Multiple Andreev Reflection and Giant Excess Noise in Diffusive Superconductor/Normal-Metal/Superconductor Junctions

We have studied superconductor/normal metal/superconductor (SNS) junctions made from a high gap (Nb) and a low gap (Al) superconductor. The normal wires (Au or Cu) have a length of about 1 $\mu$m. The Nb based junctions display very pronounced electron heating effects induced by the high thermal resistance of the NS boundaries. The Al based junctions show in addition subharmonic gap structures in the differential conductance $dI/dV$ and a pronounced peak in the excess noise at very low voltages $V$ which may be attributed to the transfer of multiple charge quanta at low $V$.