Excitonic insulator transition in the conjugated polymer polyacene

Abstract

According to molecular orbital theory, the symmetrically positioned one-dimensional (1-D) conduction and valence bands of polyacene touch at the X point. Clearly, the exciton binding energy of this semimetal exceeds the band gap so that polyacene should be a textbook case of a semimetal undergoing a transition to an excitonic insulator. We investigate qualitatively this scenario for polyacene at zero temperature ( T=0). Collectively bound electron-hole pairs (singlet- or triplet-state excitons) become mixed into the new excitonic ground state (EGS), much like pairs of electrons become mixed into the ground state of a Bardeen–Cooper–Schrieffer (BCS) superconductor. But quite unlike the latter case, the bound electron-hole pairs in the EGS can be directly photoionized by light with energy ℏ ω≥2Δ, where 2Δ is the mean excitonic energy gap of the polymer. Thus, a new (IR) optical absorption appears in the polymer, polarized along the chain direction ( σ 1( ω)≠0) and rendering the polymer an IR photoconductor. To our knowledge, the optical absorption of the excitonic insulator for ℏ ω≥2Δ has not been previously investigated. The prospect of a polymeric quasi-1-D excitonic insulator is an interesting one.