Zero bias conductance peaks in high-T-c superconductors: clues and ambiguities of two mutually excluding models

The origin of the zero bias conductance peak (ZBCP) observed in the tunneling characteristics of different high-T-c based junctions, is analyzed in the framework of the Anderson-Applebaum theory for Kondo type scattering from magnetic impurities at the barrier or in terms of midgap states induced by proximity effect at the surface of a d(x2-y2) wave superconductor. The two models mutually exclude each other since they describe a normal state or a superconducting state effect, respectively. We show that the appearance of the anomaly above or below the junction T-c, is the main indication that enable us to distinguish between the two cases. In addition to this, in the most controversial situations, the simultaneous presence at low temperatures of dips on both sides of the central structure as well as of conductance maxima at the expected values of the main high-T-c superconductor(HTS) energy gap, can be considered as indications of d-wave superconductivity. The case of ZBCP's that for decreasing temperatures merge in conductance minima, is also discussed.