Doping-induced structural phase transition in polyacetylene

We consider an anisotropic planar-rotor model on a two-dimensional triangular lattice to model the dopant ordering in polyacetylene in the process of alkali-metal doping. With the combination of a Ginzberg-Landau expansion of the free energy close to the critical temperature and total-energy calculations at zero temperature, we obtain a phase diagram in the plane of temperature and alkali-metal chemical potential, which shows various modulated phases which appear in the process of alkali-metal doping of the polymer. These results are compared with experimental observations by Winokur et al. [Phys. Rev. Lett. 58, 2329 (1987)].