Pairing mechanism and superconducting state parameters of cubic perovskiteBa0.6K0.4BiO3

We develop an effective three-dimensional dynamic interaction, which incorp orates screening of charge carriers by optical phonons and by plasmons, to discuss the nature of the pairing mechanism leading to superconductivity in cubic perovskite Ba0.6K0.4BiO3. The potassium-doped barium-bismuth oxide s ystem (Ba-K-BiO) is treated as an ionic solid containing carriers and a mod el dielectric function is set up which fulfils the appropriate sum rules on the ionic and electronic polarizabilities. The values of the coupling stre ngth and of the Coulomb interaction parameter indicate that the superconduc tor is in the strong coupling regime. Following strong-coupling theory, the superconducting transition temperature of optimally doped Ba-K-BiO is esti mated as 30 K. The energy gap ratio is slightly enhanced and the isotope ex ponent is severely reduced relative to the Bardeen-Cooper-Schrieffer values . The present analysis points to the importance of both plasmons and optica l phonons in determining the effective electron-electron interaction leadin g to superconductivity in doped cubic perovskites. (C) 2000 Elsevier Scienc e Ltd. All rights reserved.