Excitonic pairing inCuO2- andBiO3-based perovskites

Abstract

We study the ${\mathrm{BiO}}_{\mathrm{m}}$ high-${\mathrm{T}}_{\mathrm{c}}$ oxides and compare them to the ${\mathrm{CuO}}_{\mathrm{n}}$ systems (n,m=2 or 3). Both are described by a general Hamiltonian and studied by perturbation theory in the hopping energy. It is shown that a common polarization mechanism can give rise to pairing of the O holes added to the antiferromagnetic or disproportionated semiconducting states. The necessary conditions are proximity of the M (Cu or Bi) and O levels, a gap in the electronic spectrum of the system, and important nearest-neighbor M-O repulsion.