Evidence for a Josephson tunnel current in all in situ c-${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$/Pb junctions

All in situ ${\mathrm{YBu}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$/Pb and ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$/Ag/Pb planar tunnel junctions have been successfully made on c-axis-oriented ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ thin films grown by a coevaporation technique, which show very-low-leakage I-V characteristics. The junctions are hysteretic and display a Josephson current ${\mathrm{I}}_{\mathrm{c}}$ below 7.2 K (lead ${\mathrm{T}}_{\mathrm{c}}$). The only modification introduced by Ag is a substantial lowering of the normal-state resistance ${\mathrm{R}}_{\mathrm{n}}$, but the ${\mathrm{I}}_{\mathrm{c}}$ ${\mathrm{R}}_{\mathrm{n}}$ product remains roughly constant. The magnetic-field modulation of ${\mathrm{I}}_{\mathrm{c}}$ shows that the current distribution is rather homogeneous over the whole junction area (0.5 ${\mathrm{mm}}^2$), and follows the expected behavior for a short junction on a thin film (thickness <${\lambda }_{\mathrm{ab}}$). The Josephson coupling between a conventional superconductor and the high-${\mathrm{T}}_{\mathrm{c}}$ compound ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ along its c axis indicates that an s component is present in the superconducting order parameter of this material.