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

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 ($d^1\underset{‾}{\mathit{\text{C}}}$ final) states at Fermi level to the incoherent ($d^0+d^1\underset{‾}{\mathit{\text{L}}}$ 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{\mathit{L}} states resonate across the V 2$p-3d$ threshold in addition to the $d^0$ and $d^1\underset{‾}{\mathit{\text{C}}}$ states. The results support a Mott-Hubbard transition picture for the first order MIT in VO${}_2$.