Double-barrier highly asymmetric Nb-Al oxide-Al-Nb oxide-Nb structures with reproducible characteristics were fabricated. The heterocontacts with the middle Al layer thickness ranging from 4 to 6 nm exhibited a well-defined d.c. Josephson supercurrent I-c at 4.2 K and characteristic Voltages V-c = IcRN (R-N is the normal resistance, V-c defines the response time of the junction) from 0.3 to 0.4 mV. Two prominent features in the quasiparticle current-voltage curves have been observed: a so-called 'knee' in the energy-gap region and an additional (to the linear voltage dependence) current at higher biases. They are discussed within a simple Landauer-Buttiker scattering approach to the phase-coherent quasiparticle transport in a quasiballistic S-I-1-N-I-2-S heterostructure with an extremely great difference between the barrier transparencies. (C) 2001 Elsevier Science B.V. All rights reserved. [References: 20]
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