Critical Overpressure for Nucleation of⁴He Crystals in Aerogel

Abstract

We are focusing on the crystallization of 4 He in aerogels intensively, motivated by the finding of the dynamical transition of the crystal growth inside the aerogels. 1 In the experiment with aerogels contained in a flat glass tube, we found that 4 He in the aerogel crystallized by nucleation. 2 In order to confirm this nucleation, we measured the critical overpressure at which the very first 4 He crystal appears in the aerogel. 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. Anisotropic S shape characteristic to the nucleation process was successfully observed and the critical radius of the nucleated crystals was estimated. The S shape curve changes drastically through the dynamical transition temperature.

Thermal and quantum nucleation of4He crystals in aerogel

“…2 of Ref. [27]; the clear metastability of the superfluid liquid and the decay to the stable crystal phase were observed in the aerogel.…”

“…The appearance of the first crystal in the aerogel was visually observed both in the creep and avalanche regions and we recorded the critical overpressure δP c in chamber A at which the first crystal was observed. The nucleation point in the aerogel was always the same: near the bottom [27].…”