Electronic structure of strained vanadium dioxide thin films using soft X-ray spectroscopy

Despite over five decades of intense investigation, the origin of the metal-insulator transition (MIT) in VO$_2$ still presents a challenge to explain. Whether the lattice (Peierls physics) or electron-electron correlations (Mott-Hubbard physics) are responsible for the MIT has been hotly debated; more recently, the general consensus has favored a co-operative description, in which both structural and correlation effects are important and sympathetic to the transition. Key to understanding such a co-operative picture has been the behavior of VO$_2$ under doping and strain. Here, we report recent soft X-ray measurements of strained VO$_2$ thin films grown on TiO$_2$(001) and (110) substrates. We employ X-ray absorption spectroscopy and X-ray emission spectroscopy to probe the changes in both the {\em unoccupied} and {\em occupied} partial density of states across the MIT, observing distinct changes in the V $3d$-O $2p$ hybridization. Additionally, the location in energy of the unoccupied $d_{\parallel}$ state in the insulating phase is found to be dependent on the lattice strain, in agreement with the predictions of recent dynamical mean-field theory calculations. Finally, our results are discussed in the context of the origin of the MIT in VO$_2$.