Critical Overpressure for Nucleation of 4He Crystals in Aerogel

In an aerogel contained in a flat glass tube, we measured critical overpressures at which the very first 4He crystal appeared. Crystallization and melting were repeated 50 times at a constant temperature in a variable volume cell and the nucleation probability as a function of overpressure was obtained. An anisotropic S shape characteristic of the nucleation process was successfully observed.