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

TL;DR

This paper studies how antiferromagnetic order affects superconducting gaps in iron pnictides, revealing a transition from nodeless to nodal gaps driven by magnetic order and external magnetic fields, explaining recent experiments.

Contribution

It demonstrates how magnetic order reconstructs the Fermi surface, inducing angle-dependent pairing and explaining gap node formation in iron-based superconductors.

Findings

Superconducting gap remains nodeless at small magnetic order parameter M.

Accidental nodes can develop at intermediate M when Fermi surface pockets disappear.

External magnetic fields promote the formation of nodes in the superconducting gap.

Abstract

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 disappear. 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-x}K_xFe_2As_2. [J-Ph. Read et.al. arXiv:1105.2232].