Local spectroscopy of a proximity superconductor at very low temperature

We performed the local spectroscopy of a normal-metal-superconductor (N-S) junction with the help of a very low-temperature (60 mK) scanning tunneling microscope (STM). The spatial dependence of the local density of states was probed locally in the vicinity of the N-S interface. We observed spectra with a fully developed gap in the regions where a thin normal-metal layer caps the superconductor dot. Close to the S metal edge, a clear pseudo-gap shows up, which is characteristic of the superconducting proximity effect in the case of a long normal metal. The experimental results are compared to the predictions of the quasiclassical theory. (orig.)