Direction of tunneling in Pb/I/YBa2Cu3O7−x tunnel junctions

Abstract

$\mathrm{Pb}/I/\mathrm{YBCO}$ tunnel junctions have been fabricated on "$\mathrm{ab}$ edge" surfaces of YBCO single crystals (the "$\mathrm{ab}$ edge" surface is parallel to $c$ axis and normal to $a$ or $b$ axis). The characteristics of these $\mathrm{ab}$ junctions were compared to those of $c$-axis junctions. In addition, STM studies were carried out on $c$-axis YBCO surfaces (studying exposed $\mathrm{ab}$ 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.