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

TL;DR

This study observes a soft phononic mode and a novel spectral feature in Ta$_2$NiSe$_5$, indicating a combined structural and electronic transition in its excitonic insulator phase.

Contribution

It provides experimental evidence of a soft phonon mode and a new spectral feature, supporting a joint structural and electronic transition mechanism.

Findings

Observation of a soft phonon mode via Raman spectroscopy.

Detection of a novel spectral feature dominant at Tc.

Evidence supporting a combined structural and electronic phase transition.

Abstract

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 Tc. This suggests a picture of joint structural and electronic order driving the phase transition.