Tracking the Catastrophic Collapse of Hybrid Exciton-Phonon Order in a Quantum Material

Revealing the hidden interactions that bind the electronic and lattice components of a 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, exposing a striking paradox: the electronic component collapses within femtoseconds, while the periodic lattice distortion persists, challenging the very definition of a hybrid order: if the lattice distortion outlives the excitonic condensate, were they ever truly intertwined? Here we resolve this paradox by uncovering a low-frequency mode (0.13 THz) that emerges only in the ordered state and signals the presence of exciton-phonon coupling. This mode is consistent with a locked phason, a collective excitation that would arise if the coupling between the excitonic condensate and the lattice reduced its continuous phase symmetry to a discrete one, thereby giving the excitonic Goldstone mode a finite mass. Such a scenario is captured by an effective theory, which describes a shared potential landscape linking the excitonic and lattice degrees of freedom. At a critical photoexcitation threshold, the collapse of the excitonic order flattens the potential, triggering an exciton-phonon catastrophe characterized by selective overheating of the charge density wave phonon, the disappearance of the locked phason, and a sudden loss of electronic coherence. Remarkably, the lattice distortion survives this event as a dynamically trapped and non-thermal remnant, whose non-equilibrium character is confirmed by the anomalous temperature dependence of the phononic response. These findings demonstrate that the coupled potential energy landscape of cooperative orders can be manipulated to selectively dismantle complex quantum orders, advancing a new paradigm for material control through dynamical design.