Vacuum-ultraviolet reflectance and photoemission study of the metal-insulator phase transitions in VO2, V6O13, and V2O3.

Abstract

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}$${\mathrm{O}}_{5}$ are measured in order to investigate the 3d-band structures and electron-correlation effects. In the case of ${\mathrm{VO}}_{2}$, drastic changes in the 3d (${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ and ${\mathit{d}}_{\mathrm{?}}$) -band structures are found in both spectra through the metal-insulator phase transition. The ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ and ${\mathit{d}}_{\mathrm{?}}$ bands are found at the Fermi level and ${\mathit{E}}_{\mathit{B}}$=1.3 eV in the photoemission spectra of metallic ${\mathrm{VO}}_{2}$. In the insulating phase, the ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ valence band in the photoemission spectra becomes empty and a rise of the ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ conduction band by about 0.5 eV is found in the reflectance spectra. This band shift through the phase transition may be a driving force of the metal-insulator transition of ${\mathrm{VO}}_{2}$. The optical band gap between the ${\mathit{d}}_{\mathrm{?}}$ valence and ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ conduction bands is obtained as 0.7 eV in the insulating phase, to which the ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$-${\mathit{d}}_{\mathrm{?}}$ correlation energy contributes partially.The splitting of the ${\mathit{d}}_{\mathrm{?}}$ band is also found in the insulating phase. This splitting energy is about 2.5 eV, while the bandwidth of the ${\mathit{d}}_{\mathrm{?}}$ band is about 1.5 eV. This large band splitting is mainly due to the correlation energy of ${\mathit{d}}_{\mathrm{?}}$ electrons of about U(${\mathit{d}}_{\mathrm{?}}$,${\mathit{d}}_{\mathrm{?}}$)=2.1 eV. On the other hand, no drastic change is found in the 3d-band structures of ${\mathrm{V}}_{6}$${\mathrm{O}}_{13}$ and ${\mathrm{V}}_{2}$${\mathrm{O}}_{3}$ except for slight changes in the bandwidths. Furthermore, the density of states at the Fermi level is rather low in these materials even in the metallic phase. These facts support the view that the electron-correlation effects are important and Mott-type metal-insulator transitions are induced in ${\mathrm{V}}_{6}$${\mathrm{O}}_{13}$ and ${\mathrm{V}}_{2}$${\mathrm{O}}_{3}$. Resonant photoemission from V3d and O2p bands are observed as the photon energy is swept through the 3p\ensuremath{\rightarrow}3d optical-absorption transition. The resonance profiles of V3d bands show the characteristic antiresonance dip, while those of O2p bands show rather broad and simple enhancements. The multiplet structures in the 3p core photoemission spectra of ${\mathrm{VO}}_{2}$ and ${\mathrm{V}}_{2}$${\mathrm{O}}_{3}$ are quantitatively analyzed with use of results from the calculations of Yamaguchi et al.