Screened hopping conduction in ultrathin metal films

We have found that screening of the Coulomb interaction in ultrathin, quench-condensed bismuth films causes the conductivity to become simply activated. We explain this in terms of fixed-range hopping of dipoles, the dipoles consisting of localized charges in the experimental film and their images in the screening electrode. We deduce localization lengths of the order of 22 nm. Non-ohmic effects imply a scale length of the order of 700 nm which probably corresponds to the typical distance between difficult hops in the critical percolation network. We predict reversion to a temperature-dependent activation energy closer to the metal-insulator transition.