Anisotropic pairing in the iron pnictides

TL;DR

This paper investigates how spin fluctuation-induced pairing gaps vary anisotropically across the Fermi surface in iron pnictide superconductors, influenced by exchange and Hund's coupling, revealing conditions for fully gapped or nodal states.

Contribution

It provides a detailed analysis of the anisotropic pairing gap structure as a function of Hund's coupling and magnetic fluctuations, highlighting the conditions for fully gapped and nodal states in iron pnictides.

Findings

Large Hund's coupling leads to a nearly isotropic, fully gapped $s^{}$ state.

Smaller Hund's coupling results in incommensurate fluctuations and line nodes in the gap.

Variation of the gap amplitude across Fermi surfaces matches ARPES experimental observations.

Abstract

We determine the anisotropy of the spin fluctuation induced pairing gap on the Fermi surface of the FeAs based superconductors as function of the exchange and Hund's coupling $J_{H}$. We find that for sufficiently large $J_{H}$, nearly commensurate magnetic fluctuations yield a fully gapped $s^{\pm}$-pairing state with small anisotropy of the gap amplitude on each Fermi surface sheet, but significant variations of the gap amplitude for different sheets of the Fermi surface. In particular, we obtain the large variation of the gap amplitude on different Fermi surface sheets, as seen in ARPES experiments. For smaller values of Hund's coupling incommensurate magnetic fluctuations yield an $s^{\pm}$-pairing state with line nodes. Such a state is also possible once the anisotropy of the material is reduced and three dimensional effects come into play.