Effect of optical excitation on the band structure and X-ray absorption spectra of BaBiO$_3$-based high-temperature superconductors: ab initio calculation

The band structure, densities of states, and O $K$-edge X-ray absorption spectra have been calculated by the density functional theory method for BaBiO$_3$-based perovskite high-temperature superconductors at different potassium doping levels in the ground and optically excited states. It has been shown that this approach including local structural inhomogeneities caused by doping and optical irradiation allows the correct description of changes in the properties of the electron subsystem near the Fermi level. It has been shown that doping is accompanied by the appearance of hole carriers on the Bi$6s$-O$2p_{\sigma^*}$ hybridized orbital in agreement with the model of the electronic structure of bismuth-based high-temperature superconductors in terms of a spatially separated Fermi–Bose mixture. It has been found that the band structure for the excited state of the undoped compound is equivalent to the band structure of the tight binding model for a cubic lattice. The results are important for the preliminary analysis of time-resolved X-ray absorption spectra on an X-ray free electron laser at femtosecond optical excitation.