TEMPERATURE-DEPENDENCE OF THE LOWER CRITICAL-FIELD H-C1 IN BI2SR2CACU2O8-CRYSTALS(DELTA SINGLE)

Using a non-commercial dc-SQUID magnetometer we have measured the firs t flux penetration field H-p of Bi2Sr2CaCu2O8+delta (Bi2212) crystals for H perpendicular to ab. The held H-p is found to strongly depend on the sweep rate of the magnetization curve dH/dt as a result of flux c reep through surface barriers. However, for low enough sweep rates, H- p saturates towards a constant value. We identify this value with the lower critical field H-c1. The obtained temperature dependence of H-c1 has an approximately linear behavior below T similar or equal to 30 K . This is consistent with a gap with line nodes on the Fermi surface. For a relatively high fixed sweep rate dH/dt, the temperature dependen ce of H-p displays a marked upturn for T less than or similar to 40 K. Such an upturn has often been reported in the literature for strongly anisotropic HTSCs. In this work we demonstrate that the upturn result s from metastable configurations.