Zero bias anomalies in YBa₂Cu₃O₇ tunnel junctions

Reproducible planar tunnel junctions based on high-quality YBa₂Cu₃O₇ thin films are studied as a junction of crystallographic orientation. Using Pb as a counter-electrode, all junctions exhibit zero bias anomalies, the sign of which depends upon the crystallographic orientation of the film. For c-axis, or (001) films, in which the CuO₂ planes are parallel to the junction surface, a 5 mV wide dip in the tunneling conductance, which is independent of applied magnetic field up to 5 T, is observed. Conversely, for the (103) and (100) orientations, in which the CuO₂ plane points toward the junction surface, a peak in the tunneling conductance, which is strongly magnetic-field dependent, is observed. Analysis is performed in terms of spin-flip and Kondo-type scattering at the interface (Anderson and Applebaum theory).