Search

Using a torsional oscillator technique, we have investigated the onset of superfluidity of 4He in Vycor at pressures in the liquid region and at higher pressures where bulk 4He is solid. The lower pressure data trace out the onset temperature for superflow in Vycor as a function of pressure, which is roughly parallel to the bulk lambda line but about 0.25K lower. At OK, calculated values of the superfluid fraction are about 0.16 at 1 bar and 0.12 at 25 bar; these numbers contain a geometrical constant. Higher pressure data indicate that superflow can occur in Vycor at pressures as much as 20 bar above bulk solidifcation. An unexplained decrease in period is observed below the bulk solidification temperature.

Measurements of the maximum flow current in 3He-B at various pressures are presented. Their temperature dependence is in agreement with theoretical expectation, but their pressure dependence is different from that of weak coupling theory; these results may therefore be used to test predictions of strong coupling parameters.

The Rayleigh disc as a detector of hydrodynamic flow in superfluids is described. An experimental cell incorporating a novel system for detecting the deflection of the disc - the Butterfly-valve inductor (1) - has been used to observe for the first time steady thermal counterflow in superfluid 4He. We describe the extension of these experiments to superfluid 3He.

Data are presented on the specific heat of helium confined at a high externally applied pressure within the 60 A pores of vycor glass. The specific heat shows that the helium within the pores remains in the liquid state down to temperatures well below 1K. On the other hand the helium outside the vycor behaves normally, changing phase from solid to liquid as it warms.

Large scale temperature hysteresis effects have been observed in flowing superfluid 3 He-A. On cooling 3 He-A the flow acts as if it were “more superfluid” than on warming. This is observed in both the superfluid density and the dissipation present in the flow, and does not depend on whether the B transition occurs.

We report measurements of the spin diffusion coefficients of normal liquid 3He in bulk, and in 5μm diameter glass capillary tubes, as a function of pressure (0–30 bar) and temperature (3mK–15mK). The bulk experiments are well into the limiting low temperature Fermi fluid regime, and provide the first reliable measurements of the pressure variation of A = DT2. In the 5μm capillaries, deviations from this relation are found as the quasiparticle mean free path become comparable with the diameter. The results indicate that the spin momentum accommodation coefficient at this surface is less than unity.

Using a CW NMR method at a resonant frequency of 707 KHz, we have measured the nuclear magnetic susceptibility of 3 He in bulk and in a 5μm diameter glass capillary array as a function of pressure from 15 to 3 mK. Bulk values are in good agreement with previous measurements, and the capillary measurements show the surface enhancement effect. The surface enhanced susceptibility is only weakly pressure dependent. The results at low pressure scale quite accurately with previous measurements on 2μm capillaries, making it unlikely that the enhancement is due to a mean free path mechanism.