Coexistence of Andreev bound states and Josephson current in YBa2Cu3O7 break-junctions

To favour the ab-plane contribution to the tunneling current, we have used the break junction technique on highly epitaxial (dc sputtered) YBa2Cu3O7 thin films, with T(c)greater than or equal to 91K and DeltaT(c)less than or equal to0.5K. One particularly intriguing feature of the conductance spectra is the presence of a zero bias peak that can be related to the formation of Andreev bound states (ABS) in the quasi-particle density of states, according to recent models about the d-wave symmetry of the HTS order parameter. We report data in which the Josephson current and the ABS are observed simultaneously appearing as a narrow conductance peak at zero bias overimposed on a wider one. The Josephson current shows modulation in magnetic field, typical of inhomogeneous current distribution due to the fracture faceting.