Disruption, segregation, and passivation for Pd and noble-metal overlayers on YBa2Cu3O6.9

Abstract

We have investigated interfaces formed when Pd and the noble metals Cu, Ag, and Au are deposited onto polycrystalline samples of Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.9}$ fractured in an ultrahigh vacuum. Synchrotron-radiation photoemission results show that Cu and Pd overlayers leach oxygen from the underlying Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.9}$ substrate, disrupt the superconductor, and destroy electronic states near the Fermi level. Interface reactions become kinetically limited at room temperature after the deposition of ∼4 Å of Cu or Pd, significantly sooner than for the reactive metals Fe, Al, Ti, and In. The presence of Ba near the surface after the deposition of more than 100 Å of Cu and Pd reflects substrate disruption and subsequent surface segregation. In contrast, overlayers of Ag and Au do not disrupt the superconductor substrate, no segregation is observed, but the overlayers are nonuniform and the quality of passivation is in question for coverages <100 Å.