We show that the flat bands in the chiral model of magic-angle twisted bilayer graphene remain exactly flat in the presence of a perpendicular magnetic field. This is shown by an exact mapping between the model and the lowest Landau level wave functions at an effective magnetic field, in which the external field is either augmented or reduced by one flux quantum per unit cell. When the external field reaches one flux quantum per unit cell, the model exhibits a topological phase transition. These findings allow us to analyze a Jain series of fractional Chern insulators states in the exactly flat band, and to point out an unconventional dependence of the energy gap on the magnetic field.

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• Chern insulators
• Electronic structure
• Fractional quantum Hall effect
• Landau levels
• Graphene
• Superlattices