Ultrafast Electronic Band Gap Control in an Excitonic Insulator

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.