Enhanced shot noise in diffusive S/N/S junctions

摘要

Current noise measurements enligthen the transport mechanisms in mesoscopic samples in a complementary way to conductance measurements. The noise gives directly access to the charge of the current carriers and is modified by electronic interactions. This is very instructive in the context of hybrid superconductor-normal metal (S-N) structures where charge pairs generated by Andreev reflections compete with quasiparticles in the current transport. In S-N-S junctions, the shot noise is very much enhanced compared to the normal case because of the confinement of normal quasiparticles between the superconducting electrodes. The relevant time depends on the number of Andreev reflections (N-AR) necessary for quasiparticles to reach the superconducting gap Delta and escape the normal metal. It can therefore be tuned by the bias voltage because N-AR goes like 1/eV. Diffusive junctions of intermediate size (xi(Delta) < L < L-Phi) are particularly interesting because various transport regimes can then be studied from coherent (when eV < E-Th = hD/L-2) to incoherent (eV > E-Th). We present shot noise measurements in Al/Cu/Al junctions down to 100mK, that reveal a clear mesoscopic transition when the voltage equals the Thouless energy. A good agreement with semi-classical description is found at large energy when Incoherent Multiple Andreev Reflections prevail. At low voltage, the shot noise diverges which may indicates the coherent transfer of mutiple charges. No theoretical predictions exist for such a regime in diffusive junctions of intermediate size and this point remains unsolved.