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. In the recent years, there have been a noticeable renewed interest in the proximity effect appearing at the junction between a Normal metal (N) and a Superconductor (S). In particular, the transport properties of the normal metal part were shown to exhibit a striking energy dependence with a long range persistence [1]. Another aspect of the proximity effect is the local modification of the energy spectrum. In the case of a N-S junction with a finite normal metal, the density of states in the normal metal is expected to exhibit a mini-gap : the density of states is zero within an energy window around the Fermi level [2, 3]. The width of this mini-gap is smaller than the intrinsic superconducting gap of the superconductor. If two superconductors are connected to a short normal metal (S-N-S junction), the mini-gap will moreover depend on the phase difference between the two superconductors [4]. In the case of a normal metal with an infinite length, the density of states should exhibit a pseudo-gap [2,3] : the density of states is zero only at the exact Fermi level and goes approximately linearly with the energy close to the Fermi level. This behavior can be understood by arguing that some electron trajectories that travel close the interface never hit it, therefore do not couple to superconductivity and contribute to the density of states. Practically, the criteria of an infinite length for the normal metal should be understood as a length larger than the phase coherence length. The local density of states in the vicinity of a N-S junction has been measured with the help of submicron planar tunnel junctions [5]. The appearance of a pseudo-gap in the N metal as well as the inverse proximity effect on the S metal side [6] were …