OBSERVATION OF SURFACE-INDUCED BROKEN TIME-REVERSAL SYMMETRY IN YBA2CU3O7 TUNNEL-JUNCTIONS

Data from ab-oriented YBa2Cu3O7/I/Cu tunnel junctions are presented. S elf-assembled monolayers form the insulating tunnel barrier, I. The YB a2Cu3O7 features in the tunneling conductance match those of low-leaka ge ab-oriented YBa2Cu3O7/Pb junctions. Results show that the zero-bias conductance peak is an Andreev bound state (ABS) of a d-wave order pa rameter. In zero magnetic held, the ABS splits below similar to 7 K, c onsistent with the presence of a subdominant order parameter at the su rface. An applied magnetic field induces further splitting that grows nonlinearly with increasing field.