NEGATIVE-U EXTENDED HUBBARD-MODEL FOR DOPED BARIUM BISMUTHATES

We present detailed mean-field and random-phase-approximation studies of the negative-U, extended Hubbard model with a view to understanding the properties of the doped barium bismuthates. In particular, we obtain the phase diagram, the excitation spectrum, and the optical conductivity in the semiconducting phase of the bismuthates. We show by explicit calculations how this model leads to a natural explanation for the two, well-separated transport and optical gaps observed in the semiconducting phases of the bismuthates. We fix the parameters in our model by fitting these experimentally observed gaps; and with these parameter values we compute other properties of these systems. We also show how metallic screening and disorder can decrease the superconducting T-c dramatically. Our theory leads to an exotic charge-transport mechanism, dominated by charge +/-2e bosons (cooperons), in the semiconducting phases of these systems.