TiSe2: Semiconductor, semimetal, or excitonic insulator

The electronic structure of high-quality $1T-\mathrm{Ti}{\mathrm{Se}}_2$ has been investigated using angle-resolved synchrotron ultraviolet photoemission. Occupied $\mathrm{Ti} d$ states are clearly observed in small pockets around the zone-edge centers and are found to be roughly degenerate with the highest-lying $\mathrm{Se} p$ states at $\Gamma$. The results are relevant to various explanations of the $2a_0\times 2c_0$ lattice instability in this material, including a possible excitonic electron-hole interaction.