Spectroscopy of the Andreev bound state of high-temperature superconductors : Measurements of quasiparticle scattering, anisotropy and broken time-reversal symmetry

Abstract Tunneling and electron paramagnetic resonance (EPR) spectroscopies are used to investigate the quasiparticle (QP) density of states (DoS) of high-temperature superconductors. Planar tunnel junctions are formed on oriented thin films of Y 1 Ba 2 Cu 3 O 7 (YBCO) and single crystals of Ba 2 Sr 2 Ca 1 Cu 2 O 8 (BSCCO). Data are obtained as a function of crystallographic orientation, temperature, doping, damage and applied magnetic field. These data demonstrate that the observed zero bias conductance peak (ZBCP) is composed of Andreev bound states (ABS) which nucleate at an ab -plane interface of a d -wave symmetry superconductor. Tunneling into doped or ion-damaged YBCO shows that the ZBCP is weakened at the same rate as the gap-like feature, and provides a measure of the QP scattering rate below T c . An applied field causes a splitting of the ZBCP, which is due to a Doppler shift arising from the scalar product between the QP velocity and superfluid momentum, v F · P s . The dramatic hysteresis observed with increasing and decreasing applied field is consistent with the effects of strong vortex pinning at or near the interface. The magnitude of the splitting is strongly dependent on the direction of the applied magnetic field, demonstrating the highly-anisotropic transport properties of the ABS. In-plane tunneling into single crystal BSCCO also demonstrates crystallographic orientation dependence expected for a d -wave symmetry order parameter (OP). Temperature dependence in zero applied magnetic field shows the BBCP splits below ∼8K, consistent with a phase transition into a superconducting state with spontaneously-broken time-reversal symmetry (BTRS). Electron paramagnetic resonance (EPR) experiments are used to directly detect the spontaneous formation of the magnetic moments in the BTRS state.