Determination of the critical current density in the d-wave superconductor YBCO underapplied magnetic fields by nodal tunnelling

Abstract

We have studied nodal tunnelling intoY Ba2Cu3O7−x (YBCO) films under magnetic fields. The films’ orientation was such that theCuO2 planes were perpendicular to the surface with thea andb axisat 45° from the normal. The magnetic field was applied parallel to the surface and perpendicular to theCuO2 planes. The zero bias conductance peak (ZBCP) characteristic of nodal tunnelling splitsunder the effect of surface currents produced by the applied fields. Measuring thissplitting under different field conditions, zero field cooled and field cooled, revealsthat these currents have different origins. By comparing the field cooled ZBCPsplitting to that taken in decreasing fields we deduce a value of the Bean criticalcurrent superfluid velocity, and calculate a Bean critical current density of up to3 × 107 A cm−2 at low temperatures. This tunnelling method for the determination of critical currentsunder magnetic fields has serious advantages over the conventional one, as it avoids havingto make high current contacts to the sample.