A. F. Wang, X. G. Luo, Y. J. Yan, J. J. Ying, Z. J. Xiang, G. J. Ye, P. Cheng, Z. Y. Li, W. J. Hu, X. H. Chen
We measured the resistivity and magnetic susceptibility to map out the phase diagram of single crystalline NaFe$_{1-x}$Co$_x$As. The phase diagram is similar to other iron-pnictide systems, except for the absence of nonsuperconducting parent compound. Replacement of Fe by Co suppresses both the structural and magnetic transition while enhances the superconducting transition temperature ($T_{\rm c}$) and superconducting component fraction. Optimal doping level is $x$ = 0.028 and further doping of Co reduces $T_{\rm c}$. Magnetic susceptibility taken in high field exhibits linear dependence on temperature in high temperatures for the superconducting samples but deviation from the linear behavior can be observed below 200-300 K upon cooling. Analysis of the temperature dependence of the superconducting-state specific heat provides strong evidence for a two-band s-wave order parameter with gap amplitudes $\Delta_1(0)/k_{\rm B}T_{\rm c}$= 1.78 and $\Delta_2(0)/k_{\rm B}T_{\rm c}$=3.11.
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http://arxiv.org/abs/1204.3522
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