ZERO-BIAS CONDUCTANCE PEAKS IN HIGH-T-C SUPERCONDUCTORS - CLUES AND AMBIGUITIES OF 2 MUTUALLY EXCLUDING MODELS

The origin of the zero bias conductance peak (ZBCP) observed in the tu nneling characteristics of different high-T-c based junctions, is anal yzed in the framework of the Anderson-Applebaum theory for Kondo type scattering from magnetic impurities at the barrier or in terms of midg ap states induced by proximity effect at the surface of a d(x2-y2) wav e superconductor. The two models mutually exclude each other since the y describe a normal state or a superconducting state effect, respectiv ely. We show that the appearance of the anomaly above or below the jun ction T-c, is the main indication that enable us to distinguish betwee n the two cases. In addition to this, in the most controversial situat ions, the simultaneous presence at low temperatures of dips on both si des of the central structure as well as of conductance maxima at the e xpected values of the main high-T-c superconductor(HTS) energy gap, ca n be considered as indications of d-wave superconductivity. The case o f ZBCP's that for decreasing temperatures merge in conductance minima, is also discussed. (C) 1998 Elsevier Science B.V. All rights reserved .