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

Authors :: Wang, A. F.
Zhou, S. Y.
Luo, X. G.
Hong, X. C.
Yan, Y. J.
Ying, J. J.
Cheng, P.
Ye, G. J.
Xiang, Z. J.
Li, S. Y.
Chen, X. H.
Source :: Phys. Rev. B 89, 064510 (2014)
Publication Year :: 2012
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.<br />Comment: 5 pages, 4 figs