  Nanogranular metal composites, consisting of immiscible metallic and
insulating phases deposited on a substrate, are characterized by two distinct
electronic transport regimes depending on the relative amount of the metallic
phase. At sufficiently large metallic loadings, granular metals behave as
percolating systems with a well-defined critical concentration above which
macroscopic clusters of physically connected conductive particles span the
entire sample. Below the critical loading, granular metal films are in the
dielectric regime, where current can flow throughout the composite only via
hopping or tunneling processes between isolated nanosized particles or
clusters. In this case transport is intrinsically non-percolative in the sense
that no critical concentration can be identified for the onset of transport. It
is shown here that, although being very different in nature, these two regimes
can be described by treating percolation and hopping on equal footing. By
considering general features of the microstructure and of the electrical
connectedness, the concentration dependence of the dc conductivity of several
nanogranular metal films is reproduced to high accuracy within an effective
medium approach. In particular, fits to published experimental data enable us
to extract the values of microscopic parameters that govern the percolation and
tunneling regimes, explaining thus the transport properties observed in
nanogranular metal films.
