Superfluid density as a function of temperature and pressure near the lambda line

We report new results for the superfluid density in the region close to the lambda line at pressures up to 25 bars. The data were obtained from second sound velocity measurements made to within 0.9 mu K of the transition by a resonance technique. A superconducting pressure gauge was used to allow th e measurements to be performed along a constant pressure path. From these d ata the temperature and pressure dependence of the superfluid density can b e derived to test for critical exponent universality. We also report the te mperature and pressure dependence of the resonator quality factor. (C) 2000 Elsevier Science B.V. All rights reserved.