Susceptibility measurements on a system of superconducting NbC grains near the percolation threshold

Measurements of χ' and χ'' have been made at 36.7 Hz, using two dual phase lockin amplifiers, on a series of NbC-KCl composites. The measurements are made, between 4.5 and 12 K, as a function of φ (the volume fraction of the 1-3 μm NbC grains) between 0.0 and 0.40, and in AC fields between 0.0005 and 1.8 Oe. As the volume fraction φ increases χ' (4.5K) decreases slowly at first, showing a Maxwell Garnett type behaviour. The rapid decrease starts at φ between 0.18 (0.0005 Oe) and 0.24 (0.5 + Oe). The φ = 0.32 sample has a χ' of - 0.97 in low AC fields, at 4.5K, when compared to a lead sample [χ = -1.0] of the same dimensions. The samples, in the region of the rapid decrease of χ', ie. close to the percolation threshold, display χ' and χ'' against T curves which have features very similar to those of granular HiTc superconductors. The strongly current dependant features are almost certainly due to weak link point contacts forming between the hard angular NbC grains. (orig.)