Leggett's investigation of the superfluid density of solids has been extended. By employing plane waves as a basis set, one can obtain the superfluid fraction s0 as a function of the localization parameter. Solid He4 is estimated to have a s0 between 0.05 and 0.2 at zero temperature. This approach also sheds light on the superfluidity of thin films and the "index of refraction" of fourth-sound chambers. 1976 The American Physical Society.
