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Gapless Dirac surface states in the antiferromagnetic topological insulator <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>MnBi</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>Te</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math>
A topological insulator can be converted to an axion insulator when time reversal symmetry is broken, which leads to the opening of a gap in the Dirac surface state. Recently, MnBi${}_{2}$Te${}_{4}$ was predicted to host such a state upon antiferromagnetic ordering. Collecting and analyzing very detailed photoemission data, the authors …