Tracking the catastrophic collapse of hybrid exciton-phonon order in a quantum material

Abstract

Abstract Revealing the hidden interactions that bind electronic and lattice components of cooperative quantum order is central to sculpting new states of matter. This challenge is epitomized by the charge density wave material 1 T -TiSe 2 , where photoexcitation disrupts its presumed hybrid exciton-phonon order: the electronic component collapses within femtoseconds, while the periodic lattice distortion persists, challenging the definition of hybrid order. Here we resolve this paradox by uncovering a low-frequency mode (~0.13 THz) that emerges only in the ordered state and signals exciton-phonon coupling. This mode is consistent with a locked phason, a collective excitation arising when excitonic and lattice degrees of freedom share a coupled potential landscape. At a critical photoexcitation threshold, the collapse of the excitonic order flattens this potential, causing the locked phason to disappear, the charge density wave phonon to selectively overheat, and the surviving lattice distortion to become a trapped non-thermal remnant.