Optical properties of molecular conductors: One-dimensional systems with twofold-commensurate charge-density waves

Abstract

A model is presented for the optical properties in the near to far infrared of a twofold-commensurate one-dimensional system of noninteracting molecular-ion chains. Each chain may be subject to an arbitrary type of lattice distortion that doubles the periodicity of the regular chain and is composed of a lattice dimerization (LD) and/or an alternating molecular deformation (AMD). The effect of an external potential induced by the presence of nearby chains of closed-shell counterions is also accounted for. The linear coupling of the electrons to an arbitrary number of intramolecular modes and to one longitudinal acoustic phonon branch is treated in the adiabatic linear-response approximation. No direct electron-electron interaction is explicitly included but the limit case of noninteracting spinless fermions in a large-U system can be dealt with in the present scheme. The results of the model can be directly applied to the analysis of the experimental infrared data of many conducting organic radical salts of 2:1 stoichiometry where LD and/or AMD of small amplitude occur and the on-site electron-electron correlation is thought to play a major role. The model fitting of the data can be used to obtain information on the one-electron bandwidth, the individual contributions to the total gap of charge density waves components centered on the bonds and on the sites, the relevance of the counterion potential, and the strength of the electron-phonon and electron-molecular vibration interactions. Some of these potentialities as well as the ‘‘selection rules’’ governing the infrared activity of the intramolecular and intermolecular modes as phase phonons are illustrated by numerical model calculations. Self-consistent relations and practical criteria that allow one to use the minimal number of adjustable parameters in the calculations are also presented.