Charge fluctuations and electron–phonon coupling in organic charge-transfer salts with neutral–ionic and Peierls transitions

Abstract

The first-order transition of the charge-transfer complex TTF-CA (tetrathiafulvalenechloranil) is both a neutral-ionic and a Peierls transition. In related organic charge transfer complexes, cooling at ambient pressure increases the ionicity ρ in strikingly different ways, and is sometimes accompanied by a dielectric peak, that we relate to lattice stiffness, to structural and energetic disorder, and to the softening of the Peierls mode in the far-IR. The position operator P for systems with periodic boundary conditions makes possible a systematic treatment of electron-phonon interactions in extended donor-acceptor stacks in terms of correlated Peierls-Hubbard models. The IR intensity of the Peierls mode peaks at the Peierls transition at small ρ < 1/2 in soft lattices, where the dielectric constant also has a large peak. In dimerized stacks, the IR intensity of totally symmetric, Raman active, molecular vibrations is related to charge fluctuations that modulate site energies. Combination bands of molecular and Peierls modes are identified in regular TTF-CA stacks above T c . Energetic disorder can suppress the Peierls transition and rationalize a continuous crossover from small to large ρ. The TTF-CA scenario of a neutral-regular to ionic-dimerized transition must be broadened considerably in view of charge transfer salts that dimerize on the neutral side, that become ionic without a structural change, or that show vibrational evidence for dimerization at constant ρ < 1.

Phenomenology of the Neutral-Ionic Valence Instability in Mixed Stack Charge-Transfer Crystals

“…The number of CT crystals undergoing T-induced valence instabilities is very limited, the main features of the transitions being summarized in Table 1. We have excluded from the Table systems whose ionicity continuously increase by lowering T without crossing the N-I borderline, ionic systems only undergoing a Peierls transition without ionicity change [7], or systems undergoing quantum phase transitions [14]. [20] a TTF: tetrathiafulvalene; CA: chloranil; BrCl 3 BQ: bromo-trichloro-p-benzoquinone; ClMePD: 2-chloro-5-methyl-p-phenylendiamine; DCNQI: 2,5-dimethyl-dicyanoquinonediimine; DMeTTF: 4,4 -dimethyl-TTF; 2,5Br 2 Cl 2 BQ: 2,5-dichloro-3,6-dibromo-p-benzoquinone; TMB: 3,3 ,5,5 -tetramethylbenzidine; TCNQ: tetracyanoquinodimethane.…”

“…The latter class exist only at finite temperatures, as ionic regular stacks are intrinsically unstable towards dimerization [7]. Although several aspects of NIT, particularly the p-induced ones, still await full understanding, researchers interests have slowly drifted towards more application-oriented issues [9].…”

Bond-order wave phase of the extended Hubbard model: Electronic solitons, paramagnetism, and coupling to Peierls and Holstein phonons

“…The operators a † pσ , a p,σ create, annihilate electrons with spin σ in the lowest unoccupied molecular orbital of TCNQ, with equal energy ∆ = 0 and n p = 1 for a TCNQ − stack. Charge fluctuations [27] modulate ∆ and illustrate linear Holstein coupling g n to the n th totally symmetric (ts) molecular vibration. When C i symmetry is broken, ts modes become strongly IR allowed by borrowing intensity from the optical charge-transfer excitation and they are polarized along the chain.…”

“…The gs of correlated models is unconditionally dimerized when the regular array has E m = 0 and χ d (δ) = −(∂ 2 ǫ 0 /∂δ 2 ) diverges at δ = 0. Examples [27] include the ionic phase of organic charge transfer salts and the spin fluid phase of Hubbard models or Heisenberg spin chains. Dimerization is conditional at ǫ d χ d (0) = 1 when χ d (0) is finite in the neutral phase of CT salts or the CDW phase of the EHM.…”

Evaluation of the electron–TO–phonon interaction in polar crystals from experimental data

“…In particular, methods of vibrational spectroscopy have been shown to yield reliable estimates of several fundamental microscopic parameters, including relevant constants of electron-phonon interactions (e.g., [1][2][3][4][5] and references therein).…”