Superconducting Proximity Effect Probed on a Mesoscopic Length Scale

We have measured by tunneling spectroscopy the electronic density of states in a nonsuperconducting wire in good contact with a superconductor, at distances of 200, 300, and 800 nm from the interface. Closest to the interface, the density of states near the Fermi energy is reduced to 55% of its normal value. At the farthest measurement point, this dip has nearly completely disappeared. We compare our data to predictions based on the Usadel equations.