Double and Quadruple Flat Bands tuned by Alternative magnetic Fluxes in Twisted Bilayer Graphene

TL;DR

This paper demonstrates how alternative magnetic fluxes can induce four-fold degenerate flat bands in twisted bilayer graphene, expanding the possibilities for exploring correlated and topological phases.

Contribution

It introduces a method to generate and control four-fold degenerate flat bands in TBG using magnetic flux, revealing new degeneracy mechanisms and many-body states.

Findings

Four-fold degenerate flat bands can be induced by magnetic flux.

Conditions for multi-fold degeneracy are linked to band inversion and eigenvalue degeneracy.

An orbital-related intervalley coherent state is identified as the ground state.

Abstract

Twisted bilayer graphene (TBG) can host the moir\'{e} energy flat bands with two-fold degeneracy serving as a fruitful playground for strong correlations and topological phases. However, the number of degeneracy is not limited to two. Introducing a spatially alternative magnetic field, we report that the induced magnetic phase becomes an additional controllable parameter and leads to an undiscovered generation of four-fold degenerate flat bands. This emergence stems from the band inversion at $\Gamma$ point near the Fermi level with a variation of both twisted angle and magnetic phase. We present the conditions for the emergence of multi-fold degenerate flat bands, which are associated with the eigenvalue degeneracy of a Birman-Schwinger operator. Using holomorphic functions, which explain the origin of the double flat bands in the conventional TBG, we can generate analytical wave functions in the magnetic TBG to show absolute flatness with four-fold degeneracy. Moreover, we identify an orbital-related intervalley coherent state as the many-body ground state at charge neutrality. In contrast, the conventional TBG has only two moir\'{e} energy flat bands, and the highly degenerate flat bands with additional orbital channels in this magnetic platform might bring richer correlation physics.