Tunneling and percolation in metal-insulator composite materials

In many composites, the electrical transport takes place only by tunneling between isolated particles. For a long time, it was quite a puzzle how, in spite of the incompatibility of tunneling and percolation networks, these composites conform well to percolation theory. We found, by conductance atomic force microscopy measurements on granular metals, that it is the apparent cut off of the tunneling to non-nearest-neighbors that brings about this behavior. In particular, the percolation cluster is shown to consist of the nearest-neighbors subnetwork of the full tunneling network.