Competition between Thermal Fluctuations and Disorder in the Crystallization ofHe4in Aerogel

Abstract

The dynamical transition in the crystallization of 4He 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.

Formation of superfluid liquid pocket in aerogel and its solidification by cooling

“…It has been used to investigate the disorder effects on condensed phases of He [3,9,11,13,14,[22][23][24][25][26]. A silica-aerogel sample of 96% porosity was grown in situ in a flat glass tube by Panasonic Corporation.…”

“…Instead, we decided to use a variable-volume cell which allows us to grow crystals in aerogel at a constant temperature. It has succeeded in revealing the temperature effect on the crystallization dynamics [13,14,27,28]. The variable-volume cell was composed of two chambers, a high-pressure chamber (A) and a low-pressure chamber (B).…”

“…The initial condensation of 4 He gas was finished above the critical point temperature to avoid damage due to shrinkage of the aerogel caused by the gas-liquid interface [29,30]. Detailed description of the sample cell was given in publications [13,14,27,28].…”

“…Crystallization dynamics of superfluid 4 He has also been investigated visually in aerogel [13,14]. A variablevolume cell allowed us to make crystals at a constant temperature and the entire crystallization process was observed visually in the transparent aerogels.…”

“…A variablevolume cell allowed us to make crystals at a constant temperature and the entire crystallization process was observed visually in the transparent aerogels. Crystallization of 4 He in aerogels shows a dynamical phase transition at around 600 mK which is the result of the competition between thermal fluctuation and spatial disorder: crystals grow via creep at high temperatures and via avalanche at low temperatures [13]. The creep and avalanche growth was clarified as the thermal activation and macroscopic quantum tunneling, respectively, from the crystallization rate and nucleation probability measurements [14,15].…”