Coexistence of superconductivity and a spin density wave in pnictides: Gap symmetry and nodal lines

TL;DR

This paper studies how spin-density waves affect superconductivity in pnictides, revealing that $s_{\pm}$ gaps remain nodeless in the SDW state, unlike regular $s_{++}$ gaps, providing a way to distinguish between them experimentally.

Contribution

It demonstrates that $s_{\pm}$ superconductivity remains nodeless in the SDW state, contrasting with $s_{++}$, and offers a new experimental method to differentiate these gap symmetries.

Findings

$s_{\pm}$ gaps stay sign-preserving and nodeless in SDW state.

$s_{++}$ gaps develop nodes in SDW state.

Thermal conductivity data supports $s_{\pm}$ over $s_{++}$.

Abstract

We investigate the effect of a spin-density wave (SDW) on $s_{\pm}$ superconductivity in Fe-based superconductors. We show that, contrary to the common wisdom, no nodes open at the new, reconnected Fermi surfaces when the hole and electron pockets fold down in the SDW state, despite the fact that the $s_{\pm}$ gap changes sign between the two pockets. Instead, the order parameter preserves its sign along the newly formed Fermi surfaces. The familiar experimental signatures of an $s_{\pm}$ symmetry are still preserved, although they appear in a mathematically different way. For a regular $s$ case ($s_{++})$ the nodes do appear in the SDW state. This distinction suggests a novel simple way to experimentally separate an $s_{\pm}$ state from a regular $s$ in the pnictides. We argue that recently published thermal conductivity data in the coexisting state are consistent with the $s_{\pm},$ but not the $s_{++}$ state.