Anomalous impurity effects in the iron-based superconductor KFe$_2$As$_2$

TL;DR

This study investigates how cobalt impurities affect the superconducting transition temperature and thermal conductivity in KFe$_2$As$_2$, revealing unusual impurity effects that differ from other iron-based superconductors.

Contribution

It provides new insights into impurity effects in KFe$_2$As$_2$, highlighting a rapid suppression of $T_c$ and minimal change in thermal conductivity despite increased scattering.

Findings

Co impurities rapidly suppress $T_c$ to zero at x ≈ 0.04

Residual thermal conductivity $\kappa_0/T$ remains nearly unchanged with doping

Impurity effects in KFe$_2$As$_2$ differ from those in other iron-based superconductors

Abstract

High-quality K(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals have been grown by using KAs flux method. Instead of increasing the superconducting transition temperature $T_{\rm c}$ through electron doping, we find that Co impurities rapidly suppress $T_{\rm c}$ down to zero at only $x \approx$ 0.04. Such an effective suppression of $T_{\rm c}$ by impurities is quite different from that observed in Ba$_{0.5}$K$_{0.5}$Fe$_2$As$_2$ with multiple nodeless superconducting gaps. Thermal conductivity measurements in zero field show that the residual linear term $\kappa_0/T$ only change slightly with $3.4\%$ Co doping, despite the sharp increase of scattering rate. The implications of these anomalous impurity effects are discussed.