Structural contribution to light-induced gap suppression in Ta$_2$NiSe$_5$

TL;DR

This study uses ultrafast electron diffraction and first-principles calculations to show that structural changes in Ta$_2$NiSe$_5$ primarily cause light-induced gap suppression, challenging the excitonic insulator interpretation.

Contribution

It provides the first quantitative analysis linking atomic displacements to gap suppression, emphasizing the role of lattice dynamics in phase transitions of correlated materials.

Findings

Structural change accounts for most of the photoinduced gap reduction.

Atomic displacements can explain the insulating gap suppression without excitonic effects.

Lattice dynamics are crucial in understanding nonequilibrium phase transitions.

Abstract

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. Ta$_2$NiSe$_5$ 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 Ta$_2$NiSe$_5$ 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.