Non-Coulomb strong electron-hole binding in Ta2NiSe5 revealed by time- and angle-resolved photoemission spectroscopy

We reveal an ultrafast purely electronic phase transition in Ta2NiSe5, which is a plausible excitonic insulator, after excited by an ultrafast infrared laser pulse. Specifically, the order parameter of the strong electron-hole binding shrinks with enhancing the pump pulse, and above a critical pump fluence, a photoexcited semimetallic state is experimentally identified with the absence of ultrafast structural transition. In addition, the bare valence and conduction bands and also the effective masses in Ta2NiSe5 are determined. These findings and detailed analysis suggest a bare nonequilibrium semimetallic phase in Ta2NiSe5 and the strong electron-hole binding cannot be exclusively driven by Coulomb interaction.