Crystallization Onset of Liquid Pockets via Mass Flow through Solid 4He in Aerogel

What is it about?

4He crystals in a 96% porosity aerogel were grown by pressurization at 0.85 K; however, liquid pockets remained uncrystallized, even with application of a further overpressure, due to the absence of mass flow at high temperatures. The liquid pockets began to crystallize below a particular onset temperature upon cooling at the constant pressure, which indicates that mass flow occurs in the aerogel at low temperatures. The onset temperature of crystallization was investigated for various liquid pockets to examine the mass flow through 4He crystals in the aerogel.

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Why is it important?

Mass flow in 4He crystals has been an issue in the filed of quantum solids and has been actively debated recently in the context of “supersolidity”. Some experimental results have indicated that mass flow occurs in 4He crystals at low temperatures.1–8) The important issue to understand the flow mechanism is where the mass flow actually occurs.9–12) From a different perspective, we have studied the growth of 4He crystals in aerogels to investigate the effect of disorder13–15) and determined that crystallization proceeds by a temperature sweep at constant pressures.5,16) Such crystallization requires mass supply from the surrounding bulk crystals into the aerogel to compensate for the density difference between the liquid and solid. Therefore, this should sensitively reflect the mass-flowability of the bulk crystals. We have also reported the formation of uncrystallized liquid pockets in these crystals in an aerogel and their crystallization upon cooling.17) This is considered to be related to the flowability of 4He crystals in aerogels. In this work, the onset temperature for the crystallization of the liquid pockets was examined.