Tunneling into andreev bound states of YBa2Cu3O7: Observation of broken time-reversal symmetry

Planar tunneling is a powerful spectroscopic probe of superconducting YBa2C u3O7 (YBCO). In YBCO/I/Cu junctions, an organic monolayer is used to form t he insulator. The reproducible dependences of the tunneling conductance on temperature, magnetic field and ion-induced disorder show that the zero-bia s conductance peak (ZBCP) is an Andreev bound state, of the d-wave order pa rameter. In zero applied field the ZBCP splits at T similar to 8 K, signify ing a phase transition into a state with spontaneously broken time-reversal symmetry. An applied held induces further splitting that saturates at fiel ds of order 2-3 T. These data support a theoretical model that invokes the formation of sub-dominant order parameter. (C) 1998 Elsevier Science Ltd. A ll rights reserved.