Electrical conductance of a 2D packing of metallic beads under thermal perturbation

Electrical conductivity measurements on a 2D packing of metallic beads have been performed to study internal rearrangements in weakly perturbed granul ar materials. Small thermal perturbations lead to large non-Gaussian conduc tance fluctuations. These fluctuations are found to be intermittent in time , with peaks gathered in bursts. The distribution of the waiting time Delta t between two peaks is found to be a power law Delta t(-(1+alpha t)) insid e bursts. The exponent alpha(t) is independent of the head network, the int ensity of perturbation and external stress. These bursts are interpreted as the signature of individual bead creep rather than collective vault reorga nisations. We propose a simple model giving alpha(t) = 1 - zeta, where zeta is the roughness exponent of the surface of the beads.