Fermi-Bose Mixture in Ba(K)BiO3 Superconducting Oxide

We have demonstrated a new description of local electronic structure in the perovskite-like bismuthates Ba1−xKxBiO3 (BKBO) based on existence of the spatially separated Fermi-Bose mixture. We have shown that two types of charge carriers: the local electron pairs (real-space bosons) and the itinerant electrons exist in metallic compound Ba1−xKxBiO3 (x ≥ 0.37). The real-space bosons are responsible for both charge transport in semiconducting BaBiO3 and superconductivity in metallic BKBO, while the fermionic subsystem is responsible for the observed metal-insulator phase transition and appearance of the Fermi-liquid state when the percolation threshold is overcome (x ≥ 0.37). Bosons and fermions occupy different types of the octahedral BiO6 complexes, so they are separated in real space. This scenario fits well into a new quantum state of pair-density wave (PDW), actively discussed for copper-based HTSC.