Interface density of states in d-wave superconducting proximity effect through Andreev reflections

Tunneling spectra via Andreev bound states between a normal metal (N)/dx2−y2-wave superconductor (S) (in the presence of a subdominant s-wave pair potential) junction are investigated. In the present work, we employ quasiclassical Green's function methods in order to study the role of the proximity effect in detail. In the case of a high transparent contact to the (100) interface of the d-wave superconductor, the pair potential penetrates into the inside of the N due to the proximity effect, where the is-wave is not indeed at all. Then, the tunneling spectra has a very sharp zero-energy peak (ZEP). This ZEP originates from the fact that quasiparticles feel different sign of the pair potentials between normal metals and d-wave superconductors through Andreev reflections. On the other hand, in the N/S junction with the (110) interface, the induction of is-wave component in the S side leads to an induced is-wavepair potential also in the N side due to the proximity effect. In this case, the interface density of states has a dip structure. We show that the spatial dependence of pair potentials is significantly sensitive to the transparency of the junction.