Effect of Oxygen Vacancies in the SrO Planes on the Tc(x) of Bi2Sr2 –xLaxCuO6 + δ Crystals

Abstract

We report the first systematic study of the effect of oxidizing annealing on the superconducting transition temperature T_c of Bi₂Sr_{2 –x}La_xCuO_{6 + δ} crystals. Using a standard heat treatment procedure, we have obtained detailed T_c(x) data for Bi₂Sr_{2 –x}La_xCuO_{6 + δ} crystals in the range x = 0.35–0.75. Comparison of the shape of the T_c(x) curve for Bi₂Sr_{2 –x}La_xCuO_{6 + δ} with that for La_{2 –x}Sr_xCuO₄ demonstrates a factor of 4 reduction in the dependence of hole concentration in the CuO₂ planes from a La³⁺ substitution on the Sr²⁺ sites becouse oxygen content (or index) varies. This finding can be accounted for in terms of changes in the amount of oxygen vacancies in the SrO planes. In particular, the oxygen index in Bi₂Sr_{2 –x}La_xCuO_{6 + δ} varies by 0.3 per formula unit.