Activation energy of a charged soliton pinned by a dopant in polyacetylene

Summary form only given. The theoretically estimated activation energy of a charged soliton pinned by the Coulomb force of a dopant in polyacetylene is pointed out to be too large for thermal depinning. This seems to indicate that the soliton cannot be a charge carrier. In the real systems, however, the activation energy may not be so large if we consider the effects of finite concentration of dopants. The dopant concentration dependence of the activation energy of the charged soliton pinned by regularly arranged off-chain dopant ions is investigated by using the SSH model with a long ranged potential term due to dopants. For simplicity we consider a single soliton problem. The soliton width is variationally determined for its location at a pinning center and at a midpoint between neighboring pinning centers by minimizing the total energy under the constriction. The activation energy is defined as the energy difference between two states obtained in that way. It is shown that the activation energy decreases rapidly with the dopant concentration. This result can be qualitatively understood in the following way; 1) the effective potential barrier for the soliton motion is reduced because the amplitude of the spatial variation of the potential decreases with increasing concentration, and 2) the finite width of the soliton acts to average the spatial variation of the potential and therefore enhances the decrease of the effective barrier with dopant concentration. The effects of the intrachain electron-electron interactions will be also considered.