{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"8386a44f1fbdd46396a7f12f61c64eeaa4a162fb7e38bb2410cd41d65095136d","created":"2026-10-15T23:43:22Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["c937df08022b433413d0538c4b3e4094b5e3e9cb171a8a5f61f508de476e6213","ce59fb6cd812fc05906d0c60a71c75ea22bc686bcf38e7c3ef7010834d98ee7b"],"salt":"0380285b462d181de5df7fc0a4b35a7d968f59f7b83aa52efeb726fe07ed8919","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"identification","assisted_by":[],"external_id":"cm.magnetic-topological-insulators.axion-state-mnbi2te4","kind":"well-posed","literature_status":"contested","n":"2","parents":[],"plain":"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.","posed_since":"2019","precise":"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.","problem_ref":"c937df08022b433413d0538c4b3e4094b5e3e9cb171a8a5f61f508de476e6213","references":"","settled_by":"Layer-resolved Kerr or Faraday rotation, nonlocal transport, or a magnetoelectric measurement isolating the $\\theta = \\pi$ response.","status_note":"","title":"Is even-layer $\\mathrm{MnBi_2Te_4}$ an axion 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