Anomalous metallic state with strong charge fluctuations in BaxTi8 O16+δ revealed by hard x-ray photoemission spectroscopy

We have studied a charge-orbital driven metal-insulator transition (MIT) in hollandite-type ${\mathrm{Ba}}_{x}{\mathrm{Ti}}_{8}{\mathrm{O}}_{16+\ensuremath{\delta}}$ by means of hard x-ray photoemission spectroscopy (HAXPES). The Ti $2p$ HAXPES indicates strong ${\mathrm{Ti}}^{3+}$/${\mathrm{Ti}}^{4+}$ charge fluctuation in the metallic phase above the MIT temperature. The metallic phase is characterized by a power-law spectral function near the Fermi level which would be a signature of bad metal with non-Drude polaronic behavior. The power-law spectral shape is associated with the large Seebeck coefficient of the metallic phase in ${\mathrm{Ba}}_{x}{\mathrm{Ti}}_{8}{\mathrm{O}}_{16+\ensuremath{\delta}}$.