Disentangling lattice and electronic instabilities in the excitonic insulator candidate Ta$_2$NiSe$_5$ by nonequilibrium spectroscopy

TL;DR

This study uses nonequilibrium spectroscopy to disentangle the roles of lattice and electronic factors in the excitonic insulator candidate Ta$_2$NiSe$_5$, revealing electron correlations as key to its phase transition.

Contribution

It demonstrates that electron correlations are crucial in the semiconductor-to-insulator transition, independent of structural distortions, using pump-probe Raman and photoluminescence techniques.

Findings

Metastable state with suppressed gap but preserved structural distortion

Electron correlations are vital in the SI transition

Structural transition is not solely responsible for the insulator state

Abstract

Ta$_2$NiSe$_5$ is an excitonic insulator candidate showing the semiconductor/semimetal-to-insulator (SI) transition below $T_{\text{c}}$ = 326 K. However, since a structural transition accompanies the SI transition, deciphering the role of electronic and lattice degrees of freedom in driving the SI transition has remained controversial. Here, we investigate the photoexcited nonequilibrium state in Ta$_2$NiSe$_5$ using pump-probe Raman and photoluminescence (PL) spectroscopies. The combined nonequilibrium spectroscopic measurements of the lattice and electronic states reveal the presence of a photoexcited metastable state where the insulating gap is suppressed, but the low-temperature structural distortion is preserved. We conclude that electron correlations play a vital role in the SI transition of Ta$_2$NiSe$_5$.