Observation of surface-induced broken time-reversal symmetry in YBa₂Cu₃O₇ tunnel junctions

Data from ab-oriented YBa₂Cu₃O₇/I/Cu tunnel junctions are presented. Self-assembled monolayers form the insulating tunnel barrier, I. The YBa₂Cu₃O₇ features in the tunneling conductance match those of low-leakage ab-oriented YBa₂Cu₃O₇/Pb junctions. Results show that the zero-bias conductance peak is an Andreev bound state (ABS) of a d-wave order parameter. In zero magnetic field, the ABS splits below ∼7K, consistent with the presence of a subdominant order parameter at the surface. An applied magnetic field induces further splitting that grows nonlinearly with increasing field.