Four-step scenario for the phase changes in Ba1−xKxBiO3 —Melting of a three-dimensional CDW

We introduce our recent results on BaBiO3 (BBO) and its doped material, Ba1−xKxBiO3 (BKBO). The former has a three-dimensional CDW as the ground state, and its absorption spectrum is featured by direct and indirect excitons. Using a three-dimensional Peierls-Hubbard model for BBO, we reproduce appropriate energies for these two types of excitons. Next, we present a global scenario for the phase evolution in BKBO. That is, in the very light doping, the injected holes remain almost free from trapping (I), while they become bipolarons (BPʹs) when x exceeds a certain value (II). Continuing the hole doping, the long-range order of CDW is lost, but its short-range order is still remaining (III). Lastly, in the final stage of doping, the system becomes a metal (IV). Here, particular attention is paid to the difference and the similarity in the process, between the two cases with a 0.4 eV or 0.9 eV indirect gap.