POLARONS AS NUCLEATION DROPLETS IN NONDEGENERATE POLYMERS

We present a study of the nucleation mechanism that allows the decay o f the metastable phase (trans-cisoid) to the stable phase (cis-transoi d) in quasi-one-dimensional nondegenerate polymers within the continuu m electron-phonon model. The electron-phonon configurations that lead to the decay, i.e., the critical droplets (or transition state), are i dentified as polarons of the metastable phase. We obtain an estimate f or the decay rate via thermal activation within a range of parameters consistent with experimental values for the gap of the cis configurati on. It is pointed out that, upon doping, the activation barriers of th e excited states are quite smaller and the decay rate is greatly enhan ced. Typical activation energies for electron or hole polarons are alm ost-equal-to 0.1 eV and the typical size for a critical droplet (polar on) is about 20 angstrom. Decay via quantum nucleation is also studied and it is found that the crossover temperature between quantum nuclea tion and thermal activation is of order T(c) less-than-or-equal-to 40 K. Metastable configurations of nondegenerate polymers may provide exa mples for mesoscopic quantum tunneling.