Electron tunneling into single crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

We have fabricated tunnel junctions between chemically etched single crystals of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ and evaporated metal counterelectrodes that exhibit reproducible characteristics. Above the bulk critical temperature of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$, ${\mathit{T}}_{\mathit{c}}$, the conductance as a function of voltage, G(V), has a linear dependence on voltage and some asymmetry. Below ${\mathit{T}}_{\mathit{c}}$, additional structure associated with the superconducting state appears in G(V). At T≪${\mathit{T}}_{\mathit{c}}$ there is a reproducible finite zero-bias conductance, which suggests that there are states at the Fermi energy in superconducting ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. Junctions with Pb, Sn, Bi, Sb, PbBi, and Au counterelectrodes all show qualitatively similar behavior.