Effect of Coulomb correlation on hopping conductivity

The analysis of hopping conductivity requires a complete quantum-statistical description of a pair of sites between which an electron jump takes place. This description includes a double-charged energy level, the energy being increased by the Coulomb repulsion of the charges. The inclusion of this level changes the occupation numbers of single-charge states essential for hopping conductivity. Such an indirect influence of Coulomb repulsion is referred to as Coulomb correlation. It leads to a modification of Mott's law in the case of conductivity with variable range hopping. The concept of a Coulomb gap is not required for the characterization of this modification.