PARAMAGNETIC MEISSNER EFFECT IN YBA2CU3O7-DELTA - A NONRESONANT MICROWAVE-ABSORPTION STUDY

The low-field dependence of non resonant microwave absorption (MWA) in polycrystalline samples of YBa2Cu3O7-delta with 0.1 less-than-or-equal-to delta less-than-or-equal-to 0.5 has been measured at T < T(c) along with simultaneous monitoring of paramagnetic impurities in the chain copper (Cu-I) sites by EPR spectroscopy. MWA exhibits a cross-over from the normal zero-field minimum to an anomalous zero-field maximum accompanied by the gradual appearance and growth of the EPR signal as delta is changed from 0.1 to 0.5. It is to be noted that in the delta = 0.3-0.4 composition range, oxygen vacancies are disordered giving rise to magnetic moments due to isolated Cu2+ impurity ions. This correlation of the anomalous MWA with isolated paramagnetic Cu2+ impurities is consistent with the presence of pi junctions, earlier implicated in the paramagnetic Meissner effect (PME), which in turn is related to the anomalous MWA.