Disentangling Lattice and Electronic Instabilities in the Excitonic Insulator Candidate Ta₂NiSe₅ by Nonequilibrium Spectroscopy

Ta2NiSe5 is an excitonic insulator candidate showing the semiconductor or semimetal-to-insulator (SI) transition below Tc = 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 Ta2NiSe5 using pump-probe Raman and photoluminescence 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 Ta2NiSe5.