# Hall Viscosity and the Acoustic Faraday Effect

## TL;DR

This paper proposes that the acoustic Faraday effect in three-dimensional elastic media can serve as a direct probe of Hall viscosity, demonstrated through a phenomenological model of superfluid helium-3 and potential applications in other materials.

## Contribution

It establishes a theoretical link between the acoustic Faraday effect and Hall viscosity, suggesting a new method for its experimental detection in superfluid helium-3 and similar systems.

## Key findings

- Hall viscosity induces circular birefringence in transverse acoustic waves.
- The Zeeman effect can generate a non-zero Hall viscosity coefficient.
- The acoustic Faraday effect can be used to observe Hall viscosity directly.

## Abstract

For more than 20 years, observation of the non-dissipative Hall viscosity in the quantum Hall effect has been impeded by the difficulty to probe directly the momentum of the two-dimensional electron gas. However, in three-dimensional systems such as superfluid ${}^{3}\mathrm{He}\!\!-\!\!\mathrm{B}$, the momentum density is readily probed through transverse acoustic waves. We show that in a three-dimensional elastic medium supporting transverse waves, a non-vanishing Hall viscosity induces circular birefringence. Such an effect has been observed in ${}^{3}\mathrm{He}\!\!-\!\!\mathrm{B}$ in the presence of a weak magnetic field, and is known as the acoustic Faraday effect. The acoustic Faraday effect has been understood in terms of the Zeeman splitting of the excited order parameter modes which support the transverse wave propagation in the superfluid. We show that the Zeeman effect can generically lead to a non-zero Hall viscosity coefficient, and confirm this prediction using a simple phenomenological model for the ${}^{3}\mathrm{He}\!\!-\!\!\mathrm{B}$ collective modes. Therefore, we claim that the observation of the acoustic Faraday effect can be leveraged to make a direct observation of the Hall viscosity in superfluid ${}^{3}\mathrm{He}\!\!-\!\!\mathrm{B}$ in a magnetic field and other systems such as the crystalline $\mathrm{Tb}_{3}\mathrm{Ga}_{5}\mathrm{O}_{12}$ material.