Locally gauge-invariant spin response of $^3$He-$B$ films with Majorana surface states

TL;DR

This paper develops a gauge-invariant theory for the spin response of $^3$He-$B$ films, revealing how Majorana surface states and collective modes interact, with implications for NMR experiments.

Contribution

It introduces a gauge-invariant framework that accounts for fluctuation effects and Majorana states, improving upon mean-field predictions of spin response in $^3$He-$B$ films.

Findings

Spin response is suppressed above the collective mode frequency.

Majorana states cause temperature- and magnetic-field-dependent damping.

Damping effects are observable in ultra-low temperature NMR experiments.

Abstract

A locally gauge-invariant theory of the spin response of a thin film of $^3$He-$B$ film is given that describes fluctuation effects arising from the coupled dynamics of the superconducting order parameter (the collective mode) and in-gap Majorana surface states. In contrast to a mean-field calculation of the spin response, which predicts a nonzero imaginary longitudinal spin susceptibility at frequencies inside the bulk gap due to absorption from the Majorana states, our gauge-invariant theory shows that this response is strongly suppressed above the collective mode frequency and vanishes if dipole-dipole interactions are neglected. In the presence of dipole-dipole interactions, in sufficiently thin films, and at ultra-low temperatures, the Majorana states lead to a distinctive magnetic-field- and temperature-dependent damping of the collective mode, a feature that may be observable in longitudinal NMR experiments.