The Nature of Superconductivity in Ba_1-xK_xBiO₃

Conclusions

In summary, Ba_1−xK_xBiO₃ is a superconductor in the weak-to-moderate coupling limit as measured by λ. The high T_c is not due to a large value of the electron-phonon coupling constant λ, but rather to the large value of 〈ω〉 caused by the strong coupling of electrons to high energy phonons in the optic region of the phonon density of states which extends to 80 meV. Thus, with 〈ω〉 on the order of 40 meV (which is near to the value experimentally estimated by Zasadzinski et al.³⁰ and calculated by Sharai et al.³¹) and λ ≈ 1, a T_c of 30 K is expected. It is unlikely that higher values of T_c can be obtained in the weak coupling limit since the phonon spectra for all the oxides are similar and, therefore, obtaining a larger value of 〈ω〉 would be unlikely. Thus, the higher T_c copper oxides must have a different mechanism of superconductivity or, if phonons are important, the high T_c would require a transition from weak to strong coupling with a resulting larger value of λ.