Origin of superconductivity in copper niobium alloys

Superconductivity in copper-rich niobium alloys has been investigated. A controlled cooling technique was developed to provide homogeneous distributions of niobium in the copper. Two distinct microstructures were observed depending on the cooling rate. For low cooling rates (<700 degreeC/sec) and low niobium concentrations a subgrain structure of niobium is evident. These samples are superconducting with low critical current density (<10 A/cm2). For faster cooling rates (>1000 degreeC/sec) the subgrain structure is eliminated and a critical concentration for the observation of superconductivity is observed experimentally at 10--12 wt% Nb (7--9 at.% Nb). The experimentally determined critical concentration is explained by a model based on site percolation theory modified for the shape of the precipitate. The model suggests that continuous niobium chains in the copper matrix lead to the observed superconducting properties. The proximity effect, previously suggested as the basis for superconductivity in these alloys, appears to have little contribution to the appearance of superconductivity in these as-cast samples. The only contribution, if any, is to bridge gaps in the otherwise continuous niobium network