PAIR-BREAKING IN SEMICLASSICAL SINGLET SMALL-BIPOLARON HOPPING

The Holstein-Hubbard model is used to study high-temperature (T>T-phon on/3) hopping transport when it is energetically favorable for carrier s to pair as singlet small bipolarons. The semiclassical rates for one -electron transfers involving small polarons are found to be much grea ter than those that only involve small bipolarons. In particular, the most rapid one-electron-transfer processes have a small-polaron hoppin g to a vacant site and one of a small bipolaron's two carriers jumping onto the site of an adjacent small polaron. As a result, even when mo st carriers form small bipolarons rather than small polarons, one-elec tron transfers involving small polarons always dominate the de conduct ivity. The energy to thermally generate small polarons from small bipo larons thus contributes to the conductivity's activation energy. This pair-breaking energy also manifests itself in a thermally activated co ntribution to the system's paramagnetic susceptibility. In addition, t he carriers' Seebeck coefficient garners a contribution with a tempera ture dependence that is characterized by the pair-breaking energy.