Josephson tunneling of anisotropic high-Tcd-wave junctions with tiltedab-planeYBa2Cu3O7−yelectrodes

Abstract

We report measurements on high-${T}_{c}$ ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}y}$ (YBCO) Josephson ramp-edge junctions with different ab-plane orientation electrodes relatively rotated by 45\ifmmode^\circ\else\textdegree\fi{}. The ramp-edge junctions with different crystal angles against the interface boundary are fabricated on an MgO(100) substrate using a ${\mathrm{CeO}}_{2}$ seed-layer technique. The Shapiro steps under microwave irradiation of 9 GHz appear qualitatively different for different crystal angles, which supports the d-wave nature of a YBCO superconductor. The magnetic-field dependence of the Josephson maximum current shows a Fraunhofer-like pattern, which is consistent with the calculated result for the d-wave junctions. The temperature dependencies of the maximum Josephson current for different angle geometry fall off rapidly with increasing temperature and deviate much from the conventional s-wave Ambegaokar-Baratoff prediction. The results are in good qualitative agreement with the calculated results based on the d-wave pairing symmetry taking the presence of the zero energy state into account. The observed angle dependencies clearly indicate that the angle-dependent Josephson current really exists.