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)Cu2O7-delta by precise oxygen adjustment, which is accomplished by heat treatment in high P(O2), followed by quenching. Superconductivity with T(c) almost-equal-to 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.