Quantum lattice fluctuations in the ground state of one-band MX complexes

The effects of quantum lattice fluctuations on the charge density wave (CDW) of a one-dimensional one-band model for halogen-bridged mixed-valence linear chain complexes are studied by a functional integral approach. An equation for the phonon order parameter is derived within a one-loop approximation for all values of the ionic mass. We find that the dimerized CDW survives the quantum lattice fluctuations for arbitrarily small electron-lattice coupling. The result is the same as that of the Su-Schrieffer-Heeger model, although these two models are different for the interactions. The electron-phonon interactions are site-diagonal for the MX model and site-off-diagonal for the SSH model. Furthermore, in the case of finite ionic mass, the low-energy properties of the system are governed by the limit of M = 0 for a weak electron-phonon interaction, whereas they are governed by the adiabatic limit (M = ∞) for a stronger electron-phonon interaction.