Diffusion of Valley-Coherent Dark Excitons in a Large-Angle Incommensurate Moiré Homobilayer

Recent research in twistronics, particularly in small-angle twisted bilayers of transition metal dichalcogenides, has uncovered exciting phenomena like periodic arrays of excitonic quantum emitters, exotic many-body states, and long-lived interlayer excitons. However, less explored has been the physics of large-angle, incommensurate bilayers, where periodicity breaks down. In this study, we demonstrate the emergence of a brightened dark intralayer exciton in a twisted n-doped molybdenum diselenide homobilayer. This dark exciton diffuses more efficiently than bright excitons or trions, with diffusion lengths over 4 μm. Temperature-dependent spectra show a brightened dark trion, and we observe a robust valley coherence. This unique behavior is attributed to a small mixing of spin-resolved conduction bands, caused by a lack of out-of-plane reflection symmetry and strong dielectric contrast. Our findings open new possibilities for valleytronic devices using valley-robust "mixed" dark excitons.