Electron tunneling into the high-Tcsuperconductor Y-Ba-Cu-O

We have made tunneling measurements on junctions made on a polished surface of an Y-Ba-Cu-O pellet with an evaporated Pb layer as counterelectrode. We observe a Zeller-Giaever-like tunneling characteristic with a superconducting energy gap $\Delta =6\mathrm{}$ meV. This value is much smaller than the gap seen in point-contact tunneling experiments but is consistent with the gap found from infrared reflectivity measurements. We argue that this difference is due to the different superconducting properties of the surface layer and of the bulk of the material.