1(1)B(u)(-)2 (1)A(g)(+) crossover in conjugated polymers: The phase diagram of the molecular-orbital model

Journal article

Using the molecular-orbital model of conjugated polymers, a simple criterion for the relative ordering of 1 1Bu- and 21Ag+ states is provided. When the repeat unit electronic correlations increase, resulting in an increase in the triplet-singlet exchange energy, the 21Ag+ state is expected to lower relative to the 11Bu- state. This result is confirmed by density-matrix renormalization-group calculations. Close to the phase boundary there are finite-size corrections resulting in a 11Bu- and 21Ag+ crossover as a function of chain length. The nature of this crossover depends on the relative size of the excitons. A one-particle singlet excitation correlation function is shown to be a suitable means for evaluating particle-hole separations. It is further shown that the charge gap is a reliable means of determining the threshold of unbound states in the single-particle excitation channel. The biexciton triplet-triplet character of the 2Ag+ states is investigated. The triplet-triplet separation is measured via a two-particle triplet-triplet excitation correlation function. The triplet-triplet gap marks the threshold of unbound triplet-triplet states. © 1999 The American Physical Society.

Authors: Lavrentiev, M; Barford, W

• Publication status: Published
• Journal: Physical Review B
• Volume: 59
• Issue: 23
• Pages: 15048-15055
• Publication date: 1999-06-15
• ISSN: 1098-0121
• Language: English