SUSCEPTIBILITY MEASUREMENTS ON A SYSTEM SUPERCONDUCTING NBC GRAINS NEAR THE PERCOLATION-THRESHOLD

Measurements of chi' and chi'' have been made at 36.7 Hz, using two dual phase lock-in amplifiers, on a series of NbC-KCl composites. The measurements are made, between 4.5 and 12 K, as a function of phi (the volume fraction of the 1-3 mum NbC grains) between 0.0 and 0.40, and in AC fields between 0.0005 and 1.8 Oe. The phi = 0.32 sample has a chi' of -0.97, at 4.5 K, in low AC fields, when compared to a lead sample (chi = -1.0) of the same dimensions. The samples, in the region of the rapid decrease of chi', i.e., close to the percolation threshold, display chi' and chi'' versus T curves which have features very similar to those of granular high T(c) superconductors. The strongly current-dependent features are almost certainly due to weak link point contacts forming between the hard angular NbC grains.