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

TL;DR

This study investigates the solid-liquid transition and interface of helium-4 in aerogels using ultrasound, revealing effects of disorder and helium-3 on sound transmission and crystal growth.

Contribution

First measurement of sound transmission through the solid-liquid interface of helium-4 in aerogels, showing independence from temperature and helium-3 addition.

Findings

Freezing pressure is elevated in aerogels compared to bulk helium.

Sound transmission at the interface is unaffected by temperature and helium-3.

Helium-3 reduces attenuation by pinning dislocations, affecting phonon scattering.

Abstract

The freezing and melting of $^{4}$He 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 $^3$He, unlike the bulk interface. This indicates that crystal growth was suppressed by disorder, originating from silica strands. However, small amounts of $^3$He in the solid significantly decreased the attenuation, because the resultant pinning of dislocations by $^3$He suppressed phonon scattering, as is observed in bulk solid $^4$He.