Key points are not available for this paper at this time.
We observe that recent theories of phase transitions in the two-dimensional XY model predict a universal jump in the superfluid density of ^4He films as T₂ is approached from below. Specifically, we find that limₓₓ₂-ₒ (T) T=3. 5210^-9 g/cm^2^. Analogous results should hold for two-dimensional planar magnets and liquid crystals.
Nelson et al. (Mon,) studied this question.