Theory and experiments on the effects of multiparticle tunneling in Josephson junctions

The origin of hysteresis effect in the voltage region of the I-V curves of a superconducting tunnel junction near the threshold value V=(E-1+E-2)/e, where E-1 and E-2 are the energy gap values in superconductors, is discussed from both a theoretical and experimental point of view. It is shown how the actual structure of the I-V curve can be determined by multiparticle tunneling (MPT) processes. This circumstance is related to the essential role played by a large parameter (Delta/Gamma)(2/3) (Gamma and Delta being the depairing factor and the order parameter value in the superconductor, respectively), which can compensate the low barrier transparency. As a consequence, the perturbation theory in the transparency coefficient does no longer hold in such a threshold region. The competition of proximity effect in producing hysteresis in the I-V curve is also discussed. Besides the underlying physical aspects of the phenomenon, the actual structure of I-V characteristics is an issue of obvious interest for Josephson-junctions based devices. Experiments have been performed on high-quality NbN-based 6x6 mu m(2) tunnel junctions at temperatures down to 300 mK. I-V curves clearly show the presence of hysteresis at the threshold voltages corresponding to both n=1 and n=2 MPT processes in agreement with the predictions of the presented theory.