Planar tunneling spectroscopy of high-temperature superconductors: Andreev bound states and broken symmetries

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 YBa2Cu3O7 (YBCO) thin films and Bi2Sr2CaCu2O8 (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)(-y)(2)(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, phi. (C) 2003 Elsevier Science B.V. All rights reserved.