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

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.