Quasi-one-dimensional taco-shaped bands in large-angle twisted bilayer transition metal dichalcogenides

Two-dimensional moiré materials offer a powerful, twist-tunable platform for engineering electronic bands and correlations, though most studies to date have focused on small twist angles where flat bands arise from symmetry-pinned monolayer momenta. Here, we observe the surprising emergence of flat electronic bands with a distinctive quasi-one-dimensional dispersion at large twist angles in bilayer transition metal dichalcogenides that originate from the $Λ$ valley states at generic momenta between $Γ$ and $K$ points. These taco-shaped anisotropic bands result from optimal interlayer hybridization between like-spin $Λ$ valleys at the conduction band minimum in the Brillouin zone, resulting in directional band flattening at a magic twist-angle of 21.8$^{\circ}$. The bands form six anisotropic channels with a sixfold alternating spin texture reminiscent of altermagnetic textures. At low energies, the density of states shows a power-law dependence due to the quasi-one-dimensional character, enhancing the potential for correlated phases. Our results provide a new platform for correlated phenomena and broaden the scope of moiré engineering to large twist angles in 2D materials.

💡 Research Summary

This paper reports the discovery of a novel type of flat electronic band in large-twist-angle bilayers of transition metal dichalcogenides (TMDs), challenging the prevailing focus on small-angle moiré systems. The researchers demonstrate that at a specific “magic” twist angle of 21.8°, a remarkably flat band with a distinctive quasi-one-dimensional “taco-shaped” dispersion emerges at the conduction band minimum.

The key mechanism involves the interlayer hybridization of like-spin Λ valleys. In many natural TMDs like WSe2, the conduction band minimum resides not at the high-symmetry K point, but at the Λ point, located generically between the Γ and K points. The wavefunctions at the Λ valley have strong out-of-plane character, favoring interlayer coupling. At 21.8°, the momentum separation between the Λ valleys of the top and bottom layers becomes optimal, and crucially, valleys with the same spin are brought into proximity. This spin-compatible, momentum-matched condition enables strong hybridization, which drastically flattens the band along the direction connecting adjacent Λ valleys (folded into λ points in the moiré Brillouin zone), while the dispersion remains steep in the perpendicular direction, creating an anisotropic, taco-like band structure.

This flat band forms six anisotropic channels within the moiré Brillouin zone. A striking feature is its alternating spin texture: each channel possesses a uniform out-of-plane spin polarization, but the spin direction alternates between adjacent channels, creating a sixfold pattern reminiscent of altermagnetic order. The quasi-1D character of the band has a profound impact on the density of states (DOS), which exhibits a power-law divergence (D(ΔE) ∝ (ΔE)^-0.205) at low energies, unlike the constant DOS of typical 2D parabolic bands. This enhanced and diverging DOS near the band edge is a hallmark of reduced dimensionality and strongly promotes electron-electron interactions, significantly boosting the potential for correlated phases like Wigner crystals or superconductivity.

The study combines first-principles DFT calculations with a low-energy continuum model. The calculations confirm that the flat band appears uniquely at 21.8°, while angles like 13.17° and 27.8° yield dispersive bands. The model analytically captures the magic angle condition where the bandwidth is minimized, pinpointing its origin in the delicate balance of interlayer tunneling and valley separation. This work establishes large-twist-angle TMDs as a new, robust platform for moiré engineering—free from the reconstruction and disorder common in small-angle systems—and opens a frontier for exploring correlated physics intertwined with quasi-one-dimensionality and non-trivial spin textures.