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.

Figure 1

Left panel: experimental R spectra of DMTTF-CA, measured with incident and scattered light polarized along the stack axis. Thin lines refer to the high- and low-$T$ phases (green/light gray and blue/gray, respectively). Thick red/dark gray lines are used for the spectra collected just at and below NIT. Right panels: calculated IR (top) and R (bottom) spectra.

Figure 2

Left panels: experimental spectra of DMTTF-CA from Ref. 11. Green/light gray lines are the IR spectra collected with light polarized along the stack axis. Black lines are the R spectra collected with incident and scattered light polarized perpendicular to the stack. Right panels: calculated IR and R spectra. The inset shows the empirical $V(T)$ relation.

Figure 3

Polarization (left column) and polarizability (right column) vs $q$ and $\delta$. Upper (3D plot) and middle (color/grayscale map) panels show $P(q,\delta )$ and $\ln [\alpha (q,\delta )]$ calculated for $V=1.3$ ($T=120$ K). Lower panels: $P(q,\delta )$ and $\ln [\alpha (q,\delta )]$ calculated for $V=2.2$ ($T=$ 55 K). White curves show the relevant PES, in terms of equispaced (50 K) isolines starting from the equilibrium position(s) (white crosses).