Conduction in granular metals by hopping via virtual states

It is shown that the conductivity of granular metals on the insulating side of the metal-insulator transition can be essentially determined by intergrain tunnelling via virtual localized states of intermediate grains. This mechanism gives rise lo the temperature dependence of the conductivity in the form In σ ≈ N constant – (T o/T)x, where x ≈ 0.4, and to a large enhancement of the conductivity.