Vacuum-ultraviolet reflectance and photoemission study of the metal-insulator phase transitions in ${\mathrm{VO}}_2$, ${\mathrm{V}}_6$${\mathrm{O}}_{13}$, and ${\mathrm{V}}_2$${\mathrm{O}}_3$

Vacuum-ultraviolet reflectance and photoemission spectra of ${\mathrm{VO}}_2$, ${\mathrm{V}}_2$ ${\mathrm{O}}_3$, ${\mathrm{V}}_6$ ${\mathrm{O}}_{13}$, and ${\mathrm{V}}_2$ ${\mathrm{O}}_5$ are measured in order to investigate the 3d-band structures and electron-correlation effects. In the case of ${\mathrm{VO}}_2$, drastic changes in the 3d (${\mathrm{\pi }}^{\mathrm{*}}$ and ${\mathit{d}}_{\mathrm{?}}$) -band structures are found in both spectra through the metal-insulator phase transition. The ${\mathrm{\pi }}^{\mathrm{*}}$ and ${\mathit{d}}_{\mathrm{?}}$ bands are found at the Fermi level and ${\mathit{E}}_{\mathit{B}}$=1.3 eV in the photoemission spectra of metallic ${\mathrm{VO}}_2$. In the insulating phase, the ${\mathrm{\pi }}^{\mathrm{*}}$ valence band in the photoemission spectra becomes empty and a rise of the ${\mathrm{\pi }}^{\mathrm{*}}$ conduction band by about 0.5 eV is found in the reflectance spectra. This band shift through the phase transition may be a driving force of the metal-insulator transition of ${\mathrm{VO}}_2$. The optical band gap between the ${\mathit{d}}_{\mathrm{?}}$ valence and ${\mathrm{\pi }}^{\mathrm{*}}$ conduction bands is obtained as 0.7 eV in the insulating phase, to which the ${\mathrm{\pi }}^{\mathrm{*}}$-${\mathit{d}}_{\mathrm{?}}$ correlation energy contributes partially.