Phononic soft mode behavior and a strong electronic background across the structural phase transition in the excitonic insulator Ta2NiSe5

Min-Jae Kim,Tomohiro Takayama,Hidenori Takagi,Stefan Kaiser,Masahiko Isobe,Armin Schulz

doi:10.1103/physrevresearch.2.042039

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.

Highlights

The possibility of realizing the elusive state of an excitonic insulator (EI) in the zero-gap semiconductor Ta2NiSe5 has stimulated a tremendous body of experimental and theoretical work
We have performed a detailed study of the low-frequency lattice dynamics across the orthorhombic to monoclinic structural phase transition in Ta2NiSe5
Two similar Raman studies appeared as preprints [49,50] that do not report any soft mode behavior but claim a pure excitonic driven phase transition based on a low-frequency electronic background

Summary

Rapid Communications

Ta2NiSe5 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. This suggests a picture of joint structural and electronic orders driving the phase transition

Introduction

Published by the American Physical Society

Results

Conclusion