Different resistivity response to spin-density wave and superconductivity at 20 K in Ca1-xNaxFe2As2

We report intrinsic transport and magnetic properties and their anisotropy in high-quality single-crystal CaFe2As2. The resistivity anisotropy (ρc/ρab) is ∼50, less than the 150 of BaFe2As2, which arises from the strong coupling along the c-axis due to an apparent contraction of about 0.13 nm compared to BaFe2As2. In sharp contrast to the case of parent compounds ROFeAs (R = rare earth) and MFe2As2 (M = Ba and Sr), spin-density-wave (SDW) ordering (or structural transition) leads to a steep increase of resistivity in CaFe2As2. Such a different resistivity response to SDW ordering is helpful to understand the role played by SDW ordering in Fe-based high-Tc superconductors. The susceptibility behavior is very similar to that observed in single-crystal BaFe2As2. A linear temperature-dependent susceptibility occurs above the SDW transition of about 165 K. Partial substitution of Na for Ca suppresses the SDW ordering (anomaly in resistivity) and induces superconductivity at ∼20 K. (fast track communication)