A new sound mode in liquid Helium 4?

TL;DR

This paper hypothesizes a small entropy in superfluid helium-4, predicts a new 'sixth sound' mode in a superleak, and proposes an experiment to measure its velocity to determine the superfluid entropy.

Contribution

It introduces the concept of sixth sound arising from superfluid entropy and suggests an experimental method to measure this entropy in liquid helium-4.

Findings

Prediction of a new sixth sound mode in superfluid helium-4

Proposal of an experiment to measure superfluid entropy via sound velocity

Potential to set an upper limit on superfluid entropy through negative results

Abstract

This letter is based on the hypothesis of a small entropy content of the superfluid fraction of liquid helium. We show that such a superfluid entropy gives rise to a new sound mode in a ring-shaped superleak. This mode is named sixth sound. We propose an experiment by which its sound velocity and thereby the superfluid entropy can be measured. A negative experiment would yield a new upper limit for the superfluid entropy.