LOWER CRITICAL-FIELD H-C1 AND BARRIERS FOR VORTEX ENTRY IN BI2SR2CACU2O8+DELTA CRYSTALS

The penetration field H-p of Bi2Sr2CaCu2O8+delta crystals is determine d from magnetization curves for different field sweep rates dH/dt and temperatures. The obtained results are consistent with theoretical rep orts in the literature about vortex creep over surface and geometrical barriers. The frequently observed low-temperature upturn of H-p is sh own to be related to metastable configurations due to barriers for vor tex entry. Data of the true lower critical field H-cl are presented. T he low-temperature dependence of H-cl is consistent with a superconduc ting state with nodes in the gap function. [S0031-9007(98)07346-3].