The vanadium dioxide is often considered a canonical correlated material. At ~60 C, VO₂ undergoes an insulator to metal phase transition which is often ascribed as a Mott transition despite the fact that there is a large change in the crystallographic structure at the transition temperature. One piece of evidence that supports this view is the report the metallic phase can occur before the structural transition, with the material becoming more conducting whilst remaining in the low temperature monoclinic crystal structure.
This so-called monoclinic metallic state has been observed both statically and dynamically, but only indirectly. In this talk, I will present our work to investigate the existence and properties of such a state by using nanoscale resonant soft X-ray spectro-holography and time resolved diffuse X-ray scattering.
X-ray spectro-holography allows us to measure the insulator-metal phase transition in thin films on nanometer length-scales over a wide field of view. We find no evidence for a monoclinic metallic phase existing in equilibrium, but instead find that phase growth is a complex process that involves phase separation and nucleation of multiple insulating phases as well as the metallic phase, which is highly influenced by the local strain.
Furthermore, we use time resolved diffuse X-ray scattering, which enables us to measure how the phonon population changes over the whole Brillouin zone. Our results are consistent with an ultrafast change in the lattice potential directly to a metallic phase and find no evidence for an intermediate state. Together, these results suggest that it is the electron-phonon interaction which is in fact the dominate interaction that controls the insulator-metal phase transition.