Surface states in doped polymers

Surface states of doped polymers are investigated by means of the Green-function technique. We consider only the particular case in which the surface is created right next to the impurity. It is shown that the surface state that resulted from cutting the double bond is qualitatively different from that created by cutting the single bond. In the former case, there is a midgap state and two localized states outside the energy bands, while in the latter case there are only two localized states, which may occur outside the energy bands or in the energy band gap depending on the sign of the impurity potential.