Superconducting properties inLa2CuO4+δwith excess oxygen

Abstract

We have studied the thermal history dependence of the superconducting properties below and above room temperature (RT) in the ceramic ${\mathrm{La}}_2{\mathrm{CuO}}_{4+\delta }$ 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 $T_c$ (32 or 36 K) superconducting phase appears, of which $T_c$ 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 $T_c$ (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-$T_c$ superconducting phases were formed only near the surface area of the sample. These phases were homogenized by annealing at 373 K.