Quantum Crystallization and Self-Organized Criticality in Porous Materials

Abstract

Effects of disorder on the dynamics of the first order phase transition are intriguing in the low-temperature limit where thermal fluctuation ceases and novel quantum phenomena would emerge. We have been investigating crystal growth of ⁴He in silica aerogels visually. Crystallization of ⁴He in aerogel was revealed to exhibit a dynamical transition in the growth mode. Crystals grow via creeps at high temperatures and via avalanches at low temperatures due to the competition between thermal fluctuation and disorder. It was also found from a crystallization rate and a nucleation probability measurements that crystals grow via thermal activation in the high-temperature creep region and via macroscopic quantum tunneling in the low-temperature avalanche region. In the low-temperature quantum region, avalanche size distribution has a power law which indicates that the system is in a self-organized critical state. Here, we demonstrated that a crystallization on cooling at a constant pressure was possible in aerogels, using a variable volume cell to independently control the pressure and temperature. Crystallization was induced by cooling below a particular temperature, indicating that mass transport occurred from the surrounding bulk crystals into the aerogel below that onset temperature. While a possible explanation for the mass transport would be supersolidity, the onset temperature of the crystallization was lower than the reported supersolid transition temperature. Therefore, the supersolidity was found to be one of the necessary conditions for the crystallization to proceed and the stability of the crystal phase in the aerogel was another important condition that governed the crystallization. This result was the first indication of a strong connection between the crystallization in pores and the supersolidity of the surrounding crystals.