Impurity states in dopedtrans-polyacetylene

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Abstract
We assess the importance of the impurity states of a doped trans-polyacetylene chain. The impurity potential is modeled by a point charge that is located off the chain and is screened phenomenologically. The common assumption that the dopant levels of a dimerized chain closely approximate the hydrogenic levels of a point charge is invalid if the impurity is not on the chain, even if the dopant is screened by the bulk dielectric constant. Additional nonhydrogenic states occur well into the gap. The formation energies for charged kink and polaron lattice distortions are found by solving the Su-Schrieffer-Heeger model for polyacetylene with an impurity added. The impurity states severely alter the structure and states of kink and polaron distortions. Moreover, the modifications depend sensitively on the form of the dopant potential. For the dopant screened isotropically by the bulk dielectric constant, the kink distortion has gap states inconsistent with the observed midgap optical absorption. In contrast, the polaron distortion is both stable and predicts a consistent optical threshold. The nature of doping polyacetylene will remain unclear until a realistic model for the dopant and its interaction with polyacetylene has been developed.