Magnetic susceptibility due to disorder-induced neutral solitons in interacting polymer chains

We study the magnetic response due to neutral solitons induced by disorder in polymer materials. We account for interchain interactions, which, if suf ficiently strong, result in a bond-ordered phase, in which the neutral soli tons are bound into pairs. We analytically calculate the corresponding pair -size distribution. As the spins of the solitons have a distance-dependent antiferromagnetic coupling, this allows us to calculate the magnetic suscep tibility in the ordered phase. At low temperatures, the result deviates fro m the usual Curie behavior in a way that depends on the relative strength o f the disorder and the interchain interactions. We compare our results to t he observed magnetic susceptibility of transpolyacetylene and we suggest ex periments extending towards lower temperatures. [S0163-1829(99)02721-6].