Zero-temperature phase diagram of mixed-stack charge-transfer crystals

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Abstract
The zero-temperature phase diagram of mixed-stack charge-transfer crystals is investigated through a diagrammatic valence-bond technique. The direct solution of the single-chain Hubbard Hamiltonian inclusive of intersite Coulomb interactions constitutes the basis for a perturbative, adiabatic treatment of the electron–lattice-phonon and electron–molecular-vibration couplings. It is shown that intersite Coulomb interactions and electron–molecular-vibration coupling cooperate in favoring uneven distribution of the electronic charge on the sites, possibly giving rise to a discontinuous, first-order neutral-ionic (N-I) phase transition. On the other hand, the electron–lattice-phonon coupling favors an uneven distribution of the charge between the sites, yielding a Peierls-type dimerization instability. However, when all the interactions are turned on, a rather subtle feedback interplay is found, the dimerization instability range being widened by the Coulomb interactions and the electron–molecular-vibration coupling. The resulting four-dimensional phase diagram is able to account for the experimentally observed ground states and phase transitions of mixed-stack charge-transfer crystals. In particular, it is found that the boundary between N regular stack phases and I dimerized ones is, in general, very complex, as several minima in the potential-energy curve are available to the system. Consequently, the corresponding phase transition is very sensitive to the experimental conditions, a situation we believe is encountered in the famous N-I transition of tetrathiafulvalene-chloranil. In addition, preliminary calculations on dimerized stack systems show that whereas ionic regular chains are intrinsically unstable towards dimerization, the corresponding energy gain decreases as the ionicity increases. Therefore at finite temperatures ionic regular stacks may be observed, their transition temperature to the dimerized phase being expected to be lower with increasing ionicity.