Robust high-temperature topological excitonic insulator of transition-metal carbide MXenes

Topological excitonic insulators combine topological edge states and spontaneous exciton condensation, with dual functionality of topological insulators and excitonic insulators. Yet, they are very rare and little is known about their formation. In this work, we find that a mechanism dubbed as parity frustration prevents excitonic instability in usual topological insulators, and those whose band inversion is independent of spin-orbit coupling are possible candidates. We verify this by first-principles calculations on monolayer transition-metal carbides, which show a robust thermal-equilibrium exciton condensation, being sufficient for topological applications at room temperature. Such a state can be identified by angle-resolved photoemission spectroscopy and transport measurement. Our work provides not only a guide for finding more topological excitonic insulators, but also a new platform for studying the interplay between nontrivial band topology and quantum many-body effects.