Noise investigations in the subgap region of Nb-based tunnel junctions

We have investigated the quasiparticle noise phenomena of Nb/AI/AlOx/Nb Jos ephson tunnel junctions fabricated with a window geometry. In the subgap re gion the I-V characteristics display excess currents at voltages correspond ing to submultiples of the gap value. These structures can be ascribed to h igher-order processes through the tunnel barrier. In the same voltage regio n, measurements performed by using a low-frequency technique are strongly i ndicative of shot noise levels higher than the expected ideal values. (C) 2 000 Elsevier Science B.V. All rights reserved.