Superconductivity and normal state resistivity of Ba0.6K0.4BiO3: an optical phonon approach

We discuss the nature of the pairing mechanism and the physical properties associated with the normal as well as the superconducting state of cubic pe rovskites Ba0.6K0.4BiO3 using the strong coupling theory. An interaction po tential which includes the Coulomb, electron-optical phonon and electron-pl asmon interactions is developed to elucidate the superconducting state. A m odel dielectric function is constructed with these interactions fulfilling the f-sum rule. The screening parameter (mu* = 0.26) infers the poor screen ing of charge carriers. The electron-optical phonon strength (lambda) estim ated as 0.98 is consistent with an attractive electron-electron interaction and supports the moderate to strong coupling theory. The superconducting t ransition temperature of Ba0.6K0.4BiO3 is then estimated as 32 K. Ziman's f ormula of resistivity is employed to analyse and compare this with the temp erature-dependent resistivity of a single crystal. The estimated contributi on from the electron-optical phonon together with the residual resistivity clearly infers a difference when a comparison is made with experimental dat a. The subtracted data infer a quadratic temperature dependence in the temp erature domain (30 less than or equal to T less than or equal to 200 K). Th e quadratic temperature dependence of rho [= rho(exp) - (rho(0) + rho(e-ph) )] is understood in terms of 3D electron-electron inelastic scattering. The presence of these el-el and el-ph interactions allows a coherent interpret ation of the physical properties. Analysis reveals that a moderate to stron g coupling exists in the Ba0.6K0.4BiO3 system and the coupling of electrons with the high-energy optical phonons of the oxygen breathing mode will be a reason for superconductivity. The implications of the above analysis are discussed. (C) 1998 Academic Press.