Triplet Bipolarons in Disordered Conducting Polymers: An ESR Study

Electron spin resonance measurements on a disordered conducting polymer were performed from 4 K up to 400 K for the first time in order to investigate the nature of the so-called “Pauli-like” regime. The usual departure of the susceptibility from a Curie law, a shift in the g -factor value and a transition from a line broadening regime to a line narrowing one were observed for T > 200 K. We argue that all of these high-temperature ESR properties are due to thermal excitation of bipolarons from the singlet to the triplet state. The appearance of Pauli paramagnetism is ruled out for this system. A detailed discussion of the susceptibility, spin relaxation and g -factor is given.