Ultrafast Electronic Band Gap Control in an Excitonic Insulator

Abstract

We report on the nonequilibrium dynamics of the electronic structure of the layered semiconductor ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ investigated by time- and angle-resolved photoelectron spectroscopy. We show that below the critical excitation density of ${F}_{C}=0.2\text{ }\text{ }\mathrm{mJ}\text{ }{\mathrm{cm}}^{\ensuremath{-}2}$, 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 ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$, whose order parameter is connected to the gap size. This work demonstrates the ability to manipulate the band gap of ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ with light on the femtosecond time scale.