Locally gauge-invariant spin response of He3-B films with Majorana surface states

A locally gauge-invariant theory of the spin response of a thin film of He3-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 …