Voltage noise properties of Nb-(fine Nb wires)-NbN thin film short weak links

We have fabricated superconductive Nb-(fine Nb wires)-NbN short weak links. We find that their quasiparticle characteristics can be well explained using the theory of T. M. Klapwijk, G. E. Blonder, and M. Tinkham [Physica B and C 109 & 110B 1657 (1982)] based on the Andreev reflection phenomenon at two independent normal metal-superconductor interfaces for the case of a negligible effective barrier potential. For such devices, there is found no measurable excess noise associated with the Andreev reflection itself and device voltage noise clearly exhibits both thermal and shot noise limits. Although the device structure is fundamentally different, in the whole studied range of bias voltages, the voltage noise properties of our short weak links are found to be qualitatively comparable to those of small-area tunnel junctions.