AC SUSCEPTIBILITY OF YBA2CU3O7-DELTA SINGLE-CRYSTALS

We studied the mixed state ac response of a twinned YBa2Cu3O7-delta si ngle crystal. We observed a dip structure in real part and a sharp pea k in the imaginary part of the ac susceptibility at a well defined tem perature T-M. The de magnetization also shows a kink at the same tempe rature. These are interpreted as a precursor of the critical current p eak effect, whose origin is thought to be the thermal softening of the elastic moduli as the flux line lattice melts. The peak effect is acc ompanied by an unexpected magnetic history dependence of the ac suscep tibility between 50 K and T-M, which is interpreted as the indication of flux line defects. The anomalous chi falls on an universal chi'-chi '' curve, which is different from either the diffusion model or the Be an model. The peak position agrees with the reported first order melti ng line and thus might be the onset temperature of plastic motion of f lux line lattice.