Pairing mechanism and superconducting state parameters of cubic perovskite Ba 0.6K 0.4BiO 3

Abstract

We develop an effective three-dimensional dynamic interaction, which incorporates screening of charge carriers by optical phonons and by plasmons, to discuss the nature of the pairing mechanism leading to superconductivity in cubic perovskite Ba 0.6K 0.4BiO 3. The potassium-doped barium–bismuth oxide system (Ba–K–BiO) is treated as an ionic solid containing carriers and a model dielectric function is set up which fulfils the appropriate sum rules on the ionic and electronic polarizabilities. The values of the coupling strength and of the Coulomb interaction parameter indicate that the superconductor is in the strong coupling regime. Following strong-coupling theory, the superconducting transition temperature of optimally doped Ba–K–BiO is estimated as 30 K. The energy gap ratio is slightly enhanced and the isotope exponent is severely reduced relative to the Bardeen–Cooper–Schrieffer values. The present analysis points to the importance of both plasmons and optical phonons in determining the effective electron–electron interaction leading to superconductivity in doped cubic perovskites.