Superconducting properties in La2CuO4+δ with excess oxygen

Abstract

We have studied the thermal history dependence of the superconducting properties below and above room temperature (RT) in the ceramic La2CuO4+δ with excess oxygen, which were prepared by annealing in high-pressure oxygen gas or electrochemical oxidation. The diffusion of the excess oxygen is concluded not to occur below RT. In the sample with small δ, the low Tc (32 or 36 K) superconducting phase appears, of which Tc is changed by the annealing at about 200 K. This change is presumably due to some phase transition without the diffusion of the excess oxygen. On the other hand, when δ reaches some threshold, another single superconducting phase with high Tc (45 K), which is stable below 373 K at least, appears. This sample was prepared by the electrochemical oxidation at 333 K for more than 48 h, where the excess oxygen can diffuse. In the sample electrochemically oxidized at RT, both the low- and high-Tc superconducting phases were formed only near the surface area of the sample. These phases were homogenized by annealing at 373 K.

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