Photo-excited Dynamics in the Excitonic Insulator Ta2NiSe5

TL;DR

This study uses time-resolved spectroscopy to investigate the quasiparticle and phonon dynamics in Ta2NiSe5, providing evidence for its excitonic insulator state and its stability under photoexcitation.

Contribution

It demonstrates the temperature-independent gap behavior in Ta2NiSe5 and its stability against photoexcitation, supporting its classification as an excitonic insulator in the BEC-BCS crossover regime.

Findings

Intrinsic gap is nearly temperature independent up to 275 K.

The material remains stable under strong photoexcitation.

Evidence supports the excitonic insulator state in Ta2NiSe5.

Abstract

The excitonic insulator is an intriguing correlated electron phase formed of condensed excitons. A promising candidate is the small band gap semiconductor Ta2NiSe5. Here we investigate the quasiparticle and coherent phonon dynamics in Ta2NiSe5 in a time resolved pump probe experiment. Using the models originally developed by Kabanov et al. for superconductors, we show that the material's intrinsic gap can be described as almost temperature independent for temperatures up to about 250 K to 275 K. This behavior supports the existence of the excitonic insulator state in Ta2NiSe5. The onset of an additional temperature dependent component to the gap above these temperatures suggests that the material is located in the BEC-BCS crossover regime. Furthermore, we show that this state is very stable against strong photoexcitation, which reveals that the free charge carriers are unable to effectively screen the attractive Coulomb interaction between electrons and holes, likely due to the quasi one-dimensional structure of Ta2NiSe5.