Ultrafast Electronic Band Gap Control in an Excitonic Insulator

TL;DR

This study demonstrates ultrafast control of the electronic band gap in the excitonic insulator Ta$_2$NiSe$_5$ using light, revealing a transient narrowing or enhancement depending on excitation density, linked to its excitonic order.

Contribution

It provides the first direct experimental evidence of ultrafast band gap manipulation in an excitonic insulator via time-resolved photoelectron spectroscopy and theoretical Hartree-Fock modeling.

Findings

Band gap transiently narrows below critical excitation density.

Band gap enhances above critical excitation density.

Light can manipulate the band gap on femtosecond timescales.

Abstract

We report on the nonequilibrium dynamics of the electronic structure of the layered semiconductor Ta$_2$NiSe$_5$ investigated by time- and angle-resolved photoelectron spectroscopy. We show that below the critical excitation density of $F_{C} = 0.2$ mJ cm$^{-2}$, the band gap $narrows$ transiently, while it is $enhanced$ above $F_{C}$. Hartree-Fock calculations reveal that this effect can be explained by the presence of the low-temperature excitonic insulator phase of Ta$_2$NiSe$_5$, whose order parameter is connected to the gap size. This work demonstrates the ability to manipulate the band gap of Ta$_2$NiSe$_5$ with light on the femtosecond time scale.