No abstract

Low temperature transport measurements have been performed on nanofabricated Au wires a few p, m long contacting two Nb electrodes whose superconducting phase difference 50 was externally controlled. 58 was generated by passing a known supercurrent through a series array of 28 Nb-A10, -Nb Josephson junctions. The conductance of the Au wire displays a component in 58 with period 2m.These conductance oscillations arise from the change in scattering boundary conditions of quasiparticles in the Au wire at the Au-Nb interfaces due to Andreev reflection. PACS numbers: 74.50.+r, 73.50.Jt, 74.80.Fp When the size of a normal conducting system ap-

We describe the fabrication and properties of Nb-AlOx-Nb Josephson tunnel junctions having critical current densities as high as 400 kA/cm2, roughly an order of magnitude larger than any previously reported for this materials system. The quality of the junction characteristics, as indicated by the current level at subgap voltages, is considerably lower in high-Jc than in low-Jc junctions. However, over the entire high-Jc range of 20–400 kA/cm2, the quality of our junctions remains the same (i.e., subgap current scales with critical current).

We demonstrate that a large fraction of the available data relating the critical current density J, of superconducting Nb-AI0,-Nb tunnel junctions to oxidation parameters can be accounted for by a single, nearly universal dependence. For fixed oxidation temperature, J , does not depend independently on oxygen partial pressure and oxidation time, but only on their product. There are two distinct regimes in this dependence, corresponding to high and low J,.