Effects of Lattice and Molecular Phonons on Photoinduced Neutral-to-Ionic Transition Dynamics in Tetrathiafulvalene-$p$-Chloranil

TL;DR

This paper reviews how lattice and molecular phonons influence the photoinduced neutral-to-ionic transition in TTF-CA, highlighting the importance of both couplings in reproducing experimental observations and understanding the transition dynamics.

Contribution

It provides a quantitative analysis of Peierls and Holstein couplings' roles in stabilizing phases and reproducing experimental data in TTF-CA's neutral-ionic transition.

Findings

Both couplings are essential to match experimental ionicity and conductivity.

Holstein couplings stabilize the neutral phase and are crucial for the Mott insulating state.

Lattice dimerization strings are present in the neutral phase above the transition temperature.

Abstract

For electronic states and photoinduced charge dynamics near the neutral-ionic transition in the mixed-stack charge-transfer complex tetrathiafulvalene-$p$-chloranil (TTF-CA), we review the effects of Peierls coupling to lattice phonons modulating transfer integrals and Holstein couplings to molecular vibrations modulating site energies. The former stabilizes the ionic phase and reduces discontinuities in the phase transition, while the latter stabilizes the neutral phase and enhances the discontinuities. To reproduce the experimentally observed ionicity, optical conductivity and photoinduced charge dynamics, both couplings are quantitatively important. In particular, strong Holstein couplings to form the highly-stabilized neutral phase are necessary for the ionic phase to be a Mott insulator with large ionicity. A comparison with the observed photoinduced charge dynamics indicates the presence of strings of lattice dimerization in the neutral phase above the transition temperature.