Josephson coupling in untwinned YBa2Cu3O7-x/Nb d-wave junctions

Abstract

Theory predicts that d-wave superconductivity induces a significant second harmonic J2 in the Josephson current, as a result of zero-energy Andreev states (ZES) formed at the junction interface. Consequently, anomalies such as half-integer Shapiro steps should be observed. Both ZES formation and J2 are expected to be highly anisotropic as we change the tunneling orientation in the ab plane reaching their maximum for tunneling close to [110] direction and their minimum for [100] or [010] directions. We performed experiments on junctions between untwinned d-wave YBa2Cu3O7-x and Nb and found clear evidence of ZES formation for all 72 different tunneling directions in the ab plane investigated. However, in contrast to the theoretical predictions, we found no trace of half-integer Shapiro steps. That suggests J2 is insignificantly small compared to the first Josephson harmonic for all of the orientations. We believe that microscopic scale roughness, or diffusive reflection at a scale that is much smaller than the Fermi wavelength, dramatically suppresses J2 due to scattering processes. Our findings suggest that YBa2Cu3O7-x/Nb d-wave junctions have a purely sinusoidal current-phase relation which is essential to take into consideration for their implementation as qubits or π-junctions in digital circuits.