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Title: Is even-layer MnBi2Te4 an axion insulator
Plain: Thin MnBi2Te4 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: For even-layer MnBi2Te4 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.
Settled by: Layer-resolved Kerr or Faraday rotation, nonlocal transport, or a magnetoelectric measurement isolating the $\theta = \pi$ response.
Kind: well-posed
Answer type: identification
References: (empty)
Literature status: contested
Status note: (empty)
Posed since: 2019