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

Abstract

Tensile strain along the ${c}_{R}$ axis in epitaxial ${\mathrm{VO}}_{2}$ 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 ${\mathrm{TiO}}_{2}$ substrate and identify the phases of ${\mathrm{VO}}_{2}$ as a function of temperature in epitaxial ${\mathrm{VO}}_{2}/{\mathrm{TiO}}_{2}$ 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 ${c}_{R}$ axis is insufficient for describing the transition pathway of ${\mathrm{VO}}_{2}$ epitaxial films and that a strain induced increase of electron correlation effects must be considered.