Third-Sound Velocity and Onset on Grafoil

Abstract

Third-sound velocity and onset measurements have been made for ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ adsorbed on Grafoil foam. The third-sound velocity has a dependence on film thickness and temperature similar to that of third sound on glass substrates, with some distinguishing features. The film vapor pressure at onset has, to some degree, the universal temperature dependence found for other substrates. The superfluid areal density divided by the onset temperature agrees with the universal constant predicted by the Kosterlitz-Thouless-Nelson theory to within 11%.