PAIRING AND PHASE-SEPARATION IN BIO3-BASED SUPERCONDUCTORS

We study a model proposed before to give rise to a nonconventional pairing mechanism in BaPb1-xBixO3 and Ba1-xKxBiO3. This model, in which both phonons and electronic excitations play an essential role, is supported by experimental evidence. We solve exactly a BiO6 cluster and we calculate the interaction between this cluster and the rest of the system using perturbation theory in the hopping term. The holes introduced by doping form groups of n holes where n depends essentially on the values of the electron-phonon interaction, Bi-O repulsion, and Bi-O hopping. n = 2 is favored near the limit at which the disproportionated ground state becomes unstable.