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

Masino, Matteo; Castagnetti, Nicola; Girlando, Alberto

doi:10.3390/cryst7040108

Cited by 41 publications

(57 citation statements)

References 85 publications

(158 reference statements)

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“…The ionicity may increase after the transition, possibly driving the system towards large ρ, but without any additional phase transition. This is the behavior observed for DMTTF-CA and DMeTTF-2,5Br 2 Cl 2 Q [25]. Below we will discuss in some detail TTF-CA and DMTTF-CA as prototypical examples of discontinuous and continuous NIT.…”

Section: Dimerization Instability Of the Soft Latticementioning

confidence: 58%

“…Systems with t small with respect to c will be driven towards a discontinuous phase transition with an abrupt ionicity increase. Since the I-phase is unconditionally unstable towards dimerization at 0 K, if the transition temperature is sufficiently low, we expect a discontinuous jump from a neutral regular phase to an I-dimerized phase, as observed in TTF-CA and TTF-BrCl 3 Q (see the companion paper [25]). A continuous NIT is expected in systems with a larger t/ c .…”

Section: Dimerization Instability Of the Soft Latticementioning

confidence: 65%

“…The PES has a single minimum on the neutral side, two minima on the ionic side, and metastable states for very special parameter values in systems with discontinuous NIT. The PES accounts semi-quantitatively for vibrational spectra discussed in Section 4 and by Masino et al [25]. The emergence of highly anharmonic surfaces from a purely harmonic lattice and molecular vibration is an interesting consequence of coupling to correlated electrons that are delocalized along the stack.…”

Section: Discussionmentioning

confidence: 93%

“…The experimental situation is far more nuanced and complex. The IR and Raman spectra of TTF-CA or of DMTTF-CA presented in the companion work by Masino et al [25] require additional lattice phonons and more sophisticated vibrational analysis. Depending on one's perspective, more quantitative treatment either underlines the approximate nature of the microscopic model or merely the need to introduce additional parameters.…”

Section: Discussionmentioning

confidence: 99%

“…The energy of metastable domains depends on the domain length and on the boundaries, as discussed in [20]. The T-induced NIT of TTF-CA occurs at too low temperature to allow for the population of metastable domains, but the pressure-induced NIT at room temperature shows a sizable population of metastable domains, leading to coexisting N and I features in vibrational spectra as shown in Figure 20 of Masino et al [25]. While the thermal population of metastable domains is an interesting feature, the possibility to populate metastable states via photoexcitation in the so-called photoinduced NIT is even more interesting.…”

Section: Multistability and Photoinduced Nitmentioning

confidence: 98%

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Modeling the Neutral-Ionic Transition with Correlated Electrons Coupled to Soft Lattices and Molecules

2017

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Neutral-ionic transitions (NITs) occur in organic charge-transfer (CT) crystals of planar π-electron donors (D) and acceptors (A) that form mixed stacks . . . D +ρ A −ρ D +ρ A −ρ D +ρ A −ρ . . . with variable ionicity 0 < ρ < 1 and electron transfer t along the stack. The microscopic NIT model presented here combines a modified Hubbard model for strongly correlated electrons delocalized along the stack with Coulomb intermolecular interactions treated in mean field. It also accounts for linear coupling of electrons to a harmonic molecular vibration and to the Peierls phonon. This simple framework captures the observed complexity of NITs with continuous and discontinuous ρ on cooling or under pressure, together with the stack's instability to dimerization. The interplay of charge, molecular and lattice degrees of freedom at NIT amplifies the nonlinearity of responses, accounts for the dielectric anomaly, and generates strongly anharmonic potential energy surfaces (PES). Dynamics on the ground state PES address vibrational spectra using time correlation functions. When extended to the excited state PES, the NIT model describes the early (<1 ps) dynamics of transient NIT induced by optical CT excitation with a fs pulse. Although phenomenological, the model parameters are broadly consistent with density functional calculations.Keywords: neutral-ionic phase transition; charge transfer crystals; multistability; correlated electron models; structural instabilities; coupling to molecular and lattice vibrations; anharmonic vibrational spectra; polarization and polarizability; photoinduced phase transitions ForewordThe physics of strongly-correlated electron systems is characterized by a complex phenomenology that includes multiple competing phases, divergent responses, and anomalous metallic states [1]. In these materials the interplay between correlated electrons and phonons further enhances the complexity both in terms of the appearance of new phases and of amplification of materials responses. Mixed-stack charge-transfer (ms-CT) crystals offer an interesting opportunity to study strongly correlated electrons coupled to molecular vibrations and lattice modes. In these crystals, planar π-conjugated molecules with strong electron donor (D) and acceptor (A) character alternatively form face-to-face stacks. Electrons are delocalized along the stack, leading to fractional charge: . . .. . The most impressive demonstration of strong correlations in these systems is offered by the so-called neutral-ionic phase transition (NIT) [2], a transition from a valence insulator (ρ < 0.5) to a Mott insulator (ρ > 0.5), that can be induced by temperature [3], pressure [4], or even light [5][6][7][8][9]. Both continuous and discontinuous NITs are known, and a precise control of

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Section: Dimerization Instability Of the Soft Latticementioning

confidence: 58%

Section: Dimerization Instability Of the Soft Latticementioning

confidence: 65%

Section: Discussionmentioning

confidence: 93%

Section: Discussionmentioning

confidence: 99%

Section: Multistability and Photoinduced Nitmentioning

confidence: 98%

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