Abstract
The superconducting properties of oxygen annealed La 2CuO 4+δ has been studied. Three superconducting onsets have first been found, which depend on cooling process. When the sample was quenched down to 77 K from above 250 K, one superconducting onset was observed at 38 K and the zero resistance temperature was 30 K. On the other hand, when the sample was slowly cooled down to 77 K from 300 K or quenched from below 200 K, two superconducting onsets were observed at 42 K and 48 K. The three superconducting onsets are concluded to be ascribed to three superconducting phases, which probably have different lattice deformation by extra oxygens. The superconducting transition temperature of Sr doped sample is studied as a function of the Sr concentration and oxygen pressure of annealing.
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