Competition between Thermal Fluctuations and Disorder in the Crystallization of 4He in Aerogel

The dynamical transition in the crystallization of He-4 in aerogel has been investigated by direct visualization and dynamical phase diagrams have been determined. The crystal-superfluid interface in aerogel advances via creep at high temperatures and avalanches at low temperatures. The transition temperature is higher at a higher interface velocity and lower in higher porosity aerogels. The transition is due to competition between thermal fluctuations and disorder for the crystallization process.