Nature of the local pairing of charge carriers in the family of high-temperature superconductors based on BaBiO$_3$ (brief review)

A model of the local pairing of charge carriers in real space in a family of bismuthate high-temperature superconducting oxides based on X-ray absorption spectroscopy data using synchrotron and X-ray free-electron laser radiation is presented. The nature of the two band gaps and the insulator state of barium bismuthate BaBiO$_3$, the metallic state of the barium plumbate BaPbO$_3$, as well as the phase diagram and the appearance of superconductivity in lead- and potassium-doped BaBiO$_3$, has been explained. The possibility of implementing both phonon and electron pairing mechanisms is discussed.