Width dependence of resistance and currents in series arrays of superconducting niobium/indium arsenide junctions

In the present research we have fabricated and measured arrays of Nb/InAs junctions with large electrode separations (;The Nb/InAs junction arrays exhibit the Josephson effects due to phase coupling, but they also exhibit features due to the Andreev reflections. Multiple Andreev reflections between superconducting electrodes are manifested as a subharmonic gap structure in the current-voltage characteristics. In addition, there is an excess current that persists down to zero bias.;At zero bias, the differential resistance is thermally activated. The activation energy may vary from sample to sample due to uncontrolled parameters, but there is also a reproducible variation of the activation energy with width. Wider arrays show a proportionally higher activation energy than narrower arrays and are thus fundamentally different from a parallel connection of narrow arrays. This behavior appears to be related to the excess current due to multiple Andreev reflections. A simple noise model (augmented by Andreev reflections) is proposed to explain these observations.