Cooperative effects of strain and electron correlation in epitaxial VO 2 and NbO 2

We investigate the electronic structure of epitaxial VO 2 films in the rutile phase using density functional theory combined with the slave-spin method (DFT + SS). In DFT + SS, multi-orbital Hubbard interactions are added to a DFT-fit tight-binding model, and slave spins are used to treat electron correlations. We find that while stretching the system along the rutile c-axis results in a band structure favoring anisotropic orbital fillings, electron correlations favor equal filling of the t2g orbitals. These two distinct effects cooperatively induce an orbital-dependent redistribution of the electron occupations and spectral weights, driving strained VO 2 toward an orbital selective Mott transition (OSMT). The simulated single-particle spectral functions are directly compared to V L-edge resonant X-ray photoemission spectroscopy of epitaxial 10 nm VO 2/TiO 2 (001) and (100) strain orientations. Excellent agreement is observed between the simulations and experimental data regarding the strain-induced evolution of the lower Hubbard band. Simulations of rutile NbO 2 under similar strain conditions are performed, and we predict that an OSMT will not occur in rutile NbO 2. Our prediction is supported by the high-temperature hard x-ray photoelectron spectroscopy measurement on relaxed NbO 2 (110) thin films with no trace of the lower Hubbard band. Our results indicate that electron correlations in VO 2 are important and can be modulated even in the rutile phase before the Peierls instability sets in.