Flat bands correspond to the spatial localization of a quantum particle moving in a field with discrete or continuous translational invariance. The canonical example is the flat Landau levels in a homogeneous magnetic field. Several significant problems—including flat bands in moiré structures—are related to the problem of a particle moving in an inhomogeneous magnetic field with zero total flux. We demonstrate that while perfectly flat bands in such cases are impossible, the introduction of a “non-Abelian component”—a spin field with zero total curvature—can lead to perfect localization. Several exactly solvable models are constructed: (i) a half-space up/down field with a sharp one-dimensional boundary; (ii) an alternating up/down field periodic in one direction on a cylinder; and (iii) a doubly periodic alternating field on a torus. The exact solution on the torus is expressed in terms of elliptic functions. It is shown that flat bands are only possible for certain magic values of the field corresponding to a quantized flux through an individual tile. These exact solutions clarify the simple structure underlying flat bands in moiré materials and provide a springboard for constructing a novel class of fractional quantum Hall states.

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• Flat bands
• Gauge theories
• Landau levels
• Localization
• Non-Abelian gauge theories
• Quantum Hall effect
• Spin texture
• Topological phases of matter
• Topological superconductors
• Dirac semimetal
• Superlattices
• Topological materials
• Twisted bilayer graphene
• Two-dimensional electron system
• Unconventional superconductors
• Weyl semimetal
• Chiral symmetry
• Non-Abelian models