Moir\'e Superstructures in Marginally-Twisted NbSe$_2$ Bilayers

TL;DR

This study investigates the structural and electronic effects of moiré superlattices in small-angle twisted bilayers of NbSe₂, revealing domain formation, modulation of interlayer coupling, and CDW phase behavior influenced by moiré patterns.

Contribution

It provides the first detailed analysis of moiré superstructures in metallic NbSe₂ bilayers, combining multiscale modeling and DFT to understand their impact on electronic and CDW properties.

Findings

Strong reconstruction for small misalignment angles

Formation of domains and domain walls with distinct stacking

Modulation of interlayer coupling and CDW phase across the moiré superstructure

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.