Crystal structure and superconducting properties of hole-doped Ca_0.89Na_0.11FFeAs single crystals

We report for the first time on superconductivity achieved upon hole doping when Ca²⁺ is partially substituted with Na⁺ in CaFFeAs. Single crystals of Ca_0.89Na_0.11FFeAs up to 2 × 2 mm² in size with T_c = 34.5 K were grown from NaCl flux. The superconducting transition temperature, with a narrow width ∼2 K, confirmed by both magnetic and resistivity measurements, indicates the high quality of our samples. Magnetic relaxation measurements show that the flux creep in this material is larger than in other iron-based superconductors. The existence of the wide magnetic reversible region and the pronounced flux-creep effect can be attributed to the higher anisotropy of this material. (paper)