Quantum fluctuations of the electron and phonon fields in heavily doped polyacetylene

Abstract

Doped polyacetylene exhibits a transition to a metallic phase when doping concentration increases beyond a critical doping concentration. This metallic phase has not yet been understood well. When we treat both the electronic and lattice degrees of freedom within the mean field approximation, the single chain ground-state in this metallic doping regime is a charged soliton lattice with an energy gap at the Fermi energy. This theoretical result is not consistent with the experimental fact. On the other hand, the mean field approximation is not valid in this case. First, quantum lattice fluctuations are estimated to be so large that they might break the energy gap of a charged soliton lattice. However, since the theoretical result is based on the adiabatic approximation, it is not still clear whether a charged soliton lattice becomes metallic under the quantum lattice fluctuations. Moreover, since a soliton is stabilized by the adiabatic effects of electron-phonon coupling, inverse-adiabatic effects might make a soliton lattice unstable. Second, it is generally accepted that electron correlation effect is very important in the quasi one-dimensional systems such as polyacetylene. However, there has no satisfactory theory to describe the effect in this regime. Thus, we adopt the PPP model where both the Coulomb interaction between /spl pi/-electrons and the electron-phonon interaction are included. We calculate various physical quantities such as charge density, bond order and lattice displacement distributions by quantum Monte Carlo method, where electron correlation and quantum lattice fluctuations are described without any approximations. From them we investigate the effects of the quantum fluctuations on the ground-state of this model in the heavily doped regime and see whether the metallic properties can be explained by the present single chain model. Furthermore, we make a phase diagram of this model by changing some parameters, and see the effects of various interactions and competition between them.