Abstract
Although the Neutral-Ionic transition in mixed stack charge-transfer crystals was discovered almost forty years ago, many features of this intriguing phase transition, as well as open questions, remain at the heart of today’s science. First of all, there is the most spectacular manifestation of electronic ferroelectricity, in connection with a high degree of covalency between alternating donor and acceptor molecules along stacks. In addition, a charge-transfer instability from a quasi-neutral to a quasi-ionic state takes place concomitantly with the stack dimerization, which breaks the inversion symmetry. Moreover, these systems exhibit exceptional one-dimensional fluctuations, with an enhancement of the effects of electron-lattice interaction. This may lead to original physical pictures for the dynamics of pre-transitional phenomena, as the possibility of a pronounced Peierls-type instability and/or the generation of unconventional non-linear excitations along stacks. Last but not least, these mixed stack charge-transfer systems constitute a valuable test bed to explore some of the key questions of ultrafast photo-induced phenomena, such as multiscale dynamics, selective coherent excitations and non-linear responsiveness. These different aspects will be discussed through the structural and dynamical features of the neutral-ionic transition, considering old and recent results, open questions and future opportunities. In particular, we revisit the structural changes and symmetry considerations, the pressure-temperature phase diagrams and conclude by their interplay with the photo-induced dynamics.
Highlights
In this review about structural and dynamical properties we mainly focus on the case of the dimethyltetrathiafulvalene-p-chloranil (DMTTF-CA) [47] which presents two key physical features different from TTF-CA: a cell doubling in the ordered I phase and a quasi-continuous transition [48]
The analogy between the TTF-CA phase diagram and the gas-liquid-solid one is illustrative of the interplay between the charge transfer instability and the ferroelectric ordering
This places the N-I transition in an exceptional situation in the universal field of phase transitions as it combines on an equal footing isostructural transformation and symmetry breaking
Summary
Despite the Neutral-Ionic (N-I) phase transition was discovered a long time ago, and that a large number of progresses have been carried out since through numerous experimental and theoretical works, many key physical features are hand in hand with many today’s science topics: Mott physics, electronic ferroelectricity, excitations in one-dimension (1D), light or electric field pulse control,. Increase This with athe low q, stable at low pressure, to a quasi-ionic statetowith a high q, stable at high the N-I phase transition (Figure 1) has been demonstrated to originate from an unusual large increase of the degree of charge transfer, i.e., the ionicity [4,5], from a quasi-neutral molecular state with a low is in agreement with the gain under in[4,5], the Madelung electrostatic energy of the degree of charge transfer, i.e.,pressure the ionicity from a quasi-neutral molecular statewhich with acompetes low q, stable at low pressure, to a quasi-ionic state with a high q, stable at high pressure This is in with the ionization of a DA. Interactions may drive (anti)ferroelectric stack giving rise to polar DA chains, mainly in the I Mott phase and generally discussed in terms of spin-Peierls instability. Cooperative inter-stack interactions may drive (anti)ferroelectric ordering phenomena
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