Semiconductor-metal transition in doped polyacetylene

Abstract

We present studies of the stability and electronic structure of doped finite model polyenes as a function of dopant concentration. The results illustrate the nature of semiconductor-metal transition in doped polyacetylene which differs qualitatively from an inpurity-band insulator-metal transition in a conventional semiconductor. The evolution from insulating behavior to metallic behavior is characterized by four regimes: (I) accommodation of excess carriers in solitons pinned at impurity sites at low dopant concentration, (II) formation of isolated quasimetallic regions resulting from statistical fluctuations in dopant density at slightly higher concentrations, (III) formation of a dense band of localized states resulting from the disordering of an incommensurate charge density wave in the random field of the ionized impurities at still higher concentrations, and (IV) suppression of bond alternation in the presence of strong disorder due to a dense distribution of impurities at extremely high concentrations. The insulator-metal transition occurs at the onset of regime III, and is found at a ∼ 10 at.% dopant concentration in a strictly one-dimensional theory. Including three-dimensional interchain coupling the density is expected to be reduced below 5% where it is observed experimentally. Direct probes of the Fermi level state density are expected to show a smooth evolution through the transition region due to statistical fluctuations in the local dopant density.