Spatially resolved spectroscopy on a superconducting proximity structure

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 NS interface. The sample consists of a pattern of alternate stripes of Au and Nb made by UV-lithography. We used a low temperature scanning tunneling microscope to record simultaneously $dI/dV(V,x)$ curves and the topographic profile $z(x)$. Nearby the NS interface, all the spectra show a dip near the Fermi energy but depending on the geometry, different behaviours can be distinguished. First, when the characteristic size of the normal metal $L$ is much larger than the coherence length $\xi_N \simeq \sqrt{\hbar D_N/ 2\Delta}$, the spectral extension of the dip decreases from $\Delta$ at the NS interface to zero at distances $x\gg\xi_N$. Second, when $L$ is comparable to $\xi_N$ the apparent gap in the LDOS is space-independent and related to the Thouless energy.