Crystallization ofHe4in aerogel via mass flow from surrounding solidHe4

Abstract

The phase boundary between the quantum solid and liquid phases of 4 He is strongly modified in a porous material. However the phase diagram at very low temperatures remains unexplored. We have used a variable-volume experimental cell with optical access to visualize the crystallization of 4 He in silica aerogels with independent control of the pressure and temperature. The onset of crystallization was investigated in two aerogel samples with differing porosity both by pressurization at constant temperature and by cooling at constant pressure. With isothermal pressurization we have established a low temperature phase diagram for each aerogel and we find that the equilibrium crystallization pressure is reduced with increasing aerogel porosity. Crystals also began to grow in the aerogel on cooling at fixed pressure below an onset temperature, Tgrow. We found that below this temperature the crystallization rate increased with decreasing temperature. The aerogel in our cell was surrounded by bulk crystals of 4 He and surprisingly Tgrow was found to be widely distributed when the surrounding bulk crystals were repressurized. In this experimental arrangement, crystallization within the aerogel on cooling requires mass flow from these exterior bulk crystals and is strongly influenced by the disordered structure at the interface between the bulk solid and the helium within the aerogel.

Mass transport through dislocation network in solid He4

“…Upon compression the sample space is completely filled with aerogel. The melting boundary of solid 4 He grown in aerogel is found in this and another experiment [20] to be elevated to nearly 27.3 bar from the bulk melting pressure at 25.3 bar. Once the growth of solid in aerogel is completed, i.e., the liquid in the liquid-solid coexistence region is replaced completely with solid, a pressure difference across the sandwich (e.g.…”