Growth and characterization of superconducting Ca_1-xNa_xFe₂As₂ single crystals by NaAs-flux method

High-quality superconducting Ca_1-xNa_xFe_2As_2 single crystals have been successfully grown by the NaAs-flux method, with sodium doping level x = 0.4-0.64. The typical sizes of these crystals are more than 10 mm in ab-plane and \sim 0.1 mm along c-axis in thickness. X-ray diffraction, resistance and magnetization measurements are carried out to characterize the quality of these crystals. While no signature of magnetic phase transitions is detected in the normal state, bulk superconductivity is found for these samples, with a sharp transition at T_\rm c ranging from 19.8 K (x = 0.4) to 34.8 K (x = 0.64). The doping dependences of the c-axis parameter and T_\rm c are consistent with previous reports, suggesting a possible connection between the lattice parameters and superconductivity.