Polarization and polarizability in extended one-dimensional organic materials

Abstract: The modern theory of polarization in extended insulators is applied to one-dimensional models for conjugated polymers and charge transfer salts. Closed expressions for the dependence of the polarization on the site and bond energy alternations are presented for uncorrelated models, and results from exact real-space diagonalization are obtained for correlated models. Changes in polarization induced by lattice phonons or molecular vibrations are directly related to the intensity of infrared bands in the far and mid-IR, respectively. We model intensities by introducing linear electron-vibration coupling and show that coupling to delocalized electrons generates a combination band consisting of a lattice phonon and a molecular vibration. The displaced dipole operator is defined on a real-space basis allowing for the finite field calculation of linear polarizability in finite size systems with periodic boundary conditions. Size-consistency arguments are used to demonstrate that the resulting polarizability becomes exact in the thermodynamic limit, and numerical calculations demonstrate that this approach leads to reliable results that converge rapidly to the thermodynamic limit. (c) 2005 Elsevier B.V. All rights reserved.