paper

Dielectric response of modified Hubbard models with neutral-ionic and Peierls transitions

arXiv:cond-mat/0401261 · doi:10.1063/1.1665824

Abstract

The dipole P(F) of systems with periodic boundary conditions (PBC) in a static electric field F is applied to one-dimensional Peierls-Hubbard models for organic charge-transfer (CT) salts. Exact results for P(F) are obtained for finite systems of N = 14 and 16 sites that are almost converged to infinite chains in deformable lattices subject to a Peierls transition. The electronic polarizability per site, α_{el} = (\partial P/\partial F)_0, of rigid stacks with alternating transfer integrals t(1 +/- δ) diverges at the neutral-ionic transition for δ= 0 but remains finite for δ> 0 in dimerized chains. The Peierls or dimerization mode couples to charge fluctuations along the stack and results in large vibrational contributions, α_{vib}, that are related to \partial P/\partial δand that peak sharply at the Peierls transition. The extension of P(F) to correlated electronic states yields the dielectric response κof models with neutral-ionic or Peierls transitions, where κpeaks >100 are found with parameters used previously for variable ionicity ρand vibrational spectra of CT salts. The calculated κaccounts for the dielectric response of CT salts based on substituted TTFs (tetrathiafulvalene) and substituted CAs (chloranil). The role of lattice stiffness appears clearly in models: soft systems have a Peierls instability at small ρand continuous crossover to large ρ, while stiff stacks such as TTF-CA have a first-order transition with discontinuous ρthat is both a neutral-ionic and Peierls transition. The transitions are associated with tuning the electronic ground state of insulators via temperature or pressure in experiments, or via model parameters in calculations.

arXiv comment10 pages, 9 figures; J.Chem.Phys., in press

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