Electron-Hole Coupling and the Charge Density Wave Transition in ${\mathrm{TiSe}}_2$

Angle-resolved photoemission is employed to measure the band structure of ${\mathrm{TiSe}}_2$ in order to clarify the nature of the ( $2\times 2\times 2$) charge density wave transition. The results show a very small indirect gap in the normal phase transforming into a larger indirect gap at a different location in the Brillouin zone. Fermi surface topology is irrelevant in this case. Instead, electron-hole coupling together with a novel indirect Jahn-Teller effect drives the transition.