Competing order and nature of the pairing state in the iron pnictides

TL;DR

This paper investigates how the competition between magnetism and superconductivity in iron pnictides reveals the nature of their pairing state, favoring the sign-changing $s^{+-}$ pairing over the conventional $s^{++}$.

Contribution

It demonstrates that the competition between magnetic order and superconductivity can determine the pairing symmetry in iron arsenides, highlighting the role of static magnetism as an intrinsic interband Josephson coupling.

Findings

AFM and $s^{+-}$ SC can microscopically coexist or phase separate, matching experiments.

Conventional $s^{++}$ pairing cannot coexist with magnetism.

The phase diagram's sensitivity to pairing symmetry is linked to the quasiparticle spectrum and SO(5) symmetry.

Abstract

We show that the competition between magnetism and superconductivity can be used to determine the pairing state in the iron arsenides. To this end we demonstrate that the itinerant antiferromagnetic phase (AFM) and the unconventional $s^{+-}$ sign-changing superconducting state (SC) are near the borderline of microscopic coexistence and macroscopic phase separation, explaining the experimentally observed competition of both ordered states. In contrast, conventional $s^{++}$ pairing is not able to coexist with magnetism. Expanding the microscopic free energy of the system with competing orders around the multicritical point, we find that static magnetism plays the role of an intrinsic interband Josephson coupling, making the phase diagram sensitive to the symmetry of the Cooper pair wavefunction. We relate this result to the quasiparticle excitation spectrum and to the emergent SO$(5)$ symmetry of systems with particle-hole symmetry. Our results rely on the assumption that the same electrons that form the ordered moment contribute to the superconducting condensate and that the system is close to particle-hole symmetry. We also compare the suppression of SC in different regions of the FeAs phase diagram, showing that while in the underdoped side it is due to the competition with AFM, in the overdoped side it is related to the disappearance of pockets from the Fermi surface.