Evidence for a Josephson tunnel current in all in situ c-YBa2Cu3O7/Pb junctions

Abstract

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}}$${\mathrm{R}}_{\mathrm{n}}$ product remains roughly constant. The magnetic-field modulation of ${\mathrm{I}}_{\mathrm{c}}$ shows that the current distribution is rather homogeneous over the whole junction area (0.5 ${\mathrm{mm}}^{2}$), and follows the expected behavior for a short junction on a thin film (thickness ${\ensuremath{\lambda}}_{\mathrm{ab}}$). The Josephson coupling between a conventional superconductor and the high-${\mathrm{T}}_{\mathrm{c}}$ compound ${\mathrm{YBa}}_{2}$${\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.