LOWER CRITICAL-FIELD AND SURFACE-BARRIER IN SINTERED BI2SR2CACU2O8+DELTA SUPERCONDUCTOR

The saturated magnetization curves of a sintered Bi2Sr2CaCu2O8+delta s uperconductor are measured at different temperatures below T(c). The d ata are fitted by an extended critical-state model, so that the lower critical field, H(c1), and the first field for vortex entry, H-1, are obtained. With decreasing temperature, H(c1) increases almost linearly whereas H-1 > H(c1) below 50 K. Above 50 K, Bean-Livingston surface b arriers disappear and the M(H) curves show anomalies that suggest that the critical-state model is not valid.