Acoustic crystallization and heterogeneous nucleation.

By focusing a high-intensity acoustic wave in liquid helium, we have observed the nucleation of solid helium inside the wave above a certain threshold in amplitude. The nucleation is a stochastic phenomenon. Its probability increases continuously from 0 to 1 in a narrow pressure interval around P(m) + 4.7 bars ( P(m) = 25.3 bars is the melting pressure where liquid and solid helium are in equilibrium). This overpressure is larger by 2 to 3 orders of magnitude than what had been previously observed. Our result strongly supports the recent suggestion by Balibar, Mizusaki, and Sasaki that, in all previous experiments, solid helium nucleated on impurities.