Search For A Proximity Effect Induced Gap In Gold High-Tc Junctions

We have used high-resolution photoemission spectroscopy to search for a proximity effect induced superconducting gap in gold overlayers on c-axis single crystals of Bi2Sr2CaCu2O8 and a-axis thin films of YBa2Cu3O7. These two junction types give us a representative sampling of very well characterized near-ideal interfaces (gold/c-axis Bi2Sr2CaCu2O8) and junctions in which the geometry more strongly favors the existence of the proximity effect but the interfacial quality may not be as ideal (gold/a-axis YBa2Cu3O7). In neither of these junction types did we observe any evidence for a proximity effect induced gap, and we place an upper limit of approximately 5 meV on its existence in the junctions that we have studied.