Taco-shaped quasi-1D flat bands at a magic angle in twisted bilayer transition metal dichalcogenides

Strongly correlated phenomena arise from flat electronic bands, which are well known to occur in moiré systems predominantly at small twist angles. We report the surprising emergence of flat electronic bands with a distinctive quasi-one-dimensional taco-shaped anisotropic dispersion at a large angle in twisted bilayer WSe₂. Our first principles calculations show that the flat band originates from optimal interlayer hybridization between adjacent like-spin Lambda valleys. These taco-shaped valleys form six anisotropic channels that interconnect the Lambda valleys across the moiré Brillouin zone, exhibiting alternating spin polarization under sixfold rotation, reminiscent of altermagnetic textures. The conditions for band flattening are found to be strongly dependent on the twist angle. The 21.8° angle represents the precise condition for band flattening at the conduction band minimum, evidenced by a sharp van Hove singularity in the density of states. A direct consequence of the flat bands at this twist angle is the recent observation of phonon-assisted intervalley absorption peaks in reflection contrast spectra. This work broadens the scope of moiré engineering, demonstrating that moiré materials twisted at large angles can be a new platform to study novel correlated and excitonic phenomena.