Anomalous impurity effects in the iron-based superconductor   KFe$_2$As$_2$
  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. 
A. F. Wang, S. Y. Zhou, X. G. Luo, X. C. Hong, Y. J. Yan, J. J. Ying,   P. Cheng, G. J. Ye, Z. J. Xiang, S. Y. Li and X. H. Chen
cond-mat.supr-con cond-mat.str-el