Effects of quasiparticle bound states in the tunneling characteristics of YBa2Cu3O7 readjustable junctions

By carefully adjusting the distance between the two fractured electrodes, we have investigated the in-plane conductance characteristics of YBa2Cu3O7 break junctions in different tunneling regimes. In the q-p regime we have found that the conductance spectra strongly depend on the misorientation angle between the tunneling current and the YBa2Cu3O7 crystal axis. When the tunneling current along the a-a or b-b axis was favored, conductance maxima and shoulders around +/- 40 and +/- 20 mV have been measured at low temperatures. For different misorientation angles, we have found the presence of a zero bias conductance peak (ZBCP) that develops at the junction T-c as well as gap related structures at about +/- 20 mV. The amplitude of the ZBCP diverges as 1/T for temperatures below 40 K. All these features are consistent with a d-wave model for the high-T-c superconductors.