Superconductivity in BaPb1-xBixO3 cubic oxide:: A coupled charge oscillation with optical phonon model

The nature of pairing mechanism in cubic perovskite BaPbBiO3 is theoretically investigated. A model potential is developed which includes the Coulomb (e-e), electro-optical phonon (e-ph) as well as electron-plasmon (e-pl) interactions to elucidate the superconducting state. The presence of these e-e, e-ph and e-pl interactions allows a coherent interpretation of their properties. The estimated value of the coupling parameter (lambda) as 0.73 is consistent with an attractive electron-electron interaction. The screened Coulomb repulsive parameter (mu* = 0.31) suggests poor screening of charge carriers with low concentration and hence reflects the small density of states N(O). The superconducting transition temperature (T-c) of the system is calculated from the McMillan formulae and is consistent with earlier observations. The reduced energy gap of 2 Delta/KBTc is found to be 3.53 which is close to the predicted value of BCS weak coupling theory (2 Delta/KBTc = 3.5). The oxygen isotope shift of T-c is then calculated and the characteristic exponent (alpha) defined as T-c proportional to M-alpha (M is the oxygen atomic mass) is evaluated as 0.227. These observations on Ba-Pb1-xBixO3 (x = 0.25) suggest that a weak to moderate coupling exists and the coupling of electrons with the coupled charge oscillation-optical phonon mode could be a reason for the superconductivity. (C) 1998 Elsevier Science Ltd. All rights reserved.