The complex dielectric constant of a metal (superconductor)-insulator system near the percolation threshold

Measurements of the complex dielectric constant (ε' + i ε'') are performed on a series of NbC-KCl composites, at audio frequencies, near the percolation threshold. Just above Tc ε', as a function of φ (the volume concentration of the 1-3 μm Nb grains), diverges as the critical (percolation) volume fraction φc is approached from below. For φ > φc, ε' decreases rapidly and then becomes negative. At φ*, where φ*-φc ∼ 0.007, ε' = 0. Below Tc (∼ 10.5 K), in the dielectric region (φ < φc), both ε' and ε'' slowly decrease as the temperature is lowered. The magnitude of the decrease increases as φc is approached from below, but never exceeds 20% at 102 Hz or 15% at 105 Hz, for ε' at T = 4.5 K, in the samples measured. When φ ∼ φ*, ε' increases strongly as T is lowered below Tc at 102 Hz, but may decrease at 105 Hz. (orig.)