Phononic soft mode behavior and a strong electronic background across the structural phase transition in the excitonic insulator Ta_{2}NiSe_{5}

Ta_{2}NiSe_{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.