INFLUENCE OF BIAS VOLTAGE HISTORY ON CONDUCTANCE PROPERTIES OF YBACUONORMAL METAL JUNCTIONS/

We have investigated I-V characteristics and their derivatives in YBa2 Cu3O7-x/normal metal planar and point contact junctions in the voltage range up to +/-2 V. Both increases and decreases of the junction resi stance above +0.5 V and below -0.5 V (related to metal electrode), res pectively, were measured. These changes were reproducibly repeated sev eral times on the same contact, and we ascribe them to oxygen replacem ent in the tunneling barrier arising from degraded YBa2Cu3O7-x unit ce lls in the vicinity of the contact. The differential conductance of th ese junctions showed both a linear and a quadratic dependence with res pect to applied voltage polarity. These features are described in term s of inelastic processes due to strong spin interactions in the barrie r caused by oxygen replacement in unit cells of YBa2Cu3O7-x. We model the linear and the quadratic background as a superposition of elastic and inelastic transport of quasiparticles through the barrier. An abru pt change of the slope of the linear part of the differential conducta nce was found above temperature 150 K. (C) 1998 Elsevier Science B.V. All rights reserved.