Phase diagram and calorimetric properties of NaFe1-xCoxAs

We measured the resistivity and magnetic susceptibility to map out the phase diagram of single crystalline NaFe1-xCoxAs. Replacement of Fe by Co suppresses both the structural and magnetic transitions; however, it also enhances the superconducting transition temperature (T-c) and superconducting component fraction. Magnetic susceptibility exhibits temperature-linear dependence in high temperatures up to 500 K for all the superconducting samples, but such behavior suddenly breaks down for the nonsuperconducting overdoped crystal, suggesting that the superconductivity is closely related to the T-linear dependence of susceptibility. Analysis on the superconducting-state specific heat of the optimally doped crystal provides strong evidence for a two-band s-wave order parameter with gap amplitudes of Lambda(1)(0)/k(B)T(c) = 1.78 and Lambda(2)(0)/k(B)T(c) = 3.11, being consistent with the nodeless gap symmetry revealed by angle-resolved photoemission spectroscopy experiment.