Mott-Hubbard insulator-metal transition in the VO2 thin film: A combined XAS and resonant PES study

TL;DR

This study investigates the insulator-metal transition in VO2 thin films, revealing that strong electron correlations and spectral weight changes in specific orbitals drive the Mott-Hubbard transition, supported by combined XAS and PES measurements.

Contribution

It provides direct experimental evidence linking spectral weight changes in V 3d orbitals to the Mott-Hubbard transition in VO2, supported by spectral simulations.

Findings

Spectral weight changes in d_{||} states correlate with conductivity.

Strong electron-electron correlations are confirmed by spectral simulations.

The transition is characterized as a Mott-Hubbard insulator-metal transition.

Abstract

We have analyzed spectral weight changes in the conduction and the valence band across insulator to metal transition (IMT) in the VO2 thin film using X-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (PES). Through temperature dependent XAS and resonant PES measurements we unveil that spectral changes in the d$_{\|}$ states (V 3$\it{d_{x^2-y^2}}$ orbitals) are directly associated with temperature dependent electrical conductivity. Due to presence of the strong electron-electron correlations among the d$_{\|}$ states, across IMT, these states are found to exhibit significant intensity variation compared to insignificant changes in the $\pi^{\ast}$ and the $\sigma^{\ast}$ states (which are O 2$\it{p}$ hybridized V 3$\it{d}$ $e_g^{\pi}$ and $e_g^{\sigma}$ states) in the conduction band. Experimentally obtained values of the correlation parameter (U$_{dd}$ $\sim$ 5.1 eV, intra-atomic V 3$\it{d}$ correlations) and crystal field splitting (10 Dq $\sim$ 2.5 eV) values are used to simulate the V $\it{L_{2,3}}$ edge XAS spectra and an agreement between simulated and experimental spectra also manifests strong correlations. These results unravel that the IMT observed in the VO2 thin film is the Mott-Hubbard insulator-metal transition.