Bond length order parameter of conjugated polymers

It is usually assumed that the ground state lattice configuration of polyac etylene is a dimerized chain. We found that the energy can be further reduc ed by an overall chain length contraction, which leads to a new bond length order parameter, in addition to the well-known bond alternation order para meter. For quasi-particles like solitons and polarons, these two order para meters are coupled with the electron wave functions and vary in space. We g eneralize the Takayama-Liu-Maki equations in the continuum limit to incorpo rate this new order parameter. Significant modifications are found for the polaron solution. Polyacene, a ladder polymer, is also studied with the inc lusion of the bond length order parameter. The ground state is predicted to be a non-alternating structure, with zero energy gap and unusually high el ectric conductivity. (C) 1999 Elsevier Science B.V. All rights reserved.