Zn-impurity effect and interplay of $s_{\pm}$- and $s_{++}$-pairings in Fe-based superconductors

TL;DR

This paper investigates how Zn impurities affect different pairing mechanisms in Fe-based superconductors, combining theoretical modeling and experimental data to reveal impurity-induced transitions between pairing states.

Contribution

It introduces a model that captures the competition between $s_{++}$ and $s_{ ext{pm}}$ pairing, showing how Zn impurities suppress $s_{ ext{pm}}$ and can induce a transition to $s_{++}$ pairing.

Findings

Zn-impurity suppresses $s_{\pm}$ pairing significantly.

Experimental data supports impurity-induced pairing transition.

Theoretical model aligns with observed impurity effects.

Abstract

We report theoretical and experimental studies of the effect of Zn-impurity in Fe-based superconductors. Zn-impurity is expected to severely suppress sign reversed s$_\pm$ wave pairing. The experimentally observed suppression of T$_c$ under Zn-doping strongly depends on the materials and the charge carrier contents, which suggests competition of $s_{++}$ and $s_{\pm}$ pairings in Fe-base superconductors. We study a model incorporating both $s_{++}$ and $s_{\pm}$ pairing couplings by using Bogoliubov de-Gennes equation, and show that the Zn-impurity strongly suppresses $s_{\pm}$ pairing and may induce a transition from $s_{\pm}$ to $s_{++}$-wave. Our theory is consistent with various experiments on the impurity effect. We present new experimental data on the Zn-doping SmFe$_{1-x}$Zn$_x$AsO$_{0.9}$F$_{0.1}$ of T$_c=$ 50K, in further support of our proposal.