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

Abstract

The excitonic insulator is an electronically-driven phase of matter that\nemerges upon the spontaneous formation and Bose condensation of excitons.\nDetecting this exotic order in candidate materials is a subject of paramount\nimportance, as the size of the excitonic gap in the band structure establishes\nthe potential of this collective state for superfluid energy transport.\nHowever, the identification of this phase in real solids is hindered by the\ncoexistence of a structural order parameter with the same symmetry as the\nexcitonic order. Only a few materials are currently believed to host a dominant\nexcitonic phase, Ta$_2$NiSe$_5$ being the most promising. Here, we test this\nscenario by using an ultrashort laser pulse to quench the broken-symmetry phase\nof this transition metal chalcogenide. Tracking the dynamics of the material's\nelectronic and crystal structure after light excitation reveals surprising\nspectroscopic fingerprints that are only compatible with a primary order\nparameter of phononic nature. We rationalize our findings through\nstate-of-the-art calculations, confirming that the structural order accounts\nfor most of the electronic gap opening. Not only do our results uncover the\nlong-sought mechanism driving the phase transition of Ta$_2$NiSe$_5$, but they\nalso conclusively rule out any substantial excitonic character in this\ninstability.\n