The effect of energy-dependent densities of states and the importance of correlations on the conductivity of the small-polaron hopping regime in disordered systems

Abstract

We investigate, using percolation arguments, the effect of energy-dependent model densities of states on the d.c. conductivity behaviour in the small-polaron hopping regime, taking into account the fact that the lower-energy states are less ‘dense’. General expressions for T₀ and v, which determine the conductivity (In σ ˜ -(T₀/T)^v), are obtained for the high- and low-temperature regimes, ignoring or taking into account correlations between bonds due to the energy of the common site. For a constant density of states the importance of correlations is revealed by analysing experimental data previously reported for V₂O₅-P₂O₅ glasses using In σ against T^−2/5 and T^−1/4 plots as suggested by the Triberis-Friedman and the Triberis models. The agreement of the experimental data with the theory is very satisfactory.