Ultrasound study of the solid-liquid transition and solid-liquid interface of 4He in aerogels

The freezing and melting of 4He in various aerogels with porosities ranging from 92 to 97% were studied using longitudinal ultrasound. The freezing pressure, detected from changes in velocity and attenuation, was elevated compared with that of the bulk by about 0.3 MPa, and showed a weak dependence on aerogel porosity. Inside the aerogels, there was a liquid phase, a solid phase or a coexisting state depending on temperature and pressure of the sample. We report the measurement 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 that at the bulk interface. This indicates that sound attenuation at the interface is due to the disorder, originating from silica strands. Small amounts of 3He in the solid significantly decreased attenuation, because the resultant pinning of dislocations by 3He suppressed phonon scattering, as is observed in bulk solid 4He.