Oxygen isotope effects in Ba$_{1-x}$K$_{x}$BiO$_{3}$ high-temperature superconductors: Evidence for unconventional phonon-mediated pairing mechanism

The microscopic pairing mechanism for high-temperature superconductivity in magnetic copper and iron-based superconductors remains elusive despite tremendous experimental and theoretical efforts. The electron-phonon coupling constants predicted from the local density approximation (LDA) are too small to explain high-temperature superconductivity. On the other hand, high-temperature superconductivity in non-magnetic bismuth-based superconductors is believed to be phonon-mediated while the electron-phonon coupling constant predicted from the LDA is also too small (about 0.30) to explain superconductivity. We report magnetic and thermal properties of the oxygen-isotope exchanged Ba$_{1-x}$K$_{x}$BiO$_{3}$ ($x$ = 0.37 and 0.40) high-temperature superconductors to elucidate the pairing mechanism of this material. The deduced thermodynamic critical fields, electronic specific heat anomalies, superconducting transition temperatures, and magnetic penetration depths of the $^{16}$O and $^{18}$O samples are consistent with a phonon-mediated pairing mechanism with the effective electron-phonon coupling constant of about 1.0. We also show that the enhanced electron-phonon coupling constant may arise from the lattice polaronic effect, which increases the density of states at Fermi level.