Disorder-induced andreev reflections in granular metals

We have studied the transport properties of SNS structures where N is a disordered Ag film quench condensed into a narrow gap between two superconducting Pb electrodes. This setup enables us to control the disorder of the normal region by sequentially depositing layers of Ag in situ. We find that when the length of the disordered N section is (Formula presented) the samples exhibit a subgap resistance minimum, which we interpret as evidence for strong Andreev reflection processes. This feature evolves into the usual Blonder-Tinkham-Klapwijk behavior as we reduce the disorder of the N region by increasing the Ag thickness. In samples which are shorter than (Formula presented) this crossover is absent. We discuss the results within the framework of the “reflectionless tunneling” mechanism.