Ultrafast Electronic Band Gap Control in an Excitonic Insulator

We report on the nonequilibrium dynamics of the electronic structure of the layered semiconductor Ta₂NiSe₅ investigated by time- and angle-resolved photoelectron spectroscopy. We show that below the critical excitation density of FC=0.2 mJ cm⁻², the band gap narrows transiently, while it is enhanced above FC. Hartree-Fock calculations reveal that this effect can be explained by the presence of the low-temperature excitonic insulator phase of Ta₂NiSe₅, whose order parameter is connected to the gap size. This work demonstrates the ability to manipulate the band gap of Ta₂NiSe₅ with light on the femtosecond time scale.