Theory of Neutral-Ionic Transition in Organic Crystals. I. Monte Carlo Simulation of Modified Hubbard Model

Abstract

The one dimensional modified Hubbard model, which is a simplest model showing the neutral-ionic (NI) transition, is investigated by the quantum Monte Carlo simulation. The physical quantities concerning the ground state (the kinetic energy, the degree of charge transfer, the probability of the double occupancy), correlation functions and excitation energies (charge transfer gap E CT and magnetic gap E MAC ) are calculated as functions of the site energy difference Δ . It is concluded that the model shows two phases according to Δ. The ionic phase (Δ<Δ c ) is characterized by the gapless spin wave which leads to the divergence of the correlation functions in the same manner as the case of Δ=0 (the Hubbard model). When Δ exceeds Δ c , E MAG starts to rise continuously (neutral phase), while E CT has a minimum around Δ c . Within the statistical accuracy, our simulation does not exclude the appearance of the metallic state at the transition point.