BINDING-ENERGY OF 1B(U) SINGLET EXCITONS IN THE ONE-DIMENSIONAL EXTENDED HUBBARD-PEIERLS MODEL

Using a symmetrized density-matrix renormalization-group formulation, we have investigated the electronic binding energy of the lowest optic ally allowed exciton (1B(u)) within an extended Hubbard-Peierls model (with parameters U, V, and delta) for conjugated chains with N = 80 si tes. Three symmetries, the C-2, spin parity, and electron-hole symmetr ies, have been applied to construct the projector operator. Analysis o f our results on the ratios between exciton binding energies and excit on energies, sheds light on the current experimental and theoretical c ontroversy on exciton binding for polyacetylene and polyparaphenylene vinylene. We show that in the absence of dimerization, the exciton bin ding energy is vanishingly small for U/t up to 5 and V/t up to 2.3; fo r a finite dimerization. it is only when V is large enough that the ex citon gets to be significantly bound. This result questions the applic ability to conjugated polymers of the strong correlation picture, in w hich the exciton binding energy is equal to V.