Composites of the type: metal - dielectrics and superconductor - dielectrics are studied in the quasistatic approximation. The dielectric response is described by the spectral function $G(n,x)$, which contains effects of the concentration x (of metallic resp... superconductive particles) on the dielectric function,and effects of the shape. The parameter n plays the role of the depolarisation factor for dielectric materials, in metals it is a factor which includes effects like shape, and a topology of the composite. There exists a percolation transition at $ x_{c}= \frac{1}{3} $ which leads to a metallic-like for the composite with the concentration $ x > x_{c}$. At low frequencies divergence with frequency remains even when there are present dielectric particles above the percolation concentration. In superconductor case the spectral function $G(n,x)$ may include also Josephson junction effects. We assume in both cases of composites two types of spheroidal particles, metal (superconducting) ones and dielectric ones. A dielectric function is constant in both cases for the dielectric material, and a dielectric function for the metal and for the superconductor are used with well known form for metals and a classical superconductor. A percolation transition at $ x_{c}$ leads to a metallic-like absorption for the composite with $x>x_{c}$. Note that at low frequencies divergence in frequency remains even when there are present dielectric particles above $x_{c}$. Below the percolation threshold dielectric properties are modified by metalic particles. We obtain at very low temperatures and low concentrations x of the superconductor the effective dielectric constant. The absorption part is zero in our simple case. The real part of the dielectric function increases with the concentration of the superconducting spheres. The frequency dependence is quadratic, it gives low frequency tail. read more

PDF Abstract
Materials Science