Non-universal conductivity critical exponents in anisotropic percolating media: a new interpretation

The problem of non-universality of the conductivity critical exponents in anisotropic percolating systems is discussed in this paper. The authors bring simple arguments and gather experimental evidences to show that universality is observable in anisotropic media, and that its occurrence depends in the first place on the behaviour of the conductivity anisotropy. If the latter is constant within a non-vanishing region close to the percolation threshold, a single critical exponent is obtained, otherwise different apparent (and hence non-universal) exponents are derived. This finding supports the early assumptions of Smith and Lobb (Phys. Rev. B 20 (1979) 3653) concerning the narrowing of the critical region in anisotropic percolating media. Obtaining an isotropic exponent is thus the first requirement for the latter to be universal. If such an exponent is still non-universal, physical reasons other than anisotropic properties should be invoked.