Nematicity driven by hybridization in iron-based superconductors

In this Rapid Communication, we study an effective model for the normal state of iron-based superconductors. It has separate but interacting itinerant and localized degrees of freedom, originating from the d(xz) and d(yz) and from the d(xy) iron orbitals, respectively. At low temperatures, below a mean-field phase transition, these different states condense together in an excitonic order parameter. We show that, at even lower temperatures, after another phase transition, this ordered state can spontaneously break the C-4 lattice symmetry and become nematic. We propose this mechanism as an explanation of the tendency towards nematicity observed in several iron-based compounds.