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 th is material as a simple three-dimensional version of a Peierls insulat or, where the insulating gap is a consequence of the ordered distortio n of the oxygen atoms. Charge accumulates on half the atoms and deplet es from the other half. Experimentally, when holes are added to BaBiO3 by doping, it remains insulating until a very large hole concentratio n is reached, at which point it becomes superconducting. In the Rice-S neddon model, at large enough electron-phonon coupling, a mechanism fo r 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 give n, and compared with ''exact numerical results on large clusters with periodic boundary conditions.