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

Research paper by 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, X. H. Chen

Indexed on: 21 Jan '14Published on: 21 Jan '14Published in: Physics - Superconductivity


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.