Andreev bound state tunneling: spectroscopy of unconventional superconductivity

Planar tunneling spectroscopy is used to investigate the quasi-particle density of states of YBCO. The tunneling conductance, taken as a function of temperature, magnetic field, crystallographic orientation, Pr, Zn and Ni doping and ion-induced damage confirms that the observed zero-bias conductance peak (ZBCP) is an Andreev Bound State (ABS). This ABS occurs at the interface of an unconventional superconductor, that breaks the order-parameter reflectional symmetry: in this case, the (110) surface of YBCO. An applied magnetic field causes a splitting of the ZBCP, which is due to the Doppler shift arising from the scalar product of the quasiparticle velocity with the superfluid momentum, V_F·P_S. A dramatic dependence of the ZBCP splitting with the direction of the applied field demonstrates that the transport properties of the ABS are highly anisotropic with respect to the crystal axes. In zero field, the ZBCP splits below T_S approx. 8K, which is a manifestation of a phase transition into a state that breaks time-reversal symmetry.