Local spectroscopy of a superconductor-normal metal junction

At the junction with a superconductor, a normal metal acquires superconducting-like properties. The induced superconducting correlations show up as a spatially-dependent modification of the density of states. We measured the local density of states in Nb-Au heterostructures with the help of a very-low temperature (60 mK) Scanning Tunneling Microscope. Oil the superconducting dots, we obtain a fully-developed gap but with a reduced amplitude because of the inverse proximity effect. In the normal metal, we observe a pseudo-gap of decreasing width as the distance to the interface is increased. We compared our experimental results to the prediction of the quasiclassical theory.