Consistent picture of the octet-nodal gap and its evolution with doping in heavily overdoped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$

TL;DR

This paper explains the octet-nodal gap structure and its doping evolution in heavily overdoped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ using a spin-fluctuation model, highlighting the role of electron scattering and Fermi surface changes.

Contribution

It provides a unified explanation for the octet nodes and gap evolution in overdoped iron-based superconductors based on spin fluctuations and Fermi surface topology.

Findings

Octet nodes explained by spin-fluctuation mechanism.

Fermi-surface-dependent multi-gap structure identified.

D-wave pairing becomes dominant near Lifshitz transition.

Abstract

We investigate the pairing symmetry in heavily overdoped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ based on the spin-fluctuation mechanism. The exotic octet nodes of the superconducting gap and the unusual evolution of the gap with doping observed by the recent experiments are well explained in a unified manner. We demonstrate that the scatterings of electrons on the Fermi patches is mainly responsible for the incommensurate spin fluctuations and consequently the Fermi-surface-dependent multi-gap structure, since the Fermi level is close to the flat band. In addition, we find that a $d$-wave pairing state will prevail over the s-wave pairing state around the Lifshitz transition point.