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

We have studied nodal tunnelling into Y Ba2Cu3O7−x (YBCO) films under magnetic fields. The films' orientation was such that the CuO2 planes were perpendicular to the surface with the a and b axis at 45° from the normal. The magnetic field was applied parallel to the surface and perpendicular to the CuO2 planes. The zero bias conductance peak (ZBCP) characteristic of nodal tunnelling splits under the effect of surface currents produced by the applied fields. Measuring this splitting under different field conditions, zero field cooled and field cooled, reveals that these currents have different origins. By comparing the field cooled ZBCP splitting to that taken in decreasing fields we deduce a value of the Bean critical current superfluid velocity, and calculate a Bean critical current density of up to 3 × 107 A cm−2 at low temperatures. This tunnelling method for the determination of critical currents under magnetic fields has serious advantages over the conventional one, as it avoids having to make high current contacts to the sample.