Evidence for synchronized Andreev reflections in NSN devices

In mesoscopic NSN devices, in which a short superconducting region separates two metallic electrodes, the Andreev reflection process may delocalize and couple electron- and hole-states on opposite sides of the superconductor. In addition to such nonlocal (or crossed) Andreev reflections, quasiparticles may also tunnel directly between the electrodes. We have studied nonlocal transport and current correlations in Cu/Al/Cu structures. We observe that the current correlations are symmetric in applied bias and show local minima when the applied voltages at the two contacts are equal in magnitude. This behavior matches theoretical predictions for devices with intermediate interface transparency in which the nonlocal exchange of quasiparticles triggers additional synchronized Andreev reflection events at the two interfaces.