Stability of the M2 phase of vanadium dioxide induced by coherent epitaxial strain

Tensile strain along the cR axis in epitaxial VO₂ films raises the temperature of the metal insulator transition and is expected to stabilize the intermediate monoclinic M2 phase. We employ surface-sensitive x-ray spectroscopy to distinguish from the TiO₂ substrate and identify the phases of VO₂ as a function of temperature in epitaxial VO₂/TiO₂ thin films with well-defined biaxial strain. Although qualitatively similar to our Landau-Ginzburg theory predicted phase diagrams, the M2 phase is stabilized by nearly an order of magnitude more strain than expected for the measured temperature window. Our results reveal that the elongation of the cR axis is insufficient for describing the transition pathway of VO₂ epitaxial films and that a strain induced increase of electron correlation effects must be considered.