AN ANDERSON IMPURITY IN CONJUGATED POLYMERS .1. A NEW LOCALIZED STATE IN THE ELECTRONIC GAP FOR THE IMPURITY WITH U=0

An Anderson impurity in conjugated polymers is formulated in order to investigate the effects of a dopant, a carbonyl (> C = O) defect or an atomic side group. The impurity effects with no Coulomb interaction (U = 0) are studied analytically in the Takayama-Lin-Liu-Maki (TLM) model and also in the Su-Schrieffer-Heeger (SSH) model. A new localized level is found in the electronic gap. It is located at the top of the valence band when the level of the impurity is deep and in the valence band. It is at the bottom of the conduction band if the energy of the impurity level is in the conduction band. Characteristic TLM results agree remarkably well with the corresponding SSH results when the solutions in the TLM model are energetically stable against change of electron number. This agreement does not depend on whether the chain is nearly dimerized or there is a soliton excitation. Consequences for possible experimental observation of the localized state due to the real defect are discussed.