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

Measurements of χ′ and χ″ 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 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. The φ = 0.32 samples has a χ′ of -0.97, at 4.5 K, in low AC fields, when compared to a lead sample (χ = -1.0) of the same dimensions. The samples, in the region of the rapid decrease of χ′, i.e., close to the percolation threshold, display χ′ and χ″ versus T curves which have features very similar to those of granular high Tc superconductors. The strongly current-dependent features are almost certainly due to weak link point contacts forming between the hard angular NbC grains.