Conductance anomalies in a normal-metal-d-wave superconductor junction

The effects of Andreev reflection on the current-voltage characteristic and differential conductance of a junction between a normal metal and a d(xy)-wave superconductor, or equivalently, a d(x)2-(y)2-wave superconductor with a {110}-oriented surface are investigated using the Bogoliubov-de Gennes equations. Our study elucidates several important consequences of the sign change of a d-wave order parameter. In particular, a zero-bias conductance peak is obtained when an insulating barrier exists at the interface between the normal metal and the d-wave superconductor, consistent with numerous experiments performed on cuprate superconductors. If the insulating barrier is assumed to reside in the normal metal, several coherence lengths away from the superconductor surface, bound states within the energy gap, and consequent subgap resonances in the differential conductance, are predicted. The positions of these resonances are out of phase with respect to those predicted for an isotropic or anisotropic s-wave superconductor, thus providing unique signatures of pairing state symmetry.