WHAT IS THE RELEVANT LENGTH SCALE WHEN STUDYING PERCOLATING CLUSTERS - A CRITICAL-STUDY

Percolation clusters are known to be well described by the percolation correlation length xi. In fact, this length is defined by the geometr y of the clusters, and thus can only give an account of zero frequency (time independent) physical properties, such as dc conductivity or fr actal morphology of 2D or 3D clusters. Taking examples on optical prop erties of granular metal films close to the percolation threshold, we show that the relevant length scale L is not known well. It depends ex plicitly on the kind of measurement, and on the frequency omega at whi ch the physical properties are observed. We conclude that, in the regi on where L(omega) < xi, only a scaling model can give a full account o f the experimental results, and that, in the region where L(omega) > x i, an averaged behaviour can also be used.