SOLITON MODEL OF METALLIC STATE IN POLYACETYLENE

Doping of polyacetylene by more than 4-7% causes an insulator-metal transition, as evidenced by the appearance of sizable Pauli susceptibility and large conductivity. The nature of the metallic state has been the subject of wide speculation. We show that there is strong evidence, mainly from optical absorption measurements, that the soliton lattice characteristic of doped polyacetylene below the transition persists in the metallic state. Another strong piece of evidence for the soliton lattice model is the existence of samples in which free carrier absorption is present at 300 K, but disappears at low temperatures. We show that other proposed models of the metallic state (polaron lattice, uniform chain and polson model) cannot explain the data.