Phonon-mediated superconductivity in bismuthates by nonadiabatic pairing

In the present paper, impact of small Fermi energy on selected parameters superconducting state in Ba$_{1-x}$K$_{x}$BiO$_{3}$ (BKBO) is studied at $x \in (0.3, 0.4, 0.5)$. This done by employing adiabatic and non-adiabatic Eliashberg equations context available experimental data. It found that retardation, strong-coupling effects notably influence phase BKBO. particular, electron-electron interaction, approximated here Coulomb pseudopotential, argued to be reduced supplement retardation allow for phonon-mediated superconductivity. These findings are reinforced further analysis isotope effect showing reduction coefficient with respect canonical Bardeen-Cooper-Schrieffer (BCS) level, as caused interplay all mentioned above. Although physics behind appears complex, its resulting behavior comply scenario conventional superconductors. summary, obtained results confirm recent theoretical studies suggest mechanism superconductivity However, they also point out this cannot properly described BCS theory due existence somewhat unusual effects.