In heavily twinned crystals or films of YBa 2 Cu 3 O 72x (YBCO), d x 2 2y 2 pairing symmetry is expected to cause the cancellation of first-order Josephson tunneling through a YBCO-Pb tunnel junction grown on an a-b face; any residual tunneling is thus second order. As a result, microwaves at frequency f are predicted to induce steps on the current-voltage characteristic at voltages that are multiples of 1 2 ͑hf͞2e͒. Experimentally, steps are observed only at multiples of hf͞2e, suggesting that s-wave pairing is present in YBCO; however, the simultaneous presence of d-wave pairing is by no means ruled out. PACS numbers: 74.50.+r, 74.72.Bk The symmetry of the order parameter in the high transition temperature ͑T c ͒ oxide superconductors is the subject of ongoing debate. One school of thought proposes a d x 2 2y 2 symmetry (for example, [1,2]) while another proposes an anisotropic s-wave symmetry (for example, [3,4]). A series of experiments [5-8] has been performed on Josephson junctions made on the a-b faces of YBa 2 Cu 3 O 7-x (YBCO) crystals or films, with conventional low-T c superconductors as the counterelectrodes. These measurements provide evidence for a phase shift of p between the order parameters measured by transport of pairs across weak links along two orthogonal directions, building a strong case for d-wave pairing. Sun et al. [9], however, performed extensive measurements of tunneling along the c axis of YBCO and the alloys Y 12x Pr x Ba 2 Cu 3 O 7-d , using an insulating barrier and Pb as the counterelectrode. At temperatures near 1 K the current-voltage ͑I-V ͒ characteristics exhibited a Josephson supercurrent at zero voltage, and very small single-particle tunneling currents below the Pb gap. The application of a magnetic field parallel to the plane of the junction produced a well-defined Fraunhofer pattern in the critical current, and yielded values of the penetration depth l ab ͑T ͒ with a temperature dependence consistent with those obtained from earlier microwave measurements [10,11].In a single crystal with tetragonal symmetry and purely d x 2 2y 2 pairing, the first-order pair tunneling currents cancel along the c-axis direction. Any pair tunneling would have to be second order, and the current thus very weak. In an untwinned single crystal of YBCO, however, the orthorhombic crystal symmetry leads to asymmetric d-wave pairing so that the first-order c-axis pair tunneling is not expected to vanish. The situation is different yet again in a heavily twinned single crystal or thin film. Here, the phase of the order parameter is maintained throughout the sample, but the amplitude variation along the a and b axes is expected to ensure that the net first-order tunneling cur-rent in the c-axis direction is zero. We note that any tunneling transitions in the a-b directions would contribute only to second-order supercurrents; again, first-order components cancel by symmetry. Since the c-axis tunneling experiments demonstrating the existence of a nonzero Josephson supercurrent [9] were performed m...

Pb/I/YBCO tunnel junctions have been fabricated on ''ab edge'' surfaces of YBCO single crystals ͑the ''ab edge'' surface is parallel to c axis and normal to a or b axis͒. The characteristics of these ab junctions were compared to those of c-axis junctions. In addition, STM studies were carried out on c-axis YBCO surfaces ͑studying exposed ab planes͒. We conclude from the combined results that the tunneling current in our previously studied c-axis junctions is along the c direction and is the result of c-axis coupling. c-axis Josephson coupling between a conventional superconductor and YBCO provides strong evidence for the existence of an s-wave component in the YBCO order parameter.