FIELD-INDUCED VARIATIONS OF THE MICROWAVE SURFACE IMPEDANCE OF YBA2CU3O7 CRYSTALS NEAR T-C

The microwave surface impedance Z(s) of a YBa2Cu3O7 single crystal is investigated as a function of the magnetic field H-0 at different temp eratures close to T-c. It Is shown that Z(s) depends on H-0 for applie d fields higher as well as lower than H-c1. A model is discussed in wh ich the field dependence of Z(s) below H-c1 is ascribed to the field-i nduced variations, within the penetration depth, of the partial concen trations of both normal and condensate electrons. The field dependence of Z(s) above H-c1 is explained using the Coffey and Clem model.