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

Abstract

The origin of hysteresis effect in the voltage region of the I−V curves of a superconducting tunnel junction near the threshold value V=(E1+E2)∕e, where E1 and E2 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 (Δ∕Γ)2∕3 (Γ and Δ 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 6×6μm2 tunnel junctions at temperatures down to 300mK. 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.

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