Identifying axion insulator by quantized magnetoelectric effect in antiferromagnetic MnBi_{2}Te_{4} tunnel junction

The intrinsic magnetic topological insulator MnBi_{2}Te_{4} is believed to be an axion insulator in its antiferromagnetic ground state. However, the 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 functions, we theoretically demonstrate the quantized magnetoelectric current in a tunnel junction of atomically thin MnBi_{2}Te_{4} sandwiched between two contacts, which is a smoking-gun signal that unambiguously confirms antiferromagnetic MnBi_{2}Te_{4} to be an axion insulator. Our predictions can be verified directly by experiments.