Optimization of superconducting transition temperature in (Pb_0.7Cu_0.3)Sr₂(Ca_0.5Y_0.5)Cu₂O_7+δ by postannealing

We demonstrate systematic studies of the optimization of the superconducting transition temperature in (Pb_0.7Cu_0.3)Sr₂(Ca_0.5Y_0.5)Cu₂O_7+δ by annealing in an oxygen atmosphere. As-sintered samples with a superconducting midpoint transition temperature, T_c(midpoint), of 18 K were prepared by solid state reaction at 1000°C for 1 hr in oxygen and then quenched in air. Subsequently, the samples were annealed in the temperature range between 700 and 1000°C for 12 hr in oxygen and then rapidly quenched in air. The ultimate T_c(midpoint) increased from 30 to 48 K with an increase in the annealing temperatures from 700 to 800°C, but a further increase in the annealing temperature of the samples from 800 to 1000°C resulted in a decrease in T_c(midpoint). Based on our studies, we believe that the optimum T_c(midpoint) of 48 K arises from the removal of excess oxygen in the insulating slabs of (Pb,Cu)O in (Pb_0.7Cu_0.3)Sr₂ (Ca_0.5Y_0.5)Cu₂O_7+δ and this leads the material to approach an optimum hole concentration.