Exotic Physics in the Negative- U , Extended-Hubbard Model for Barium Bismuthates?

We point out possibilities for exotic physics in barium bismuthates, from a detailed study of the negative- U , extended-Hubbard model proposed for these systems. We emphasize the different consequences of electronic and phononic mechanisms for negative U . We show that, for an electronic mechanism, the semiconducting phases must be unique, with their transport properties dominated by charge ± 2 e Cooperon bound states . This can explain the observed difference between the optical and transport gaps. We propose other experimental tests for this novel mechanism of charge transport.