▎ 摘　　要

Twisted bilayer graphene (TBG) is remarkable for its topological flat bands, which drive strongly interacting physics at integer fillings, and its simple theoretical description facilitated by the Bistritzer-MacDonald Hamiltonian, a continuum model coupling two Dirac fermions. Because of the large moire unit cell, TBG offers the unprecedented opportunity to observe reentrant Hofstadter phases in laboratory -strength magnetic fields near 25 T. This Letter is devoted to magic angle TBG at 2 pi flux where the magnetic translation group commutes. We use a newly developed gauge-invariant formalism to determine the exact single-particle band structure and topology. We find that the characteristic TBG flat bands reemerge at 2 pi flux, but, due to the magnetic field breaking C2zT, they split and acquire Chern number +1. We show that reentrant correlated insulating states appear at 2 pi flux driven by the Coulomb interaction at integer fillings, and we predict the characteristic Landau fans from their excitation spectrum.