Theory of Neutral-Ionic Transition in Organic Crystals. II. Effect of the Intersite Coulomb Interaction

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Abstract
The effects of the Coulomb interaction V (Madelung energy) between nearest neighbor sites on the neutral-ionic (NI) transition are studied. The transition becomes discontinuous due to this collective interaction. The degree of charge transfer ρ, charge transfer gap E CT , magnetic gap E MAG and imaginary-time current-current correlation function (which is related to the charge transfer absorption spectrum) are calculated by the quantum Monte Carlo simulation. From these results, we propose a new kind of elementary excitation–neutral-ionic domain wall (NIDW)–discussing the breakdown of the conventional exciton picture near the transition point.