We study here the stability of soliton and polaron excitations in a single strand of trans-polyacetylene as a function of the strength of the electron-electron correlations as well as the doping level. We proceed by first solving exactly the continuum version of the Su, Schrieffer, and Heeger Hamiltonian for the single-particle states that arise when electrons are added to a single polymer chain. The role of on-site (U), nearest-neighbor (V), and bond repulsion (W) Coulomb interactions are then obtained from a first-order perturbative calculation with the exact single-particle states. By minimizing the total energy, we are able to determine the relative stability of polaron and soliton configurations. We show that, at a fixed doping level, a polaron lattice is favored over a soliton configuration provided that U and V exceed critical values. However, as the doping level is increased, we show that the critical values of U and V at which a soliton lattice converts to a polaron lattice increase significantly beyond experimentally accepted estimates. W is also shown to favor solitons. We estimate that at a doping level corresponding to the insulator-metal transition (5%), U=4 eV and V=0.4 eV, a soliton lattice is 0.6 eV lower in energy than the corresponding polaron lattice. If we argue that the insulator-metal transition in polyacetylene results from the onset of a polaron lattice at 5%, then our work places restrictions on the magnitude of such effects as interchain coupling which have been proposed in the literature as a principal player in the metal transition in polyacetylene.

• Received 10 September 1993

DOI:https://doi.org/10.1103/PhysRevB.49.5149