ELECTRON-TUNNELING INTO SINGLE-CRYSTALS OF YBA2CU3O7-DELTA

We have fabricated tunnel junctions between chemically etched single crystals of YBa2Cu3O7-delta and evaporated metal counterelectrodes that exhibit reproducible characteristics. Above the bulk critical temperature of YBa2Cu3O7-delta, T(c), the conductance as a function of voltage, G(V), has a linear dependence on voltage and some asymmetry. Below T(c), additional structure associated with the superconducting state appears in G(V). At T << T(c) there is a reproducible finite zero-bias conductance, which suggests that there are states at the Fermi energy in superconducting YBa2Cu3O7-delta. Junctions with Pb, Sn, Bi, Sb, PbBi, and Au counterelectrodes all show qualitatively similar behavior.