Superconductivity at 67 K in (Pb,Cu)Sr₂(Ca,Y)Cu₂O₇ by precise adjustment of oxygen

We demonstrate that high-temperature superconductivity (HTSC) with T_c=67 K can be induced in (Pb,Cu)Sr₂(Y,Ca)Cu₂O_7-δ by precise oxygen adjustment, which is accomplished by heat treatment in high P(O₂), followed by quenching. Superconductivity with T_c≊17 K was previously reported in this compound obtained by quenching from high temperature in 1 atm of oxygen. Since quenching leaves less oxygen than in annealed samples, it appears paradoxical that HTSC can be induced in this compound by high-pressure oxygen treatment. We find that, while oxidation in high P(O₂) produces HTSC, excess oxygen taken up during subsequent annealing at lower temperature destroys the superconductivity; so quenching is required for high T_c. Thermogravimetric analysis shows that weakly bound oxygen is taken up into several distinct sites, one of which is required for the appearance of HTSC. However, occupation of the most weakly bound sites is detrimental to HTSC, at least in (Pb,Cu)Sr₂(Y,Ca)Cu₂O₇. The appearance and then destruction of high-temperature superconductivity by adding successive amounts of oxygen in different sites may be a powerful tool to explore the mechanism of high-temperature superconductivity.