Identifying Axion Insulator by Quantized Magnetoelectric Effect in antiferromagnetic MnBi₂Te₄ Tunnel Junction

Intrinsic magnetic topological insulator MnBi₂Te₄ is believed to be an axion insulator in its antiferromagnetic ground state. However, direct identification of axion insulators remains experimentally elusive because the observed vanishing Hall resistance, while indicating the onset of the axion field, is inadequate to distinguish the system from a trivial normal insulator. Using numerical Green's function method, we theoretically demonstrate the quantized topological magnetoelectric current in a tunnel junction with atomically thin MnBi₂Te₄ sandwiched between two metalic contacts, which is a smoking-gun signal that unambiguously confirms antiferromagnetic MnBi₂Te₄ to be an axion insulator. Our predictions can be verified directly by experiments.