Solitons and Twistons in Conducting Polymers

Abstract

An important property of polyacetylene (PA) is the spontaneous violation of symmetry, which manifests itself in an alternation of bond lengths between the carbon atoms in molecule. Because of twofold degeneracy of the ground state domain walls may appear between the equivalent phases. Such walls in the molecular chain are quasiparticles of the type of topological solitons, or kinks¹’². For an isolated chain kink can be located at any point. But in a system of interacting chains a divergence of a pair of kinks or the displacement of one of the kink from the end of the chain corresponds to the nucleation on this chain of another phase, which results in a loss of surface energy — the confinement energy W = Fx (x-the distance between kinks, or the distance from a kink to the end of the chain)²’³. Thus the single kink should be localized at the ends of chains, and for charge solitons the confinement to bipolarons is possible^4,5. However as can be seen from the experimental data⁶ the spins are delocalized at least by a distance of 10² A. Since the interaction between the chains is not very small the plausibility of a soliton mechanism for PA appears questionable.