Effective action for the superfluid Fermi gas

In this paper we revisit the derivation of the Landau expansion for superfluid 3He. Following Nijhoff, Capel and Den Breems we adopt a model interaction which contains an L = 1 pairing interaction and a contact term of the Hubbard type. We formulate the problem in the language of the path integral and construct the Gibbs free energy as a functional of the order parameter. In the mean-field approximation there is agreement with the exact results of Capel et al. Applying standard perturbation theory, we show that the contribution of the Hubbard interaction to the Landau coefficients is represented by a sum of 1PI diagrams.