Anomalous metallic state with strong charge fluctuations in BaxTi8O16+δ revealed by hard x-ray photoemission spectroscopy

Abstract

We have studied a charge-orbital driven metal-insulator transition (MIT) in hollandite-type BaxTi8O 16+δ by means of hard x-ray photoemission spectroscopy (HAXPES). The Ti 2p HAXPES indicates strong Ti 3+ /Ti 4+ charge fluctuation in the metallic phase above the MIT temperature. The metallic phase is characterized by 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 BaxTi8O 16+δ .

Strongly correlated oxides for energy harvesting

“…Together with the result of the thermal conductivity as discussed below, the figure of merit ZT at 300 K is maximized at n = 0.22 (ZT~0.05). According to photoemission spectroscopy, the low-energy electronic structure of this series of compounds is dominated by the polaron state, and the experimental absolute values of the Seebeck coefficient are consistent with those calculated with the Mott formula for the Seebeck coefficient, where the density of states is obtained from a photoemission spectrum [12].…”