Low-temperature transport properties of granular Pb films below the percolation threshold

The author studies the transport properties of disconnected Pb films from 77 down to 0.05 K, at zero and high applied magnetic fields. The conductivity between separated metal clusters is found to be dominated by voltage injection and not by thermal excitation of electrons at low temperatures. This explains temperature-independent resistance tails in both the superconductivity and the normal state of the films, observed at low temperatures in this and in other experiments. I-V characteristics were found to follow a power law I varies as Vⁿ, n>2, at low temperatures, consistent with a space-charge-limited current model for the conductivity.