Charge and Lattice Dynamics in Excitonic Insulator Ta2NiSe5 Investigated Using Ultrafast Reflection Spectroscopy

An excitonic insulator, in which an optical gap is formed by exciton condensation, is expected to exhibit large photoresponses. Here, we report the results of femtosecond reflection spectroscopy covering a wide infrared region on a typical excitonic insulator, Ta2NiSe5, which gives direct information not only about a change of excitonic gap but also a metallization. With a weak excitation, the excitonic gap decreases due to the weakening of excitonic effect by photocarriers. With a strong excitation, a Drude-like high-reflection band appears, showing an excitonic-insulator-to-metal transition. Analyses of spectral and temporal changes of reflectivity revealed that the metallization is accompanied by large structural changes. This demonstrates an important role of electron-lattice interaction on the stabilization of excitonic phase.

Automated summary

In this study, the optical properties of the excitonic insulator Ta2NiSe5 were investigated using femtosecond reflection spectroscopy. The results showed that changes in excitation intensity lead to variations in the excitonic gap and a transition from an excitonic insulator to a metal. Analysis of the reflectivity also revealed a correlation between metallization and changes in the lattice structure.