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 = I
cRN (R-N is the normal resistance, V-c defines the response time of the jun
ction) from 0.3 to 0.4 mV. Two prominent features in the quasiparticle curr
ent-voltage curves have been observed: a so-called 'knee' in the energy-gap
region and an additional (to the linear voltage dependence) current at hig
her 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 t
he barrier transparencies. (C) 2001 Elsevier Science B.V. All rights reserv
ed.