Observation of Surface-Induced Broken Time-Reversal Symmetry inYBa2Cu3O7Tunnel Junctions

Abstract

Data from $\mathrm{ab}$-oriented ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7/\mathrm{I}/\mathrm{Cu}$ tunnel junctions are presented. Self-assembled monolayers form the insulating tunnel barrier, I. The ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ features in the tunneling conductance match those of low-leakage $\mathrm{ab}$-oriented ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7/\mathrm{Pb}$ 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 $\sim 7\mathrm{K}$, 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.