Electronic states intrans-polyacetylene with site-type impurities in the coherent-potential approximation

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Abstract
Disorder effects on the electronic states in trans-polyacetylene are investigated when site-type impurities are randomly distributed at designated interstitial positions. Use is made of the method previously applied to the bond-type impurities. The generalized Takayama–Lin-Liu–Maki model is studied with the help of the coherent-potential approximation. The dimerization of the lattice is assumed to be represented by a uniform order parameter and the electronic states are half-filled. The order parameter, electronic band structure, density of states, and the total energy are calculated as functions of impurity concentration and strength of the impurity potential. At low concentrations, an impurity band is formed close to either the valence or the conduction band. As the concentration increases, it is connected and absorbed by the latter. When the impurity strength is large enough, the order parameter vanishes in a region around the concentration of 50% together with the energy gap. A phase diagram is given. It has symmetric structure about the 50% concentration.