Diatomic Polymers, Mixed-Stack Compounds, and the Soliton Lattice

Abstract

The continuum electron-phonon system is studied for ${(\mathrm{AB})}_{x}$-type polymers with an average charge of $1+\stackrel{^}{\ensuremath{\rho}}$ per site. For $\stackrel{^}{\ensuremath{\rho}}=0$ spontaneous bond dimerization and soliton excitations are possible if $\ensuremath{\alpha}<{\ensuremath{\Delta}}_{0}$, where $\ensuremath{\alpha}$ is the intrinsic gap and ${\ensuremath{\Delta}}_{0}$ is the spontaneous dimerization gap when $\ensuremath{\alpha}=0$. Recently observed neutral-to-ionic transitions in mixedstack organic compounds are reinterpreted as the onset of such bond dimerization. For $\stackrel{^}{\ensuremath{\rho}}\ensuremath{\ne}0$ spontaneous order and a soliton lattice are found even if $\ensuremath{\alpha}>~{\ensuremath{\Delta}}_{0}$. The electronic gap then has a maximum at a finite $\stackrel{^}{\ensuremath{\rho}}$.