Abstract

© The Author 2016.Charge density wave formation, a key physics issue for materials, arises from interactions among electrons and phonons that can also lead to superconductivity and other competing or entangled phases. The prototypical system TiSe 2, with a particularly simple transition and no Kohn anomalies caused by electron-phonon coupling, is a fascinating but unsolved case after decades of research. Our angle-resolved photoemission measurements of the band structure as a function of temperature, aided by first-principles calculations, reveal a hitherto undetected but crucial feature: a electronic order in each layer sets in at ∼232 K before the widely recognized three-dimensional structural order at ∼205 K. The dimensional crossover, likely a generic feature of such layered materials, involves renormalization of different band gaps in two stages.