Anharmonic excitations, time correlations and electric conductivity

We study the influence of anharmonic mechanical excitations of a classical ionic lattice on its electric properties. First, to illustrate salient features, we investigate a simple model, an one-dimensional (I D) system consisting of ten semiclassical electrons embedded in a lattice or a ring with ten ions interacting with exponentially repulsive interactions. The lattice is embedded in a thermal bath. Tile behavior of the velocity autocorrelation function and the dynamic structure factor of the system are analyzed. We show that in this model the nonlinear excitations lead to long lasting time correlations and, correspondingly, to an increase of the conductivity in a narrow temperature region, where the excitations are supersonic soliton-like. In the second part we consider the quantum statistics of general ion-electron systems with arbitrary dimension and express - following linear response transport theory - the quantum-mechanical conductivity by means of equilibrium time correlation functions. Within the relaxation time approach an expression for the effective collision frequency is derived in Born approximation, which takes into account quantum effects and dynamic effects of the ion motion through the dynamic structure factor of the lattice and the quantum dynamics of the electrons. An evaluation of the influenec of solitons predicts for I D-lattices a conductivity increase in the temperature region where most thermal solitons are excited, similar as shown in the classical Drude-Lorentz-Kubo framework. (C) 2007 WILEYNCH Vertag GmbH & Co. KGaA, Weinheim.