Tracking the Catastrophic Collapse of Hybrid Exciton-Phonon Order in a Quantum Material
Revealing the interactions binding 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 1T-TiSe$_2$, where photoexcitation disrupts its presumed hybrid exciton-phonon order. This exposes a paradox: the electronic component collapses within femtoseconds while the periodic lattice distortion persists. If the lattice distortion outlives the excitonic condensate, were they truly intertwined? Here we resolve this by uncovering a low-frequency mode (approx. 0.13 THz) emerging only in the ordered state, signaling exciton-phonon coupling. This mode is consistent with a locked phason -- a collective excitation arising if coupling between the excitonic condensate and lattice reduces continuous phase symmetry to a discrete one, giving the excitonic Goldstone mode finite mass. This is captured by an effective theory describing a shared potential landscape. At a critical threshold, the collapse of excitonic order flattens the potential, triggering an exciton-phonon catastrophe: selective overheating of the charge density wave phonon, disappearance of the locked phason, and sudden loss of electronic coherence. Remarkably, the lattice distortion survives as a dynamically trapped, non-thermal remnant, confirmed by the anomalous temperature dependence of the phononic response. These findings demonstrate that coupled potential energy landscapes can be manipulated to selectively dismantle complex quantum orders, advancing material control through dynamical design.