DIRECTION OF TUNNELING IN PB I/YBA2CU3O7-X TUNNEL-JUNCTIONS/

Pb/I/YBCO tunnel junctions have been fabricated on ''ab edge'' surface s of YBCO single crystals (the ''ab edge'' surface is parallel to c ax is and normal to a or b axis). The characteristics of these ab junctio ns were compared to those of c-axis junctions. In addition, STM studie s were carried out on c-axis YBCO surfaces (studying exposed ab planes ). 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.