Analysis of superconductivity in Ba1−xKxBiO3 on the basis of indirect-exchange mediated pairing

After a critical survey of current interpretations of superconductivity in (Ba,K)BiO3, we present a quantitative analysis of this system, with emphasis on dependence of the critical temperature on potassium content and on pressure (external or ‘chemical’). Adopting a BCS formalism, Cooper-pair formation is assumed not to be phonon-mediated, but to arise from indirect-exchange interactions between conduction electrons via oxygen anions (oxygen valence band), by analogy with previous analyses of cuprate high-TC superconductors. Numerical calculations are carried through with, and without, accounting for shrinkage of the lattice with increasing potassium content. Results for pressure gradients of TC, at varying doping, and TC under pressures up to 20 GPa, are compared with experimental data. Extension of the analysis to the bismuthate Ba(Pb,Bi)O3 is outlined.