Planar tunneling spectroscopy of high-temperature superconductors: Andreev bound states and broken symmetries (original) (raw)

Quasiparticle planar tunneling spectroscopy is shown to be a powerful, phase-sensitive spectroscopic tool for the study of unconventional superconductivity. Several familiar and novel junction fabrication techniques on YBa 2 Cu 3 O 7 (YBCO) thin films and Bi 2 Sr 2 CaCu 2 O 8 (BSCCO) single crystals are used for our systematic studies of the tunneling conductances, which are obtained as a function of crystallographic orientation, applied magnetic field (magnitude and orientation) atomic substitution and surface damage. All these results confirm that the observed zero-bias conductance peak (ZBCP) is comprised of quasiparticle (QP) Andreev bound states (ABS), which arise from reflection-symmetry breaking at the interface of a superconductor whose superconducting order parameter (OP) exhibits d-wave, or d x 2-y 2 , symmetry. Consistency in the observation of the splitting of the ZBCP is presented: The splitting of the ZBCP observed in applied field, and the spontaneous splitting observed at lower temperatures in zero field, occur concomitantly in a given junction, and that observation of this splitting is dependent upon the length-scale of the surface disorder and the magnitude of the tunneling cone, /.