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
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 transition temperature $T_{\rm c}$ through electron doping, we find that Co doping suppresses the superconductivity very rapidly, with $T_{\rm c}$ down to zero at only $x$ = 0.042. Such an effective suppression of $T_{\rm c}$ by Co doping is distinctly different from that observed in Ba$_{0.5}$K$_{0.5}$Fe$_2$As$_2$ system. Thermal conductivity measurements show the universal heat conduction for $x$ = 0 and 0.034 superconducting samples, namely a nearly constant residual linear term $\kappa_0/T$ in zero field, despite that the scattering rate increases by 70 times. All these evidences support a d-wave superconducting state in KFe$_2$As$_2$.
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http://arxiv.org/abs/1206.2030
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