Thickness dependence of electronic and crystal structures in VO2 ultrathin films: suppression of the collaborative Mott-Peierls transition
Abstract
Through in~situ photoemission spectroscopy, we investigated the change in the electronic and crystal structures of dimensionality-controlled VO2 films coherently grown on TiO2(001) substrates. In the nanostructured films, the balance between the instabilities of a bandlike Peierls transition and a Mott transition is controlled as a function of thickness. The characteristic spectral change associated with temperature-driven metal-insulator transition in VO2 thick films holds down to 1.5 nm (roughly corresponding to five V atoms along the [001] direction), whereas VO2 films of less than 1.0 nm exhibit insulating nature without V-V dimerization. These results suggest that the delicate balance between a Mott instability and a bandlike Peierls instability is modulated at a scale of a few nanometers by the dimensional crossover effects and confinement effects, which consequently induce the complicated electronic phase diagram of ultrathin VO2 films.