Doping Dependence Of The Superconductivity Of (Ca1-Xnax)Fe2as2

Single-crystalline CaFe2As2 and (Ca1-xNax)Fe2As2 polycrystals (0 < x < 0.66) are synthesized and characterized using structural, magnetic, electronic transport, and heat capacity measurements. These measurements show that the structural/magnetic phase transition in CaFe2As2 at 165 K is monotonically suppressed by the Na doping and that superconductivity can be realized over a wide doping region. For 0.3 < x < 0.36, the magnetic susceptibilities indicate the possible coexistence of the spin density wave (SDW) and superconductivity. Superconducting phases corresponding to the Na doping level in (Ca1-xNax)Fe2As2 for nominal x = 0.36, 0.4, 0.5, 0.6, and 0.66, with Tc = 17, 19, 22, 33, and 34 K, respectively, are identified. The effects of the magnetic field on the superconductivity transitions for x = 0.66 samples are studied from which high upper critical fields H-c2 approximate to 103 T is deduced. A phase diagram of the SDW and superconductivity as a function of the doping level is thus presented.