Acoustic nucleation of solid helium 4 on a clean glass plate

It's have used a focused acoustic wave to nucleate solid helium. 4 on a. clean,glass plate. From the reflectance of light at, the glass/helium interface, we measured the amplitude of the acoustic wave in the focal region. Nucleation was found to be stochastic, occurring 4.3 +/- 0.2 bar above the equilibrium. melting pressure. This overpressure is 2 to 3 orders of magnitude larger than found in previous nucleation studies where favorable defects or impurities must have been present. From the statistics of nucleation and its temperature dependence above 300 mK, we have estimated the activation energy E for the nucleation on the,glass plate; we have found E/k(B) T = 10. This value is consistent, with a thermally activated nucleation on a single site at the glass/helium interface. the also found a crossover to a quantum nucleation regime below 300 mK. We finally discuss some implications of these results for the homogeneous nucleation of solid helium and the search of a liquid-solid spinodal limit.