Effect of Ag on the surface barrier for depletion of oxygen from polycrystalline YBa2Cu3O7 oxides

We have investigated the effect of evaporated Ag on the depletion of oxygen from polycrystalline YBa2Cu3O7- in a He ambient atmosphere by in situ resistivity experiments and surface spectroscopies. Our data indicate an increase in the rate of out-diffusion of oxygen due to the presence of the Ag, which lowers the surface barrier for the out-diffusion of oxygen by 0.20.03 eV. We have performed x-ray photoelectron spectroscopy (XPS) analysis of the in situ fractured surfaces to indicate the presence of Ag at the internal surfaces in the oxide. XPS data from Ag-free in situ scraped samples show oxygen accumulation in the near-surface region upon vacuum annealing. This effect is prevented by evaporation of a few monolayers of Ag. Changes in the cutoff energies of the ultraviolet photoelectron spectra indicate that a submonolayer coverage of Ag is sufficient to produce a change in the work function of 0.3 eV, corresponding to the lowering of the surface barrier.