Observation of Josephson pair tunneling between a high-${\mathit{T}}_{\mathit{c}}$ cuprate (${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$) and a conventional superconductor (Pb)

Josephson tunneling currents have been observed on low leakage Pb/insulator/${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Pr}}_{\mathit{x}}$ ${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ tunnel junctions. The junctions are planar with tunneling into the c axis. We have studied these Josephson currents on junctions with resistances varying by more than 2 orders of magnitude. From the magnetic field modulation of the Josephson current, the low temperature value and the temperature dependence from 1.3 to 4.2 K of the c-axis penetration depth were measured. This observation of Josephson coupling along the c axis between a conventional superconductor (Pb) and a cuprate raises questions about the proposed d-wave nature of the superconducting order parameter in the cuprates.