Fig. 4: Charge-density-wave (CDW) orders on the interior of monolayer (ML) NbSe₂ near the well-ordered zigzag (ZZ) edges.

a Scanning tunneling microscopy (STM) images at the ZZ edge of ML NbSe₂. The entire regions exhibit clear 3 × 3 superlattices. The range of edge states are marked in the panel. b Schematic diagram of edge states and edge-CDW interference interactions at the CDW edge. The in-plane CDW modulations can be described as the sum of three individual plane waves CDW_1,2,3, which are connected by a threefold rotation symmetry. In the direction perpendicular to the edge, the incident and elastic scattered waves can be simplified to sin (kx + φ) and −sin (kx − φ), respectively. The interference of the plane waves by the incident and elastic scattered processes results in the stripe-like charge density, depending on the phase φ of the plane wave. c–e Atomically resolved scanning tunneling spectroscopy (STS) maps recorded at the location marked by the black rectangle in panel a under the energies of −0.2, 0.2, and 0.3 eV, respectively. In the bulk of defect-free NbSe₂, the CDW contrast in the STS maps always exhibits a 3 × 3 CDW order, along with the strong charge modulation within each CDW supercell. While on the interior of NbSe₂ near the edges, the CDW contrast changes from 3 × 3 CDW orders to nearly one-dimensional stripe phases. f–h Simulations of edge-induced CDW interference with φ = 0 π, 0.6 π, and 1.8 π, respectively.