Real-space observation of a time-reversal invariant topological state in twisted bilayer InSe.

The moiré superlattice formed by van der Waals (vdW) stacking of two-dimensional (2D) monolayers with a twist angle can give rise to a variety of novel correlated physical phenomena. In this study, we propose that a 2D semiconductor composed of vdW layers with band edge states extended into the interlayer region can host time-reversal-invariant topological states, as demonstrated by density functional theory and tight-binding calculations. This emergent physics in moiré superlattices is experimentally verified in a twisted bilayer InSe with a twisting angle of θ = 7.34°. Using scanning tunneling microscopy/spectroscopy (STM/STS), we observed topological edge modes associated with the Z2 topological metal state at the moiré domain boundary, while these states are absent in the bilayer without twisting. Our theoretical predictions and experimental discoveries advance the field of moiré physics and twistronics, offering a promising strategy for the creation of moiré topological devices.