Density of States and Isotope Effect in BiO Superconductors: Evidence for Non-Phonon Mechanism

We report measurements of the physical properties of the 30K superconductor Ba sub (0.6) K sub (0.4) BiO sub 3, including upper and lower critical fields and normal state susceptibility. From the results and from comparison with BaPb sub (0.75) Si sub (0.25) O sub 3 and band structure calculations it is concluded that the BiO superconductors are in the weak coupling limit, implying that the anomalously high T sub c's are not due to strong electron-phonon coupling. The oxygen isotope effect is the same for both compounds, given by alpha = 0.21+-0.03. We speculate that "phononic" effects in these materials are actually only indicative of dressed electronic excitations.