RANDOM SOLITONS IN POLYACETYLENE AND THE INSULATOR METAL TRANSITION

A chain with a random arrangement of dimers, i.e. pairs of sites with energy epsilon(b) while all other sites have energy epsilon(a), has been shown to have a set of delocalized states. The number of delocalized states is N1/2, where N is the number of sites on the chain. It has been proposed that the solitons or bipolarons produced by doping conducting polymers are equivalent to the dimers defined above and the resulting delocalized states are responsible for the insulator-metal transition found in these polymers. We point out that polyacetylene chains and solitons have quite different properties from the chains and dimers specified above. In the metallic state the solitons are reflectionless for all electron energies, not just a finite range of energies. Although a random soliton distribution has been shown to lead to an insulator-metal transition at a dopant concentration even smaller than that found experimentally, allowing the chain to relax in order to minimize the energy removes the randomness and reopens the gap. Using the observed infrared absorption due to solitons, we show that the soliton distribution cannot depart much from uniformity at room temperature and below for doping concentration less than about 6%.