Spatially resolved spectroscopy on superconducting proximity nanostructures

Abstract

We investigated the local density of states (LDOS) of a normal metal (N) in good electrical contact with a superconductor (S) as a function of the distance x to the $\mathrm{NS}$ interface. The sample consists of a pattern of alternate strips of Au and Nb. We used a low-temperature scanning tunneling microscope to record simultaneously $dI/dV(V,x)$ curves and the topographic profile $z(x).$ Nearby the $\mathrm{NS}$ interface, all the spectra show a dip at the Fermi energy but, depending on the geometry, different behaviors can be distinguished. First, when the characteristic size of the normal metal L is much larger than the coherence length ${\ensuremath{\xi}}_{N}=\sqrt{\ensuremath{\Elzxh}{D}_{N}/2\ensuremath{\Delta}},$ the spectral extension of the dip decreases from $\ensuremath{\Delta}$ at the $\mathrm{NS}$ interface to zero at distances $x\ensuremath{\gg}{\ensuremath{\xi}}_{N}.$ Second, when L is comparable to ${\ensuremath{\xi}}_{N},$ the apparent gap in the LDOS is space independent.