critical temperature well below 1 K. Wetting and superfluid onset phase diagrams for4He on rubidium and potassium are presented and compared with that of4He on cesium.

Adsorption of 3He on Cesium

Adsorption isotherms of3He on cesium substrates have been measured in the temperature range from 0.2 K to 1.5 K. At liquid-vapor coexistence3He wets cesium at all temperatures studied. Step-like features are found in the isotherms which are similar to the prewetting transitions of4He on Cs substrates, but the width of these steps is ∼20 times wider for3He than for4He. In the case of3He on Cs, the steps are located at a chemical potential about 0.6 K below liquid-vapor coexistence. If the low temperature behavior is interpreted to be first order prewetting, the prewetting critical point temperature is 0.6±0.1 K.

Contact Angle of Superfluid Helium Droplets on a Cesium Surface

We present pictures of superfluid drops on several cesium surfaces prepared in different ways. These pictures are used to determine the temperature dependence of the contact angle. The results show that the temperature dependence of the helium-cesium surface tensions depends on the wetting temperature. The relationship between these results and other recent measurements of the temperature dependence of the contact angle is discussed.

Direct Optical Imaging of Superfluid 4He Droplets on a Cesium Surface

The recent discovery that4He does not wet the surface of cesium makes possible the study of superfluid droplets. We have used a long focal length microscope to study superfluid4He droplets on a cesium surface. We find that4He droplets do not flow freely over the surfaces we prepare. Rather, they remain stationary even when the surface is inclined by as much as 10° to the horizontal. The contact angle, the angle between the surface and the liquid-vapor interface is also highly hysteretic. At 1.2 K, where the advancing angle is near 25°, the receding angle varies between 0° and 7°, depending on the cesium film.

Adsorption of3He on cesium surfaces

Vapor pressure isotherms of3He on Cs are reported which show that, in contrast to4He,3He weis Cs for 1.26K<T<2K. Although the isotherms are distinctly different from those taken on strong binding substrates, we have observed no evidence for a first order prewetting transition. The data suggest that the prewetting critical point is substantially below T=1.26K.

Matching the resistivity of Si:Nb thin film thermometers to the experimental temperature range

The useful temperature range of semiconducting resistive thermometers is limited by declining sensitivity at high temperature and inconveniently high resistivity at low temperatures. The useful temperature range of sputtered thin film Si:Nb resistance thermometers is controlled by the Nb concentration. We have developed a simple technique for controlling the Nb concentration during the fabrication process. We describe an extremely sensitive family of thermometers with useful temperature ranges that overlap and span temperatures from below 1 K to above 450 K.

Wetting transitions of binary liquid mixtures at a weakly attractive substrate

The adsorption of a binary liquid mixtures onto a weakly attractive substrate has been studied in the vicinity of bulk liquid phase separation. A wetting transition between two different kinds of wetting films has been found. Below the wetting transition temperature, homogeneous films of one liquid phase wet the substrate. Above the wetting temperature, the wetting films contain both liquid phases. The surface phase transition line associated with this wetting transition is found to extend to both sides of the bulk phase separation line. On one side it is a prewetting line, and on the other side it becomes a line of triple point induced dewetting transitions. The experimental system used to study this behavior was liquid helium mixtures on a cesium substrate.

The solid-liquid surface tension at a helium/cesium interface

The existence of a wetting transition for helium adsorbed on cesium provides an experimental opportunity to investigate the solid-liquid surface tension. We have extracted the temperature dependence of the solid-liquid surface tension at the4He/Cs interface from our earlier measurements of the wetting temperature of pure4He on thin layers of cesium overlaying gold. More recently we have measured the wetting temperature as a function of the3He concentration for3He-4He mixtures. An analysis of these experiments paralleling Andreevs investigation of the free surface reveals3He bound states at the helium/cesium interface.

Non Diffusive Mobility of Solid Hydrogen Films

We have used a “hole-burning” technique to study the mobility of thin solid films of H2 and D2 for temperatures between 1.6 K and 5 K. Even at low temperatures where transport through the vapor is negligible, the solid films remain mobile. The transport is thermally activated with an activation energy of 19 K for H2 and 38 K for D2. The time dependence of the regrowth shows that surface transport is not due to simple diffusion.