Out-of-equilibrium nucleation in the solidification of helium

We observed nucleation in a first-order phase transition in which a nonequilibrium (metastable) solid phase nucleates and grows from overpressured superfluid He-4 in preference to the equilibrium solid phase. After a time varying from a fraction of a second to tens of minutes, the stable phase nucleates independently from the liquid, and the nonequilibrium solid phase melts. We examined the possibility of accounting for these events in terms of the known differences in the interfacial free energies between the superfluid and the two solid phases (stable and metastable). The experimental results are not consistent with this explanation, however, and we discuss other possibilities.