Conductivity and noise critical exponents in thin films near the metal-insulator percolation transition

We report on experiments on thin silver films evaporated and ion milled at liquid-nitrogen temperature near the percolation threshold. We provide evidence for the universality of the critical exponent for the conductivity in two dimensions by showing that the critical exponents are the same in the two geometries studied. In contrast, the noise critical exponent κ is found to be different in the two model systems. For the case of the evaporated films κ=1.2_-0.1+0.3 in very good agreement with theory. For the ion-milled films, the critical exponent is κ=2.7±0.3, which is much lower than expected from theory.