The freezing and melting of 4He in various aerogels with porosities ranging from 92 to 97% was studied using longitudinal ultrasound. The freezing pressure, detected by a change in velocity and attenuation, was elevated from that of bulk by about 0.3 MPa, and showed little dependence on aerogel porosity. We report measurements of the transmission of sound through the solid-liquid interface in aerogel, for the first time, which was independent of temperature and unaffected by a small addition of 3He, unlike the bulk interface. This indicates that crystal growth was suppressed by disorder, originating from silica strands. However, small amounts of 3He in the solid significantly decreased the attenuation, because the resultant pinning of dislocations by 3He suppressed phonon scattering, as is observed in bulk solid 4He.
@article{arxiv.0806.3816,
title = {Ultrasound study of the solid-liquid transition and solid-liquid interface of $^4$He in aerogels},
author = {Koichi Matsumoto and Hiroyuki Tsuboya and Keiichi Yoshino and Satoshi Abe and Hiroyuki Tsujii and Haruhiko Suzuki},
journal= {arXiv preprint arXiv:0806.3816},
year = {2009}
}