Josephson and tunneling experiments in in-situ Pb/YBCO junctions

Abstract

Josephson tunnel junctions between a conventional superconductor (Pb) and a High Tc material YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-(delta</SUB> ) (YBCO) have been reproducibly obtained in a film based planar geometry. Using a quasi-MBE method to grow high quality thin films, and making in-situ Pb/YBCO junctions along different crystallographic orientations, we studied the Josephson coupling along the (001) and (103) directions, together with the excitation spectra through quasi-particle tunneling. We evidenced that the coupling along the c- direction is small but finite, compatible with a small s-wave component in YBCO, and that a Zero Bias Conductance Peak is present when tunneling in the ab-plane, as expected for an Andreev Bound State (ABS) at a surface of a dominantly d-wave superconductor. The role of twin boundaries in c-axis experiments is discussed, and the coupling between the ABS with the Josephson current as well.