The onset of solidification of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ in porous Vycor is identified from changes in transverse sound velocity. From observed overpressures required to initiate freezing (ranging from 18 bars at 1.3 K to 30.7 bars at 2.38 K), the interfacial energy between liquid and solid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ was estimated. The superfluid transition up to 35.5 bars was also observed.

• Received 12 November 1982

DOI:https://doi.org/10.1103/PhysRevLett.50.425