Phonon-mediated superconductivity in bismuthates by nonadiabatic pairing

Abstract

Here, the impact of small Fermi energy on the selected parameters of the superconducting state in Ba1−xKxBiO3 (BKBO) is studied at x∈(0.3,0.4,0.5). This is done by employing the adiabatic and nonadiabatic Eliashberg equations in the context of the available experimental data. It is found that the retardation, strong-coupling, and the nonadiabatic effects notably influence the superconducting phase in BKBO. In particular, the electron-electron interaction, approximated here by the Coulomb pseudopotential, is argued to be reduced by the nonadiabatic effects that supplement retardation and allow for the phonon-mediated superconductivity. These findings are reinforced by further analysis of the Coulomb pseudopotential cutoff and the isotope effect. The former confirms that signatures of the nonadiabatic effects are visible even when high-order pseudopotential terms are considered, whereas the latter analysis shows a reduction of the isotope coefficient with respect to the canonical BCS level, as caused by the interplay of the retardation and strong-coupling effects. In summary, obtained results confirm recent theoretical and experimental studies which suggest a phonon-mediated mechanism of superconductivity in BKBO. However, they also point out that this phase cannot be properly described by the conventional theory due to the existence of somewhat unusual effects.

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