Superconductivity with repulsion. A variational approach

We revisit the stability of the superconducting state within mean-field theory in the presence of repulsive pairing interactions, focusing on multiband systems where such channels naturally arise. We show that, when repulsion is present, the self-consistent BCS solution appears as a saddle point of the conventional mean-field free energy, casting doubt on its physical stability, but that this apparent pathology simply reflects the use of a non-variational functional. Recasting the problem with Bogoliubov’s variational principle restores a free energy that is bounded from below and places the BCS solution at a genuine minimum. Using a two-band toy model relevant to iron-based superconductors, we demonstrate the stability of the s± state and clarify how projection schemes that rely only on the interaction matrix can misidentify the attractive eigenmode that drives pairing. Our results clarify the instability issue highlighted by Aase et al. [Aase, Johnsen, and Sudbø, Phys. Rev. B 108, 024509 (2023)] and provide a consistent foundation for analyzing fluctuations in the presence of repulsive interactions.