Low frequency acoustics in solid $^4$He at low temperature

TL;DR

This study investigates the low frequency acoustic properties of solid helium-4 at very low temperatures, revealing temperature-dependent changes in elastic properties and highlighting the influence of sample history.

Contribution

It provides detailed measurements of elastic moduli and Poisson's ratio in solid helium-4, showing how these properties vary with temperature and sample preparation.

Findings

Poisson's ratio increases from 0.28 to 0.35 as temperature rises from below 20 mK to 0.7 K.

Sample history significantly affects the temperature dependence of elastic properties.

Crystals grown at the melting curve exhibit more abrupt changes at lower temperatures.

Abstract

The elastic properties of hcp $^4$He samples have been investigated using low frequency (20 Hz to 20 kHz) high sensitivity sound transducers. In agreement with the findings of other workers, most samples studied grew very significantly stiffer at low temperature; Poisson's ratio $\nu$ was observed to increase from 0.28 below 20 mK to $\sim 0.35$ at 0.7 K. The span of the variation of $\nu$ varies from sample to sample according to their thermal and mechanical history. Crystals carefully grown at the melting curve show a different behavior, the change in $\nu$ taking place at lower $T$ and being more abrupt.