Moiré Superstructures in Marginally-Twisted NbSe$_2$ Bilayers

Abstract: The creation of moir\'e superlattices in twisted bilayers of two-dimensional crystals has been utilised to engineer quantum material properties in graphene and transition metal dichalcogenide (TMD) semiconductors. Here, we examine the structural relaxation and electronic properties in small-angle twisted bilayers of metallic NbSe$2$. Reconstruction appears to be particularly strong for misalignment angles $\theta_P$ < 2.9$^o$ and $\theta{AP}$ < 1.2$^o$ for parallel (P) and antiparallel (AP) orientation of monolayers' unit cells, respectively. Multiscale modelling reveals the formation of domains and domain walls with distinct stacking, for which density functional theory (DFT) calculations are used to map the shape of the bilayer Fermi surface and the relative phase of the CDW order in adjacent layers. We find a significant modulation of interlayer coupling across the moir\'e superstructure and the existence of preferred interlayer orientations of the CDW phase, necessitating the nucleation of CDW discommensurations at superlattice domain walls.