Abstract
The neutral-ionic phase transition in TTF-CA was investigated by steady-state and time-resolved infrared spectroscopy. We describe the growth of high-quality single crystals and their characterization. Extended theoretical calculations were performed in order to obtain the band structure, the molecular vibrational modes and the optical spectra along all crystallographic axes. The theoretical results are compared to polarization-dependent infrared reflection experiments. The temperature-dependent optical conductivity is discussed in detail. We study the photo-induced phase transition in the vicinity of thermally-induced neutral-ionic transition. The observed temporal dynamics of the photo-induced states is attributed to the random-walk of neutral-ionic domain walls. We simulate the random-walk annihilation process of domain walls on a one-dimensional chain.
Highlights
A large number of ultrafast pump–probe experiments have been performed on TTF-CA in order to explore the photo-induced phase transition (PIPT) that occurs in this one-dimensional charge-transfer compound, most of them confined to the femto- and picoseconds time range [35,58,69,70,71,72,73,74,75,76,77,78,79,80]
The photon energy corresponds in this case to the energy hωCT of the charge-transfer band/exciton (CT) which we have identified in the spectrum as a broad maximum at 5200 cm−1 (0.65 eV)
The concept of an electronically induced phase transition [104] has already been introduced in Section 5 above, where we suggested that the creation of a domain can be considered as the excitation of neutral-ionic domain-wall (NIDW) pairs, which are the lowest excitation state in TTF-CA
Summary
Due to the small charge transfer, the crystals glimmer greenish and transparent because the total absorption and reflection spectrum is composed of the single spectrum of the quasi-neutral TTF0 and CA0 molecules whose electronic excitations in the visible range are blue-shifted. For that reason, it is often called the green phase in literature. The strong dimerization and the charge transfer lead to the creation of strong electric dipoles between the TTF and CA molecules This was demonstrated by dielectric measurements showing a sharp feature in 1 at the transition temperature TNI. To make a conclusion on this “exotic” state, it will be necessary to perform polarized infrared studies on single crystals under high pressure
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