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

Tunneling and electron paramagnetic resonance (EPR) spectroscopies are used to investigate the quasiparticle (QP) density of states (DoS) of high-temp erature superconductors. Planar tunnel junctions are formed on oriented thi n films of Y1Ba2Cu3O7 (YBCO) and single crystals of Ba2Sr2Ca1Cu2O8 (BSCCO). Data are obtained as a function of crystallographic orientation, temperatu re, 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 ZBC P is weakened at the same rate as the gap-like feature, and provides a meas ure of the QP scattering rate below T-c. An applied field causes a splittin g of the ZBCP, which is due to a Doppler shift arising from the scaler prod uct between the QP velocity and superfluid momentum, V-F.P-S The dramatic h ysteresis observed with increasing and decreasing applied field is consiste nt 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 ap plied magnetic field, demonstrating the highly-anisotropic transport proper ties of the ABS. In-plane tunneling into single crystal BSCCO also demonstr ates crystallographic orientation dependence expected for a d-wave symmetry order parameter (OP). Temperature dependence in zero applied magnetic fiel d shows the ZBCP splits below similar to 8K, consistent with a phase transi tion into a superconducting state with spontaneously-broken time-reversal s ymmetry (BTRS). Electron paramagnetic resonance (EPR) experiments are used to directly detect the spontaneous formation of the magnetic moments in the BTRS state.