${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_5$ became one of the most investigated candidate materials for hosting an excitonic insulator ground state. Many studies describe the corresponding phase transition as a condensation of excitons breaking a continuous symmetry. This view got challenged recently pointing out the importance of the loss of two mirror symmetries at a structural phase transition that occurs together with the semiconductor—excitonic insulator transition. For such a scenario an unstable optical zone-center phonon at low energy is proposed to drive the transition. Here we report on the experimental observation of such a soft mode behavior using Raman spectroscopy. In addition we find a novel spectral feature, likely of electronic or joint electronic and phononic origin, that is clearly distinct from the lattice dynamics and that becomes dominant at $T_c$. This suggests a picture of joint structural and electronic orders driving the phase transition.

• Received 10 July 2020
• Revised 30 October 2020
• Accepted 11 November 2020

DOI:https://doi.org/10.1103/PhysRevResearch.2.042039

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Published by the American Physical Society