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

The continuum electron-phonon system is studied for ${\mathrm{}\left(\mathrm{AB}\right)\mathrm{}}_x$-type polymers with an average charge of $1+\hat{\rho }$ per site. For $\hat{\rho }=0\mathrm{}$ spontaneous bond dimerization and soliton excitations are possible if $\alpha <{\Delta }_0$, where $\alpha$ is the intrinsic gap and ${\Delta }_0$ is the spontaneous dimerization gap when $\alpha =0\mathrm{}$. Recently observed neutral-to-ionic transitions in mixedstack organic compounds are reinterpreted as the onset of such bond dimerization. For $\hat{\rho }\ne 0$ spontaneous order and a soliton lattice are found even if $\alpha >~{\Delta }_0$. The electronic gap then has a maximum at a finite $\hat{\rho }$.