EVIDENCE FOR A JOSEPHSON TUNNEL CURRENT IN ALL IN-SITU C-YBA2CU3O7 PBJUNCTIONS/

All in situ YBu(2)Cu(3)O(7)/Pb and YBa2Cu3O7/Ag/Pb planar tunnel junct ions have been successfully made on c-axis-oriented YBa2Cu3O7 thin fil ms grown by a coevaporation technique, which show very-low-leakage I-V characteristics. The junctions are hysteretic and display a Josephson current I-c below 7.2 K (lead T-c). The only modification introduced by Ag is a substantial lowering of the normal-state resistance R(n), b ut the I(c)R(n) product remains roughly constant. The magnetic-field m odulation of I-c shows that the current distribution is rather homogen eous over the whole junction area (0.5 mm(2)), and follows the expecte d behavior for a short junction on a thin film (thickness <lambda(ab)) The Josephson coupling between a conventional superconductor and the high-T-c compound YBa2Cu3O7 along its c axis indicates that an s compo nent is present in the superconducting order parameter of this materia l.