Bond disproportionation and electron-phonon driven oxygen-bismuth hybrid hole pairing in bismuth perovskites

In this talk, we will present the recently developed theory^1-3 describing the bismuth perovskites ABiO₃ (A = Sr or Ba) as bond- (as opposed to charge-) disproportionated insulators where Bi-O hybridization and the presence of oxygen holes lead to formation of molecular orbitals and strong polaronic effects, which might be the prime mechanism driving these materials towards superconductivity upon doping. In particular, we will first use electronic structure methods to demonstrate that oxygen holes indeed condense into A_1g molecular orbitals centered at collapsed BiO₆ octahedra and then to derive an appropriate tight-binding (TB) model, as well as to estimate electron-phonon coupling and an effective attraction between the two holes in the A_1g symmetry in this collapsed “BiO₆ molecule”, emphasizing that these two parameters are strong enough to sustain polaronic superconductivity. We will also explore the parameter space of the TB model, finding a cross-over from the bond- into the charge-disproportionated insulating phase, as well as a metallic phase with a non-bonding character of the oxygen holes.
1. K. Foyevtsova et al., PRB 91, 121114(R) (2015).
2. A. Khazraie et al, 97, 075103 (2018).
3. A. Khazraie et al, arXiv:1807.07168; to appear in PRB.