Ultrafast Band and Gap Renormalization Induced by Excitonic Electron-Hole Liquid in Monolayer MoS2

Extreme-UV time-resolved photoemission spectroscopy is used to study the ultrafast band and gap dynamics in photon-excited monolayer \ce{MoS2} on HOPG with carrier densities below Mott threshold. We observe band renormalizations for both valence and conduction bands at K valley of the ML \ce{MoS2} and realize an unexpected increase of the band gap up to 50 meV with simultaneously enhancement of band effective mass at ultrafast timescale. We also observe a transient satellite band emerging closely above the valence band maximum. We resort to the quasi-equilibrium theory of electron-hole liquid to calculate the ML \ce{MoS2} spectral functions dressed by the photoexcited excitons. Our theoretical results agree well with the experimental observations, revealing the intriguing complex correlations between the electronic structure and excitonic quasi-particles in ML \ce{MoS2}.