Metallic polyacetylene is a soliton lattice

Abstract

We present a series of calculations which suggest that a soliton lattice persists into the metallic phase of polycetylene and can account for the onset of the Pauli susceptibility at a doping level of 5%. The starting point for analysis is the continuum version of the SSH Hamiltonian. We solve exactly for the single-particle states that arise when n-doped 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. We minimize the total energy and hence are able to assess the relative stability of soliton and polaron configurations. We show that as the doping level approaches metal concentrations, 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. In fact, we estimate that a doping level corresponding polaron lattice. We then show that the bound-state soliton levels merge to fill the gap sufficiently that the Pauli susceptibility becomes non-zero and comparable to the corresponding experimental values.