Conductance spectroscopy of high-Tc single-grain-boundary junctions in the YBa2Cu3O7-δ system

We have measured the conductance spectra of YBa2Cu3O7-δ thin-film grain-boundary junctions. For voltages between 2 and 12 meV there are two sets of sharp structures. One appears at nearly regular multiples of 2.4 mV, out to 9.5 mV; the conductance peak at 9.5 mV is split in a magnetic field parallel to the film ab planes. A second set of peaks with spacing 0.1-0.2 mV extends from low voltages to V∼11.5 mV; these peaks are insensitive to magnetic fields but merge into the background noise at about 50 K. At higher voltages (V>12 mV) peaks at roughly the phonon energies appear. We speculate on the origins of these features. © 1993 The American Physical Society.