Cooperative Exciton-Phonon Bose-Einstein Condensation in an Excitonic Insulator*

The excitonic insulator is an exotic phase of matter in which excitons spontaneously form and collectively undergo Bose-Einstein condensation. Recently, increasing evidence has shown that this ground state is stabilized in the layered transition metal chalcogenide Ta₂NiSe₅ (TNS). Distinctive signature of exciton condensation is the pronounced flattening of the valence band top with decreasing temperature, signaling the opening of an additional many-body gap, as well as a coherent amplitude-like response observed in optical pump-probe data. Due to its direct bandgap, TNS is believed to realize the pure excitonic insulator state, free from the complications of coexisting density-wave orders or strong coupling to other degrees of freedom. Here, we reveal that a cooperative exciton-phonon mechanism lies instead at the origin of the condensate in TNS. Specifically, we use time- and angle-resolved photoemission spectroscopy to show that the vibrational degrees of freedom play a crucial role in the photoinduced melting of the exciton Bose-Einstein condensate. Our results open new routes towards the selective manipulation of the excitonic insulating state via specific modes of the crystal lattice.