Three-dimensional band structure of highly metallic Na_0.8WO ₃ by angle-resolved photoemission spectroscopy

Three-dimensional electronic structure of highly metallic sodium tungsten bronze, Na_0.8WO₃, is investigated by high-resolution angle-resolved photoemission spectroscopy. The experimentally determined valence-band structure along the momentum directions both parallel and perpendicular to the surface has been compared with the results of ab initio band-structure calculation. The angle-resolved photoemission spectroscopy spectra for different photon energies reveal that possibly the oxygen vacancies in the system are responsible for the evolution of density of states at the top of Γ point in experimental valence band. The band dispersion around Γ (X) point leading to an electronlike Fermi surface is well predicted by the band calculation. As we move from bulk-sensitive to more-surface-sensitive photon energy, we found emergence of Fermi surfaces at X (M) and M (R) points similar to the one at Γ (X) point, suggesting the reconstruction of surface due to rotation/deformation of WO₆ octahedra.