Fermi Surface Nesting Inba1−xkxbio3

We have measured three-dimensional momentum densities in ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}\mathrm{Bi}{\mathrm{O}}_{3}$ using Compton scattering, where $x=0.10$ (insulating) and 0.37 (metallic). The momentum densities reconstructed from experimental Compton profiles exhibit significant differences between the metallic and insulating samples. The momentum densities in the $x=0.10$ insulating sample are interpreted to represent the filled, polyhedral Brillouin zone for fcc-type crystals, displaying a feature of the perfectly nested Fermi surface. The momentum densities in the $x=0.37$ metallic sample resemble those predicted by band theory based on the local-density approximation, but still show an unusual feature due to Fermi-surface nesting. Those features, which showed some signatures in the two-dimensionally reconstructed data that we have previously reported, have been direct and compelling in the three-dimensionally reconstructed data in this study.