  Ti2O3 exhibits a unique metal-insulator transition (MIT) at approximately 450
K over a wide temperature range of ~ 150 K. This broad MIT accompanied by
lattice deformation differs from the sharp MITs observed in most other
transition-metal oxides. A long-standing issue is determining the role of
electron-electron correlation in the electronic structure and MIT of Ti2O3.
However, the lack of information about the band structure of Ti2O3 has hindered
investigating the origin of its unusual physical properties. Here, we report
the electronic band structure of insulating Ti2O3 films with slight hole doping
by angle-resolved photoemission spectroscopy (ARPES). ARPES showed clear band
dispersion on the surface of single-crystalline epitaxial films. The
experimentally obtained band structures were compared with band-structure
calculation results based on density functional theory (DFT) with generalized
gradient approximation + U correction. The obtained band structures are in good
agreement with the DFT calculations at U = 2.2 eV, suggesting that
electron-electron correlation plays an important role in the electronic
structure of Ti2O3. Furthermore, the detailed analyses with varying U suggest
that the origin of the characteristic MIT in Ti2O3 is a semimetal-semimetal or
semimetal-semiconductor transition caused by changes in the Fermi surface due
to lattice deformation.
