Disorder effects in multiorbital $s_{\pm}$-wave superconductors: Implications for Zn-doped BaFe$_2$As$_2$ compounds

TL;DR

This study investigates how nonmagnetic Zn impurities affect the superconducting properties of $s_{}$-wave iron pnictides, revealing impurity-induced pair breaking and suppression of superconductivity consistent with recent experimental findings.

Contribution

It models Zn impurities as extended defects in a two-orbital superconductor, demonstrating their significant impact on superconductivity and challenging traditional theories.

Findings

Zn impurities cause a gradual gap filling in the density of states.

Superfluid density and order parameter are dramatically suppressed with impurity concentration.

Superconductivity is fully suppressed near 10% impurity concentration.

Abstract

Recent experiments on Zn-doped 122-type iron pnictides, Ba(Fe$_{1-x-y}$Co$_y$Zn$_x$)$_2$As$_2$, are challenging our understanding of electron doping the 122s and the interplay between doping and impurity scattering. To resolve this enigma, we investigate the disorder effects of nonmagnetic Zn impurities in the strong (unitary) scattering limit on various properties of the system in the $s_{\pm}$-wave superconducting pairing state. The lattice Bogoliubov-de Gennes equation (BdG) is solved self-consistently based on a minimal two-orbital model with an extended range of impurity concentrations. We find that Zn impurity is best modeled as a defect, where charge is mainly localized, but scattering is extended over a few lattice sites. With increasing Zn concentration the density of states shows a gradual filling of the gap, revealing the impurity-induced pair breaking effect. Moreover, both the disorder configuration-averaged superconducting order parameter and the superfluid density are dramatically suppressed towards the dirty limit, indicating the violation of the Anderson theorem for conventional s-wave superconductors and the breakdown of the Abrikosov-Gorkov theory for impurity-averaged Green's functions. Furthermore, we find that the superconducting phase is fully suppressed close to the critical impurity concentration of roughly $n_\text{imp}\approx 10%$, in agreement with recent experiments.