Is even-layer $\mathrm{MnBi_2Te_4}$ an axion insulator?
In plain words
Thin $\mathrm{MnBi_2Te_4}$ with an even number of layers shows zero Hall resistance and high longitudinal resistance at zero field, which fits an axion insulator but also an ordinary insulator.
Precise statement
For even-layer $\mathrm{MnBi_2Te_4}$ in its antiferromagnetic ground state, determine whether $\theta = \pi$, with half-quantized surface Hall conductance $\pm e^{2}/(2h)$ on the top and bottom surfaces, or whether the state is trivial ($\theta = 0$). The zero Hall plateau alone does not discriminate. An answer is an identification from a surface-resolved or theta-sensitive measurement.
What would settle it
Layer-resolved Kerr or Faraday rotation, nonlocal transport, or a magnetoelectric measurement isolating the $\theta = \pi$ response.
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