Transition to a spin-gap state in a doped conducting polymer is reported for the first time. A decrease in the ESR spin susceptibility was observed near 200 K in trans-polyacetylene doped with perchloric ion ( ${\mathrm{ClO}}_4^{-}$). The thermoelectric power did not show any corresponding anomaly, while the activation energy of the resistivity was decreased slightly on the spin-gap formation. These results suggest that the electronic state undergoes a spin-charge separation and the carriers are actuated by lower energy. It was also shown, by the lattice heat capacity measurement, that the lattice structure, connected to the framework for the soliton lattice, is changed remarkably at the initial stage of doping.

• Received 4 September 1998

DOI:https://doi.org/10.1103/PhysRevLett.82.2151