Conductivity near the percolation threshold
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In a mixture between a dielectric and a metallic component, the conductivity and the dielectric constant of this mixture show a critical behavior if the fraction of the metallic component reaches the percolation threshold. The behavior of the conductivity near this percolation threshold will show a smooth changeover from the conductivity of the dielectric component to the conductivity of the metallic component and can be described using two critical exponents s and t, whereas the dielectric constant will diverge if the threshold is approached from either side. To include the frequency dependent behavior, a resistor-capacitor model (R-C model) is used.
For describing such a mixture of a dielectric and a metallic component we use the model of bond-percolation. On a regular lattice, the bond between two nearest neighbors can either be occupied with probability or not occupied with probability . There exists a critical value . For occupation probabilities an infinite cluster of the occupied bonds is formed. This value is called the percolation threshold. The region near to this percolation threshold can be described by the two critical exponents and (see Percolation critical exponents).
With these critical exponents we have the correlation length,
and the percolation probability, P:
For the description of the electrical percolation, we identify the occupied bonds of the bond-percolation model with the metallic component having a conductivity . And the dielectric component with conductivity corresponds to non-occupied bonds. We consider the two following well-known cases of a conductor-insulator mixture and a superconductor–conductor mixture.
In the case of a conductor-insulator mixture we have . This case describes the behaviour, if the percolation threshold is approached from above:
Below the percolation threshold we have no conductivity, because of the perfect insulator and just finite metallic clusters. The exponent t is one of the two critical exponents for electrical percolation.
In the other well-known case of a superconductor-conductor mixture we have . This case is useful for the description below the percolation threshold:
Now, above the percolation threshold the conductivity becomes infinite, because of the infinite superconducting clusters. And also we get the second critical exponent s for the electrical percolation.
conductivity near the percolation threshold
In the region around the percolation threshold, the conductivity assumes a scaling form:
At the percolation threshold, the conductivity reaches the value:
Values for the critical exponents
In different sources there exists some different values for the critical exponents s, t and u in 3 dimensions:
|Efros et al.||Clerc et al.||Bergman et al.|
The dielectric constant also shows a critical behavior near the percolation threshold. For the real part of the dielectric constant we have:
The R-C model
Within the R-C model, the bonds in the percolation model are represented by pure resistors with conductivity for the occupied bonds and by perfect capacitors with conductivity (where represents the angular frequency) for the non-occupied bonds. Now the scaling law takes the form:
This scaling law contains a purely imaginary scaling variable and a critical time scale
which diverges if the percolation threshold is approached from above as well as from below.
- A. L. Efros and B. I. Shklovskii, Critical Behaviour of Conductivity and Dielectric Constant near the Metal-Non-Metal Transition Threshold, Phys. Status Solidi B 76, 475 (1976)
- J. P. Clerc, G. Giraud, J. M. Laugier and J. M. Luck, The electrical conductivity of binary disordered systems, percolation clusters, fractals and related models, Adv. Phys. 39, 191 (1990)
- D. J. Bergman and D. Stroud, Physical Properties of Macroscopically Inhomogeneous Media, hg. von H. Ehrenreich und D. Turnbull, Bd. 46, Solid State Physics (Academic Press inc., 1992)