2-PHASE COEXISTENCE AND SEMIMETALLIC STATES IN CONDUCTING POLYMERS

We study a simple model of conducting polymers, such as polyacetylene, which consists of a quasi-one-dimensional coupled electron-phonon system with short-range electron-electron repulsions. The problem is solved in the Hartree-Fock approximation. A variety of experimentally measurable properties of the system, especially the infrared absorption spectrum, are computed. (i) As a function of doping, the interchain coupling causes the system to undergo a phase transition from a correlated soliton-lattice state to a metallic state, described by a weakly interacting but highly anisotropic three-dimensional Fermi liquid. The result is consistent with a previous renormalization-group treatment. (ii) When interactions with counterions are included, the nonmetal-metal transition is very sensitive to the arrangement of counterions relative to the polymer chain: a minor change in the arrangement will either stabilize or destabilize the soliton state allowing the coexistence of the metallic and the soliton states. (iii) For realistic three-dimensional band structures, a semimetallic phase intervenes the nonmetallic (Peierls) and metallic (Fermi-liquid) phases. © 1994 The American Physical Society.