Polaron defects in a charge density wave: a model for lightly doped BaBiO3

A model for BaBiO₃ was introduced by Rice and Sneddon, which treats this material as a simple three-dimensional version of a Peierls insulator, where the insulating gap is a consequence of the ordered distortion of the oxygen atoms. Charge accumulates on half the atoms and depletes from the other half. Experimentally, when holes are added to BaBiO₃ by doping, it remains insulating until a very large hole concentration is reached, at which point it becomes superconducting. In the Rice-Sneddon model, at large enough electron-phonon coupling, a mechanism for insulating behavior of doped samples is formation of small polarons or bipolarons which trap carriers in bound states in the Peierls gap. A variational calculation of the polaron binding in this model is given, and compared with “exact” numerical results on large clusters with periodic boundary conditions.