Abstract
We correlate electron transport data directly with energy band structure measurements in vanadium oxide thin films with varying V-O stoichiometry across the ${\text{VO}}_{2}$ metal-insulator transition. A set of vanadium oxide thin films were prepared by reactive dc sputtering from a V target at various oxygen partial pressures (${\text{O}}_{2}$ p.p.). Metal-insulator transition (MIT) characteristic to ${\text{VO}}_{2}$ can be seen from the temperature dependence of electrical resistance of the films sputtered at optimal ${\text{O}}_{2}$ p.p. Lower and higher ${\text{O}}_{2}$ p.p. result in disappearance of the MIT. The results of the near edge x-ray absorption fine structure spectroscopy of the $\text{O}\text{ }K$ edge in identical VO films are presented. Redistribution of the spectral weight from ${\ensuremath{\sigma}}^{\ensuremath{\ast}}$ to ${\ensuremath{\pi}}^{\ensuremath{\ast}}$ bands is found in the vanadium oxide films exhibiting stronger ${\text{VO}}_{2}$ MIT. This is taken as evidence of the strengthening of the metal-metal ion interaction with respect to the metal-ligand and indirect V-O-V interaction in vanadium oxide films featuring sharp MIT. We also observe a clear correlation between MIT and the width and area of the lower ${\ensuremath{\pi}}^{\ensuremath{\ast}}$ band, which is likely to be due to the emergence of the ${d}_{||}$ band overlapping with ${\ensuremath{\pi}}^{\ensuremath{\ast}}$. The strengthening of this ${d}_{||}$ band near the Fermi level only in the vanadium oxide compounds displaying the MIT points out the importance of the role of the ${d}_{||}$ band and electron correlations in the phase transition.
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