Fig. 4: Oxygen-vacancy-induced isotropic orbital occupancy in VO₂ templates.

Polarization-dependent x-ray absorption spectroscopy (XAS) at the V L_2,3-edges at 270 K for two heterostructures composed of 2.5-nm-thick TiO₂ films grown on VO₂ templates under different \(pO_2\) (a H24mT, b H6mT). Unlike large difference of XAS signal in H24mT at 270 K due to the orbital polarization with V-V dimerization in the monoclinic VO₂, almost no effect on the XAS signal in H6mT was observed at 270 K; this result indicates isotropic orbital filling in H6mT sample even at 270 K. The XLD (I_|| – I_⊥) are also shown at both 270 K and 320 K for c H24mT and d H6mT. Oxygen vacancies driven by directional ionic transport in H6mT tend to increase the crystal symmetry to close to rutile structure by weakening of V-V dimerization, so selective filling of d_|| (inset of c.) changes to isotropic orbital occupancy of d_|| and π* (inset of d) in VO₂ templates; the oxygen-vacancy-driven isotropic occupancy leads to metallization at 270 K.