POLARONS AS NUCLEATION DROPLETS IN NONDEGENERATE POLYMERS

We present a study of the nucleation mechanism that allows the decay of the metastable phase (trans-cisoid) to the stable phase (cis-transoid) in quasi-one-dimensional nondegenerate polymers within the continuum electron-phonon model. The electron-phonon configurations that lead to the decay, i.e., the critical droplets (or transition state), are identified as polarons of the metastable phase. We obtain an estimate for the decay rate via thermal activation within a range of parameters consistent with experimental values for the gap of the cis configuration. It is pointed out that, upon doping, the activation barriers of the excited states are quite smaller and the decay rate is greatly enhanced. Typical activation energies for electron or hole polarons are almost-equal-to 0.1 eV and the typical size for a critical droplet (polaron) is about 20 angstrom. Decay via quantum nucleation is also studied and it is found that the crossover temperature between quantum nucleation and thermal activation is of order T(c) less-than-or-equal-to 40 K. Metastable configurations of nondegenerate polymers may provide examples for mesoscopic quantum tunneling.