Dimerization transition versus neutral-ionic transition in organic mixed-stack compounds

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Abstract
The neutral-ionic transition in organic mixed-stack compounds is studied by using the finite-size extrapolation procedure. It is shown that when neglecting the electron-phonon interaction, the low-ionicity phase has a nondegenerate ground state and all excitations have a finite gap, while the high-ionicity phase has a highly degenerate ground state and both the charge-transfer and spin-flip excitations are gapless. The transition is of first or second-order depending on the strength of the overlap integral. When electron-phonon coupling is taken into account, these excitations acquire a gap even in the ionic phase and the order parameter is then the dimerization. The phase boundaries are shifted considerably.