Drude conductivity of a granular metal

We present a complete derivation of the analog to Drude conductivity in granular metals using diagrammatic methods. The derivation of Drude conductivity in a homogeneous metal using Kubo linear response theory is somewhat involved, with the evaluation of a singular double integral being required to cancel the diamagnetic response. We first provide a history and careful derivation of this standard result, before analyzing the equivalent situation in the model of a granular metal as a lattice of metallic grains connected by tunneling. A singular triple integral occurs here, and its evaluation gives the result expected from the Einstein relation and Fermi’s golden rule.