METAL-INSULATOR TRANSITION OF Na_xWO₃ STUDIED BY ANGLE-RESOLVED PHOTOEMISSION SPECTROSCOPY

Abstract

The electronic structure of sodium tungsten bronzes NaxWO 3 is investigated by highresolution angle-resolved photoemission spectroscopy (ARPES). The ARPES spectra measured in both insulating and metallic phases of NaxWO 3 reveals the origin of metalinsulator transition (MIT) in sodium tungsten bronze system. It is found that in insulating NaxWO 3 the states near the Fermi level (E F ) are localized due to the strong disorder caused by the random distribution of Na + ions in WO 3 lattice. Due to the presence of disorder and long-range Coulomb interaction of conduction electrons, a soft Coulomb gap arises, where the density of states vanishes exactly at E F . In the metallic regime the states near E F are populated and the Fermi level shifts upward rigidly with increasing electron doping (x). Volume of electron-like Fermi surface (FS) at the Γ(X) point of the Brillouin zone gradually increases with increasing Na concentration due to W 5d t 2g band filling. A rigid shift of the Fermi energy is found to give a qualitatively good description of the Fermi surface evolution. As we move from bulk-sensitive to more surface sensitive photon energy, we found the emergence of Fermi surfaces at X(M ) and M (R) points similar to the one at the Γ(X) point in the metallic regime, suggesting that the reconstruction of surface was due to rotation/deformation of WO 6 octahedra.

Hydrogen in tungsten trioxide by membrane photoemission and density functional theory modeling

“…The discovery of the electrochromic properties of thin WO 3 films led to the development of a number of applications such as smart windows, displays, and variable mirrors [4,5]. As the above-discussed constraints for hydrogen do not apply here, the electronic structure of alkali-metal-intercalated tungsten bronzes, e.g., Na x WO 3 , are well characterized [6,7]. With the help of electronic structure modeling [8,9], there is consensus about the underlying physical phenomena of the chemical modification induced by alkali-metal intercalation [10].…”

Observation of anomalous temperature dependence of spectrum on small Fermi surfaces in aBiS2-based superconductor

“…The disorder-induced self-traping of polarons in Na 0.025 WO 3 also shows nontrivial temperature dependence of the spectral function [34,35]. The Na x WO 3 and BiS 2 systems bear an interesting resemblance in their electronic structures.…”

Proton polarons in HxWO3 by synchrotron photoemission and DFT modelling

“…As the above discussed constraints for hydrogen do not apply here, the electronic structure of alkali-metal intercalated tungsten bronzes, e.g. Na x WO 3 , are well characterized [6,7]. Together with electronic structure modeling [8,9], there is consensus about the underlying physical phenomena of the chemical modification induced by alkali-metal intercalation [10].…”