Observation of Surface-Induced Broken Time-Reversal Symmetry in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>YBa</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi>Cu</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi>O</mml:mi></mml:mrow><mml:mrow><mml:mn>7</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>Tunnel 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 $\ensuremath{\sim}7\mathrm{K}$, 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.