The intrinsic magnetic topological insulator MnBi₂Te₄ is believed to be an axion insulator in its antiferromagnetic ground state (with even number of septuple layers). 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 functions, PI-Cheng’s group theoretically demonstrated the quantized magnetoelectric current in a tunnel junction of atomically thin MnBi₂Te₄ sandwiched between two metallic contacts, which is a smoking-gun signal that unambiguously confirms antiferromagnetic MnBi₂Te₄ as an axion insulator.

This article has been published in Physical Review Research and is available here.