Electron-Hole Coupling and the Charge Density Wave Transition inTiSe2

Angle-resolved photoemission is employed to measure the band structure of TiSe2 in order to clarify the nature of the (2 × 2 × 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