Diffusion of valley-coherent dark excitons in a high-angle incommensurate Moir\'e homobilayer

Abstract

The last few years have witnessed a surge in interest and research efforts in the field of twistronics, especially in low-angle twisted bilayers of transition metal dichalocogenides. These novel material platforms have been demonstrated to host periodic arrays of excitonic quantum emitters, interlayer excitons with long lifetimes, and exotic many-body states. While much remains to be known and understood about these heterostructures, the field of large-angle, incommensurate bilayers is even less explored. At twist angles larger than a few degrees, the presence of periodicity in these bilayers becomes chaotic, making the systems essentially aperiodic and incommensurate in nature due to the limitations of fabrication techniques. In this work, we demonstrate the emergence of a brightened dark intralayer exciton in twisted n-doped molybdenum diselenide homobilayer. We show that this dark exciton diffuses across the excitation spot more efficiently as compared to bright trions or excitons, reaching diffusion lengths greater than 4 microns. Temperature-dependent spectra provide corroborative evidence and reveal a brightened dark trion. Almost inexplicably, this dark exciton showcases a robust valley coherence, which we attribute to a small mixing of the spin-resolved conduction bands due to an absence of out-of-plane reflection symmetry arising from a strong dielectric contrast. Our results reveal some of the richness of the physics of these large-angle systems while uncovering new opportunities for valleytronic devices that may utilize these more valley-robust "mixed" dark excitons.