Superconductivity in Ba1−x KxBiO3: Possible scenario of spatially separated Fermi-Bose mixture

Abstract

We propose a new scenario for the metal-insulator phase transition and superconductivity in the perovskite-like bismuthates Ba1−x KxBiO3 (BKBO) based on our EXAFS studies. We show that two types of charge carriers, the local pairs (real-space bosons) and the itinerant electrons, exist in the metallic compound Ba1− x KxBiO3 (x≥0.37). The real-space bosons are responsible for the charge transport in semiconducting BaBiO3 and for superconductivity in the metallic BKBO. The appearance of the Fermi liquid state as the percolation threshold is overcome (x≥0.37) explains the observed metal-insulator phase transition. Because bosons and fermions occupy different types of the octahedral BiO6 complexes, they are separated in real space, and therefore, the spatially separated Fermi-Bose mixture of a new type is likely to be realized in the bismuthates. The nature of superconductivity is consistently explained in the framework of this scenario. A new superconducting oxide Ba1−x LaxPbO3 has been successfully synthesized to check our conclusions.