Hysteresis in AC magnetic susceptibility and onset of field penetration in bulk granular YBa2Cu3Oy superconductors

Hysteresis in AC magnetic susceptibility of bulk granular YBa2Cu3Oy superconductors was investigated as a function of DC magnetic field and temperature. The pronounced irreversible behavior in the low field field regime allows a new approach to determine the onset of magnetic field penetration and an averaged lower critical field of the superconducting grains. A complete field-screening state in the intergranular region was observed for the first time. The temperature dependence of the lower critical field in both the intra- and intergranular regions is consistent with the BCS pairing mechanism with a modified gap parameter 2Δ(0)=7.0 kTc. (orig.)