Anomalous density of states in a metallic film in proximity with a superconductor

We investigated the local electronic density of states in superconductor-normal metal (Nb-Au) bilayers using a very low temperature (60 mK) scanning tunnel microscope. High-resolution tunneling spectra measured on the normal metal (Au) surface show a clear proximity effect with an energy gap of reduced amplitude compared to the bulk superconductor (Nb) gap. Within this minigap, the density of states does not reach zero and shows clear subgap features. We show that the experimental spectra cannot be described with the well-established Usadel equations from the quasiclassical theory.