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 ${\mathrm{Ba}}_{0.6}$ ${\mathrm{K}}_{0.4}$Bi${\mathrm{O}}_3$, including upper and lower critical fields and normal-state susceptibility. From the results and from comparison with Ba${\mathrm{Pb}}_{0.75}$ ${\mathrm{Bi}}_{0.25}$ ${\mathrm{O}}_3$ and band-structure calculations it is concluded that the BiO superconductors are in the weak-coupling limit, implying that the anomalously high $T_c\text{'}\mathrm{s}$ are not due to conventional, strong electron-phonon coupling. A sizable oxygen isotope effect of very similar magnitude is found for both compounds ($\alpha =0.2\mathrm{}-0.25$). We speculate that "phononic" effects in these materials are actually only indicative of dressed electronic excitations mediating superconducting pairing.