DIELECTRIC-CONSTANT MEASUREMENTS IN A SYSTEM OF NBC GRAINS NEAR THE PERCOLATION-THRESHOLD

Measurements of the complex dielectric constant (epsilon' + iepsilon'') on a series on NbC-KCl composites in a wide range of concentrations are performed as a function of phi (the volume fraction of the 1-3 mum NbC grains) at frequencies of 10(2), 10(3), 10(4) and 10(5) Hz. Frequency scaling of epsilon = epsilon' + iepsilon'' at the metal-insulator transition is different from one which follows from the scaling theory of an ideal percolation system. We observe two different values of the critical volume fraction of metal. The first critical concentration, phi(c1), is a cross-over point where the dielectric-constant frequency dependence changes and the loss factor is on the order of unity. The temperature behavior of the complex dielectric constant below the superconducting transition temperature T(c) reveals a transformation of a system of isolated NbC grains into a system of weakly coupled tunneling junctions at phi(c1). The expected divergence of epsilon' is observed as the second critical volume concentration phi(c2) > phi(c1) is approached. At this concentration a cross-over from the capacitive tunneling junction medium to a truly metallic state occurs. At phi > phi(c2), epsilon' decreases rapidly as a function of phi and becomes negative at phi - phi(c2) approximately 0.01, due to the negative effective real dielectric constant of the percolation metallic cluster which spans the system.