Cooperative Effects of Strain and Electron Correlation in Epitaxial VO2 and NbO2

TL;DR

This study combines DFT and slave spin methods to explore how strain and electron correlation influence the electronic structure of VO₂, revealing an orbital-selective Mott transition and comparing predictions with experimental spectroscopy data.

Contribution

It introduces a combined DFT+SS approach to analyze strain effects on electron correlation and orbital behavior in VO₂, predicting an OSMT in strained VO₂ but not in NbO₂.

Findings

Strain induces orbital-dependent electron redistribution in VO₂.

Electron correlation favors equal orbital filling, promoting OSMT.

Experimental spectra agree with the simulated evolution of the Hubbard band.

Abstract

We investigate the electronic structure of the epitaxial VO$_2$ films in the rutile phase using the density functional theory combined with the slave spin method (DFT+SS). In DFT-SS, the multiorbital Hubbard interactions are added to a DFT-fit tight-binding model, and we employ the slave-spin method to treat the electron correlation. We find that while stretching the system along the rutile $c$-axis results in a band structure favoring an anisotropic orbital fillings, the electron correlation favors an equal electron filling among $t_{2g}$ orbitals. These two distinct effects cooperatively induce interesting orbital-dependent redistributions of the electron occupations and the spectral weights, which pushes the 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 as VO$_2$ are performed, and we predict that OSMT will not occur in rutile NbO$_2$. Our results indicates that the electron correlation in VO$_2$ is important and can be modulated even in the rutile phase before the Peierls instability sets in.