Effect of Annealing Temperature on the Properties of K 0.8 Fe 2+x Se 2

K0.8Fe2+xSe2 single crystals (x = -0.3, 0, 0.3) have been successfully grown by one-step method. The as-grown crystals all don’t show superconductivity; however, the superconductivity appears after annealing. The data of R-T and M-T indicate that the Fe- content and annealing-temperature have visibly effect on the superconducting properties of K0.8Fe2+xSe2 single crystals. The crystal of K0.8Fe2.3Se2, annealed at 400°C for 1 hour, demonstrates the best superconducting properties. Furthermore, the pinning potential of K0.8Fe2.3Se2 is visibly lower than that of BaK-122, indicating different pinning mechanism caused by large size non-superconducting phase in K0.8Fe2.3Se2.