Porosity dependence of sound propagation in liquid $^4$He filled aerogel

TL;DR

This study investigates how the porosity of aerogel affects sound propagation in liquid helium-4, revealing the influence of aerogel on superfluid transition and phonon interactions.

Contribution

It provides a detailed analysis of sound velocity and absorption in aerogel-liquid helium-4 systems across different porosities, highlighting the role of phonon-strand collisions.

Findings

Superfluid transition identified as an absorption peak suppressed by aerogel.

No phonon-roton interaction peak observed at 1 K in porous aerogel.

Phonon-aerogel strand collisions significantly affect acoustic properties.

Abstract

Longitudinal sound wave propagation has been studied in an aerogel-liquid $^4$He system for various porosities of aerogel. The superfluid transition was identified as the absorption peak, whose magnitude was suppressed by aerogel. The sound velocity was analyzed within a hydrodynamic theory in both normal and superfluid phases. The absorption peak due to phonon-roton interaction around 1 K was not observed even with the most porous aerogel. The low temperature sound velocity and attenuation show that direct collisions of phonons with aerogel strands plays an important role in the acoustic properties.