The effects of interstitial oxygen on superconducting electronic phases in strontium and oxygen co-doped La_1.937Sr_0.063CuO_4+\delta

Strontium and oxygen co-doped La_1.937Sr_0.063CuO_4 + \delta superconductor with T_\rm c\approx 40 K, which is obtained by oxidizing strontium-doped starting ceramic sample La_1.937Sr_0.063CuO_4 in NaClO solution, is annealed under different conditions to allow interstitial oxygen to redistribute. The evolution of the intrinsic superconducting property with the oxygen redistribution is studied in detail by magnetic measurements in various fields. It is found that there occurs the electronic phase separation from the single superconducting phase with T_\rm c \approx40 K into two coexisting superconducting states with values of T_\rm c: 15 and 40 K or of 15 and 35 K in this system, depending on annealing condition. Our results indicate that the 15, 35 and 40 K superconducting phases associated with the excess oxygen redistribution are all thermodynamically meta-stable intrinsic states in this Sr/O co-doped cuprate.