Superconductivity in Ba1-xKxBiO3: Possible scenario of spatially separatedFermi-Bose mixture

We propose a new scenario for the metal-insulator phase transition and supe rconductivity in the perovskite-like bismuthates Ba1-xKxBiO3 (BKBO) based o n our EXAFS studies. We show that two types of charge carriers, the local p airs (real-space bosons) and the itinerant electrons, exist in the metallic compound Ba1-xKxBiO3 (x greater than or equal to 0.37). The real-space bos ons are responsible for the charge transport in semiconducting BaBiO3 and f or superconductivity in the metallic BKBO. The appearance of the Fermi liqu id state as the percolation threshold is overcome (x : 0.37) explains the o bserved metal-insulator phase transition. Because bosons and fermions occup y different types of the octahedral BiO6 complexes, they are separated in r eal space, and therefore, the spatially separated Fermi-Bose mixture of a n ew type is likely to be realized in the bismuthates. The nature of supercon ductivity is consistently explained in the framework of this scenario. A ne w superconducting oxide Ba1-xLaxPbO3 has been successfully synthesized to c heck our conclusions. (C) 2001 MAIK "Nauka/Interperiodica".