Quasiparticle dispersion of a 2D layer of BaBiO3

We use the Momentum Average approximation to find the spectral function of the quasiparticle that forms when a hole is doped into a parent layer of BaBiO3. To model this system, we include the 6s Bi and the p_x/y O ligand orbitals, and treat the phonons as dispersionless, quantum Einstein modes on the oxygens only. The hole-phonon coupling is of Peierls/SSH origin, arising from the changes in the s-p and p-p hopping amplitudes due to oxygen displacements. In the undoped ground state, this electron-phonon coupling is responsible for driving a breathing-mode distortion of the O octahedra, leading to insulating behavior. Our calculation investigates how this distortion is modified in the vicinity of the quasiparticle, as the doped hole acquires its own phonon cloud. We also calculate the spectral function for two doped holes, to study the possibility of bipolaron formation. A stable bipolaron could provide a direct explanation for the relatively high T_c observed in the superconducting bismuthates.