Direct evidence of overdamped Peierls-coupled modes in TTF-CA temperature-induced phase transition

TL;DR

This study provides direct experimental evidence of overdamped Peierls-coupled phonon modes in TTF-CA, revealing a soft-mode behavior and electron-phonon interactions near the neutral-ionic phase transition.

Contribution

It offers the first detailed optical analysis of phonon behavior and electron-phonon coupling in TTF-CA during its phase transition, highlighting overdamped modes and softening phenomena.

Findings

Overdamped lowest-frequency phonon near transition temperature.

Spectral weight shifts towards lower-frequency phonons.

Soft-mode behavior observed only for the lowest phonon.

Abstract

In this paper we elucidate the optical response resulting from the interplay of charge distribution (ionicity) and Peierls instability (dimerization) in the neutral-ionic, ferroelectric phase transition of tetrathiafulvalene-chloranil (TTF-CA), a mixed-stack quasi-one-dimensional charge-transfer crystal. We present far-infrared reflectivity measurements down to 5 cm-1 as a function of temperature above the phase transition (300 - 82 K). The coupling between electrons and lattice phonons in the pre-transitional regime is analyzed on the basis of phonon eigenvectors and polarizability calculations of the one-dimensional Peierls-Hubbard model. We find a multi-phonon Peierls coupling, but on approaching the transition the spectral weight and the coupling shift progressively towards the phonons at lower frequencies, resulting in a soft-mode behavior only for the lowest frequency phonon near the transition temperature. Moreover, in the proximity of the phase transition, the lowest-frequency phonon becomes overdamped, due to anharmonicity induced by its coupling to electrons. The implications of these findings for the neutral-ionic transition mechanism is shortly discussed.