Photoemission evidence for a Mott-Hubbard metal-insulator transition in VO$_2$

TL;DR

This study uses advanced photoemission spectroscopy to provide evidence that the metal-insulator transition in VO₂ is driven by a Mott-Hubbard mechanism involving spectral weight transfer and gap formation.

Contribution

It offers direct spectroscopic evidence supporting a Mott-Hubbard transition in VO₂, with detailed analysis of spectral weight transfer and electronic structure parameters.

Findings

Spectral weight shifts from coherent to incoherent states across the MIT.

Gap formation is linked to spectral weight transfer in V 3d states.

Resonant PES reveals multiple V 3d states resonating at the threshold.

Abstract

The temperature ($T$) dependent metal-insulator transition (MIT) in VO$_2$ is investigated using bulk sensitive hard x-ray ($\sim$ 8 keV) valence band, core level, and V 2$p-3d$ resonant photoemission spectroscopy (PES). The valence band and core level spectra are compared with full-multiplet cluster model calculations including a coherent screening channel. Across the MIT, V 3$d$ spectral weight transfer from the coherent ($d^1\underbar{\it {C}}$ final) states at Fermi level to the incoherent ($d^{0}+d^1\underbar{\it {L}}$ final) states, corresponding to the lower Hubbard band, lead to gap-formation. The spectral shape changes in V 1$s$ and V 2$p$ core levels as well as the valence band are nicely reproduced from a cluster model calculations, providing electronic structure parameters. Resonant-PES finds that the $d^1\underbar{\it{L}}$ states resonate across the V 2$p-3d$ threshold in addition to the $d^{0}$ and $d^1\underbar{\it {C}}$ states. The results support a Mott-Hubbard transition picture for the first order MIT in VO$_2$.