Abnormal minigap induced by superconducting proximity effects in a metallic film

The scattering matrix approach is applied to study tunneling spectra in normal metal/normal metal/superconductor double junctions. The abnormal minigap in tunneling conductance is attributed to the phase coherence of electrons and Andreev-reflected holes in the middle metallic film. We consider a combination of the coherent tunneling with the sequential tunneling and the calculated results provide a reasonable explanation for the abnormal minigap observed in recent experiments.