Symmetry breaking and geometric confinement in VO2: Results from a three-dimensional infrared nano-imaging

Epitaxial strain can play an important role in controlling the local phase dynamics of transition metal oxides. With scattering-type scanning near-field optical microscopy, we visualize the three dimensional landscape of phase inhomogeneity in strained VO2 films grown on [100]R TiO2 substrates. We demonstrate that three different symmetries are spontaneously broken in the vicinity of the VO2 phase transition: (1) Monoclinic-tetragonal (rutile) crystal symmetry breaking due to the structural phase transition, (2) in-plane (x-y plane) rotational symmetry breaking due to the formation of periodic strain domains, and (3) out-of-plane (z-axis) mirror symmetry breaking at the film cross-section due to substrate-induced epitaxial strain.