Abstract
Diamagnetic moments for two Bi2Sr2CaCu2O8 + δ crystals were measured at different fields H and temperatures. For the higher fields two distinct transition temperatures Tg and TJ are seen, with Tg > TJ. By increasing H the line Tg(H) shifts very slowly while TJ(H) shifts much faster to lower temperatures, displaying a clear upward curvature well described by a theory based on Josephson coupling between superconducting clusters. We show further that TJ(H) is dependent on sample homogeneity, which is correlated with oxygen distribution in the high-Tc superconductors.