Coulomb problem in iron-based superconductors

We discuss the role of strong Coulomb interactions in iron-based superconductors (FeSCs). The presumed s± character of these superconductors means that the condensate is not symmetry protected against Coulomb repulsion. Remarkably, the transition temperatures and the excitation gap are quite robust across the large family of iron-based superconductors, despite drastic changes in Fermi-surface geometry. The Coulomb problem is to understand how these superconductors avoid the strong on-site Coulomb interaction of the iron atoms, while maintaining a robust transition temperature. Within the dominant space of t2g orbitals, on-site repulsion in the FeSCs forces two linearly independent components of the condensate to vanish. This raises the possibility that iron-based superconductors might adapt their condensate to the Coulomb constraints by rotating the pairing state within the large manifold of entangled extended s-wave gap functions with different orbital and momentum space structure. We examine this "orbital and k-space flexibility" (OKF) mechanism using both Landau theory and microscopic calculations within a multiorbital t-J model. Based on our results, we conclude that OKF necessitates a large condensate degeneracy. One interesting possibility raised by our results is that a resolution to the Coulomb problem in FeSCs might require a reconsideration of triplet pairing.