Functional Renormalization Group Analysis of $\eta$-Pairing in Iron-based Superconductors
Using functional renormalization group (FRG) approach, we analyze the $\eta$ pairing in a five d-orbital model for iron-pncitides. We find that the $\eta$ pairing between two hole pockets is a leading instability at low energy when the two hole pockets are close to a nesting condition. The result suggests that the system can spontaneously enter into a superconducting state with both normal pairing and $\eta$ paring and the $\eta$ pairing is driven by the inter-orbital antiferromagnetic exchange coupling while the normal pairing is driven by the intra-orbital antiferromagnetic ones. An effective model at low energy is proposed to account for the $\eta$ pairing instability.