Size-Dependent Nonuniversal Conductivity in Nanometer-Size Co Cluster Assemblies

We studied the two-dimensional assembling process of nanometer-size Co clusters on substrates in terms of the percolation theory. We demonstrate the nonuniversal behavior of the electrical conductivity depending strongly on the cluster size, d : the conductivity critical exponent increases from 1.1 to 1.8 with increasing d from 6 to 13 nm. The soft-percolation model, which takes into account the wide distribution of connectivity between clusters, is a candidate to explain this nonuniversality.