OBSERVATION OF SUBGAP STRUCTURES IN HIGH-QUALITY NB/AL-ALOX/NB JOSEPHSON TUNNEL-JUNCTIONS

We have performed I-V measurements on high-quality Nb/Al-AlOx/Nb Josephson junctions. All curves exhibit subgap structures. Some junctions exhibit in the I-V curves the (DELTA1-DELTA2)/e singularity, due to the presence of niobium films with different gaps. In these devices we observed sharply separated structures corresponding to the series 2DELTA1/ne and 2DELTA2/ne, with n less-than-or-equal-to 3 corresponding to the tunneling of two and three electrons. The respective amplitudes of the two structures at n = 2 allow one to conclude that the phenomenon is a multiparticle tunneling process.