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arXiv · 0712.3267

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

A. Girlando·M. Masino·A. Painelli·N. Drichko·M. Dressel·A. Brillante·R. G. Della Valle·E. Venuti
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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.

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A. Girlando, M. Masino, A. Painelli, N. Drichko, M. Dressel, A. Brillante, R. G. Della Valle, E. Venuti. 2007-12-19. Direct evidence of overdamped Peierls-coupled modes in TTF-CA temperature-induced phase transition. https://doi.org/10.1103/physrevb.78.045103

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