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The excitonic insulator is an electronically-driven phase of matter thatemerges upon the spontaneous formation and Bose condensation of excitons.Detecting this exotic order in candidate materials is a subject of paramountimportance, as the size of the excitonic gap in the band structure establishesthe potential of this collective state for superfluid energy transport.However, the identification of this phase in real solids is hindered by thecoexistence of a structural order parameter with the same symmetry as theexcitonic order. Only a few materials are currently believed to host a dominantexcitonic phase, Ta$_2$NiSe$_5$ being the most promising. Here, we test thisscenario by using an ultrashort laser pulse to quench the broken-symmetry phaseof this transition metal chalcogenide. Tracking the dynamics of the material'selectronic and crystal structure after light excitation reveals surprisingspectroscopic fingerprints that are only compatible with a primary orderparameter of phononic nature. We rationalize our findings throughstate-of-the-art calculations, confirming that the structural order accountsfor most of the electronic gap opening. Not only do our results uncover thelong-sought mechanism driving the phase transition of Ta$_2$NiSe$_5$, but theyalso conclusively rule out any substantial excitonic character in thisinstability.

The spontaneous symmetry breaking in Ta$_2$NiSe$_5$ is structural in nature