Giant Proximity Effects in Confined Superfluid 4He

Figure 2: Confinements. An scanning electron microscope image of (2 μm)3 boxes etched out of thermally grown silicon oxide. These boxes are filled with liquid helium creating an extremely well defined array of uniform confinements. Figure 1: Confinement Cell. A diagram (not to scale) of a cell used to confine liquid helium. This consists of an array of (2 μm)3 boxes etched in a thermally grown oxide on a 2-inch silicon wafer. A second wafer has an outer wall and several support posts etched into a 31.7 nm oxide. This wafer forms a uniform film used to fill the (2 μm)3. After patterning of the silicon wafers they are bonded together and form a confinement cell to be filled with helium. Previous measurements of the specific heat of 4He confined in all three spatial dimensions suggested a coupling between neighboring confinements through the tiny channels used to fill them [1]. Measurements of the superfluid fraction in the filling channels also suggested an enhancement attributed to their proximity to the larger regions of superfluid [1]. Recent measurements of confinement structures designed at the Cornell NanoScale Facility (CNF) have confirmed these effects [2], and begun to address their nature [3]. Summary of Research: Using the facilities at CNF, we are able to etch various geometries out of thermally grown silicon oxide. Then, through direct wafer bonding of two patterned wafers, we have been able to construct extremely uniform and well­ characterized confinement cells (see Figures 1 and 2). Measurements of the specific heat and superfluid fraction of liquid helium in these confinements has allowed, for the first time, observation of coupling between neighboring confinements as well as proximity effects on a thin film in equilibrium with larger regions of superfluid [2]. Specific heat data for an array of (1 μm)3 confinements showed various anomalies [4]. It was suggested that these anomalies could be explained by a coupling between the confinements through the small channels used to fill them [1]. This coupling reveals itself as an enhancement in the specific heat. A measurement of (2 μm)3 confinements has allowed us, via scaling, to quantify this enhancement (Figure 3) [2]. The (2 μm)3 measurement also identified proximity effects on the 31.7 nm film used to fill the (2 μm)3 boxes [2]. These proximity effects include an increase in both the superfluid fraction and the specific heat of the film, as well as an increase in the temperature of the specific heat maximum, and the