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

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

Figure 1

(Color online) (a) Valence-band spectra measured across the MIT using hard x ray $\left(h\nu =7937\text{eV}\right)$ compared with the cluster model calculation. Inset shows an expanded view (0–3 eV) of the $\text{V}3d$ states near $E_F$. (b) The main final-state configuration spectra in the metallic phase and the insulating phase.

Figure 2

(Color online) Resonant PES across the $\text{V}2p-3d$ threshold in (a) the metallic phase and (b) the insulating phase. (c) $\text{V}2p$ XAS spectra.

Figure 3

(Color online) (a) $\text{O}1s$, (b) $\text{V}2p$, and (c) $\text{V}1s$ core-level spectra measured across the MIT using hard x ray $\left(h\nu =7937\text{eV}\right)$ compared with the cluster calculations for the metallic phase and the insulating phase.