Gap nodes induced by coexistence with antiferromagnetism in iron-based superconductors

We investigate the pairing in iron pnictides in the coexistence phase, which displays both superconducting and antiferromagnetic orders. By solving the pairing problem on the Fermi surface reconstructed by long-range magnetic order, we find that the pairing interaction necessarily becomes angle dependent, even if it was isotropic in the paramagnetic phase, which results in an angular variation of the superconducting gap along the Fermi surfaces. We find that the gap has no nodes for a small antiferromagnetic order parameter M, but may develop accidental nodes for intermediate values of M, when one pair of the reconstructed Fermi surface pockets disappears. For even larger M, when the other pair of reconstructed Fermi pockets is gapped by long-range magnetic order, superconductivity still exists, but the quasiparticle spectrum becomes nodeless again. We also show that the application of an external magnetic field facilitates the formation of nodes. We argue that this mechanism for a nodeless-nodal-nodeless transition explains recent thermal conductivity measurements of hole-doped (Ba _1-xK _x)Fe ₂As ₂.