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 Ba0.6K0.4BiO3, including upper and lower critical fields and normal-state susceptibility. From the results and from comparison with BaPb0.75Bi0.25O3 and band-structure calculations it is concluded that the BiO superconductors are in the weak-coupling limit, implying that the anomalously high Tc's are not due to conventional, strong electron-phonon coupling. A sizable oxygen isotope effect of very similar magnitude is found for both compounds (=0.2-0.25). We speculate that "phononic" effects in these materials are actually only indicative of dressed electronic excitations mediating superconducting pairing.