SUPERCONDUCTIVITY IN BAPB1-XBIXO3 CUBIC OXIDE - A COUPLED CHARGE OSCILLATION WITH OPTICAL PHONON MODEL

The nature of pairing mechanism in cubic perovskite BaPbBiO3 is theore tically investigated. A model potential is developed which includes th e Coulomb (e-e), electro-optical phonon (e-ph) as well as electron-pla smon (e-pl) interactions to elucidate the superconducting state. The p resence of these e-e, e-ph and e-pl interactions allows a coherent int erpretation of their properties. The estimated value of the coupling p arameter (lambda) as 0.73 is consistent with an attractive electron-el ectron 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 superconducti ng transition temperature (T-c) of the system is calculated from the M cMillan formulae and is consistent with earlier observations. The redu ced energy gap of 2 Delta/KBTc is found to be 3.53 which is close to t he predicted value of BCS weak coupling theory (2 Delta/KBTc = 3.5). T he oxygen isotope shift of T-c is then calculated and the characterist ic exponent (alpha) defined as T-c proportional to M-alpha (M is the o xygen 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 Els evier Science Ltd. All rights reserved.