Critical charge fluctuations and emergent coherence in a strongly correlated excitonic insulator

TL;DR

This paper uses polarization-resolved Raman spectroscopy to identify and analyze excitonic modes in Ta$_2$NiSe$_5$, revealing strong correlations and coherence effects that clarify the nature of the excitonic insulator phase.

Contribution

It provides the first direct spectroscopic evidence of a quadrupolar excitonic mode in Ta$_2$NiSe$_5$ and explores its critical behavior near the phase transition.

Findings

Identification of a quadrupolar excitonic mode

Softening of the mode near the phase transition

Emergence of coherent superpositions at low temperatures

Abstract

Excitonic insulator is a coherent electronic phase that results from the formation of a macroscopic population of bound particle-hole pairs - excitons. With only a few candidate materials known, the collective excitonic behavior is challenging to observe, being obscured by crystalline lattice effects. Here we use polarization-resolved Raman spectroscopy to reveal the quadrupolar excitonic mode in the candidate zero-gap semiconductor Ta$_2$NiSe$_5$ disentangling it from the lattice phonons. The excitonic mode pronouncedly softens close to the phase transition, showing its electronic character, while its coupling to non-critical lattice modes is shown to enhance the transition temperature. On cooling, we observe the gradual emergence of coherent superpositions of band states at the correlated insulator gap edge, with strong departures from mean-field theory predictions. Our results demonstrate the realization of a strongly correlated excitonic state in an equilibrium bulk material.