An excitonic insulator is a material that hosts an exotic ground state, where an energy gap opens due to spontaneous condensation of bound electron-hole pairs. Ta2NiSe5 is a promising candidate for this type of material, but the coexistence of a structural phase transition with the gap opening has led to a long-standing debate regarding the origin of the insulating gap. Here we employ MeV ultrafast electron diffraction to obtain quantitative insights into the atomic displacements in Ta2NiSe5 following photoexcitation, which has been overlooked in previous time-resolved spectroscopy studies. In conjunction with first-principles calculations using the measured atomic displacements, we find that the structural change can largely account for the photoinduced reduction in the energy gap without considering excitonic effects. Our work illustrates the importance of a quantitative reconstruction of individual atomic pathways during nonequilibrium phase transitions, paving the way for a mechanistic understanding of a diverse array of phase transitions in correlated materials where lattice dynamics can play a pivotal role.
@article{arxiv.2508.12363,
title = {Structural contribution to light-induced gap suppression in Ta$_2$NiSe$_5$},
author = {Zijing Chen and Chenhang Xu and Chendi Xie and Weichen Tang and Qiaomei Liu and Dong Wu and Qing Xu and Tao Jiang and Pengfei Zhu and Xiao Zou and Jun Li and Zhiwei Wang and Nanlin Wang and Dong Qian and Alfred Zong and Dao Xiang},
journal= {arXiv preprint arXiv:2508.12363},
year = {2025}
}