Correlated electrons in soft lattices: Raman scattering evidence of the nonequilibrium dielectric divergence at the neutral-ionic phase transition

A broad low-frequency Raman band is observed in the proximity of the temperature-induced neutral-ionic phase transition (NIT) of a mixed-stack charge-transfer crystal, 4,4'-dimethyltetrathiafulvalene-chloranil (DMTTF-CA). Adopting a modified Hubbard model with coupling to lattice phonons and molecular vibrations, we propose a time-correlation-function approach to the calculation of the Raman and infrared spectra of these quasi-one-dimensional systems. The approach provides a unifying picture for several puzzling features observed in vibrational spectra across the NIT, and ascribes the anomalous intensity of the Raman band to the divergent polarizability of the electronic system.