Topologically protected moire exciton at a twist-boundary in a van der Waals heterostructure*

A twin boundary in one of the layers of a twisted van der Waals heterostructure separates regions with near opposite inter-layer twist angles. In a MoS₂/WSe₂ bilayer, the regions with R_h^h and R_h^X stacking registry that defined the sub-lattices of the moiré honeycomb pattern would be interchanged across such a twist boundary. In that case, we demonstrate that topologically protected chiral moiré exciton states are confined at the twist boundary. These are one-dimensional and uni-directional excitons with opposite velocities for opposite spins, enabling intrinsic, guided, and far reaching spin-polarized exciton currents.