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Ta$_2$NiSe$_5$ became one of the most investigated candidate materials forhosting an excitonic insulator ground state. Many studies describe thecorresponding phase transition as a condensation of excitons breaking acontinuous symmetry. This view got challenged recently pointing out theimportance of the loss of two mirror symmetries at a structural phasetransition that occurs together with the semiconductor-excitonic insulatortransition. For such a scenario an unstable optical zone-center phonon at lowenergy is proposed to drive the transition. Here we report on the experimentalobservation of such a soft mode behavior using Raman spectroscopy. In additionwe find a novel spectral feature, likely of electronic or joint electronic andphononic origin, that is clearly distinct from the lattice dynamics and thatbecomes dominant at Tc. This suggests a picture of joint structural andelectronic order driving the phase transition.

physical review research

Phononic soft mode and strong electronic background behavior across the  structural phase transition in the excitonic insulator Ta$_2$NiSe$_5$ (with  Erratum)