Fig. 2: Observation of an isolated flat band throughout the entire Brillouin zone.

a Dispersion image along the Z-Γ-Z direction and corresponding integrated energy distribution curve (EDC). The color scale is enhanced above −0.8 eV, and the EDC is multiplied by 1000 for better visibility of the weak signal of the CB. The peak width of the flat band is larger than the energy resolution, possibly owing to the intrinsic broadening or roughness of the cleaved surface. b Curvature-filtered dispersion map in 2D momentum space. The color scale indicates photoelectron intensity processed by the 1D curvature filter along the energy direction. c Full dispersion images along all high-symmetry momentum directions in the entire Brillouin zone. The color scale indicates photoelectron intensity in (a and c). d Extracted dispersion of the flat band across the Brillouin zone from the data in (c).