Oxygen-terminated Ti3C2 MXene as an excitonic insulator

Excitonic insulators originate from the formation of bound excitons (electron–hole pairs) in semiconductors and provide a solid-state platform for quantum many-boson physics. We determined the excitonic insulator phase of Ti3C2O2 monolayer from its indirect quasiparticle band structure and from the precise evaluation of the relative value of the fundamental bandgap vs the momentum-indirect excitonic binding energy. The excitonic insulator is stable over the ±4% range of compressive and tensile biaxial strain. The energy region relevant for the optical absorption is strongly strain-dependent.