Classification of High-Ordered Topological Nodes towards Moir\'e Flat Bands in Twisted Bilayers

TL;DR

This paper classifies topological nodes in twisted bilayer systems with chiral symmetry, identifying conditions for flat band emergence and revealing multiple degeneracy possibilities that could lead to richer correlated physics.

Contribution

It provides a systematic classification of topological nodes and their locations in twisted bilayer systems, linking them to the emergence of flat bands beyond Dirac cones.

Findings

Flat bands are locked at zero energy across the Brillouin zone.

Multiple degeneracy levels in flat bands, including four-, six-, and eight-fold.

Conditions for flat band emergence based on node classification and location.

Abstract

At magic twisted angles, Dirac cones in twisted bilayer graphene (TBG) can evolve into flat bands, serving as a critical playground for the study of strongly correlated physics. When chiral symmetry is introduced, rigorous mathematical proof confirms that the flat bands are locked at zero energy in the entire Moir\'e Brillouin zone (BZ). Yet, TBG is not the sole platform that exhibits this absolute band flatness. Central to this flatness phenomenon are topological nodes and their specific locations in the BZ. In this study, considering twisted bilayer systems that preserve chiral symmetry, we classify various ordered topological nodes in base layers and all possible node locations across different BZs. Specifically, we constrain the node locations to rotational centers, such as {\Gamma} and M points, to ensure the interlayer coupling retains equal strength in all directions. Using this classification as a foundation, we systematically identify the conditions under which Moir\'e flat bands emerge. Additionally, through the extension of holomorphic functions, we provide proof that flat bands are locked at zero energy, shedding light on the origin of the band flatness. Remarkably, beyond Dirac cones, numerous twisted bilayer nodal platforms can host flat bands with a degeneracy number of more than two, such as four-fold, six-fold, and eight-fold. This multiplicity of degeneracy in flat bands might unveil more complex and enriched correlation physics.