Density of States and Isotope Effect in BiO Superconductors: Evidence for Nonphonon Mechanism

We report measurements of the physical properties of the 30-K superconductor Ba0.6{K}0.4$Bi${O}₃$, including upper and lower critical fields and normal-state susceptibility. From the results and from comparison with Ba${Pb}0.75Bi0.25{O}₃$ and band-structure calculations it is concluded that the BiO superconductors are in the weak-coupling limit, implying that the anomalously high ${T}c's$ are not due to conventional, strong electron-phonon coupling. A sizable oxygen isotope effect of very similar magnitude is found for both compounds (${α}=0.2{-}0.25$). We speculate that "phononic" effects in these materials are actually only indicative of dressed electronic excitations mediating superconducting pairing.