  We investigated the local electronic density of states in
superconductor-normal metal (Nb-Au) bilayers using a very low temperature (60
mK) STM. 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 mini-gap, the density
of states does not reach zero and shows clear sub-gap features. We show that
the experimental spectra cannot be described with the well-established Usadel
equations from the quasi-classical theory.
