We have fabricated tunnel junctions between chemically etched single crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and evaporated metal counterelectrodes which exhibit reproducible characteristics. Above the bulk critical temperature of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$, $T_c$, the conductance, G(V), has a linear dependence with voltage and has some asymmetry. Below $T_c$, additional structure associated with the superconductivity 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 superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$. Junctions with Pb, Sn, Bi, Sb, PbBI, and Au counterelectrodes all show qualitatively similar behavior.

• Received 26 May 1989

DOI:https://doi.org/10.1103/PhysRevLett.63.1008