{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"ad2ca6b5a7812addb6b1594ccd58be36a4a8b5bbd4376faad9251cb58426b18d","created":"2026-10-03T07:18:00Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"eb57f466699c987ffef942df92db780519ac97bf630b34c83fa5b84262b09efc","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"value","assisted_by":[],"external_id":"cm.magnetic-topological-insulators.quantized-magnetoelectric","kind":"well-posed","literature_status":"partially-resolved","n":"1","parents":[],"plain":"An axion insulator should become electrically polarized in a magnetic field by an amount fixed only by the fine-structure constant, and this static effect has never been measured directly.","posed_since":"2008","precise":"For an axion insulator with $\\theta = \\pi$ (magnetic topological insulator sandwich or even-layer MnBi2Te4), measure the static (dc) magnetoelectric response $P = (\\theta \\alpha / 4\\pi^2) B$ in Gaussian units with $\\alpha = e^2/(\\hbar c)$, i.e., $\\alpha/(4\\pi)$ per unit field at $\\theta = \\pi$, or the equivalent magnetization induced by an electric field. An answer is the measured coefficient in units of $\\alpha/(4\\pi)$ with accuracy better than 10%.","problem_ref":null,"references":"","settled_by":"Capacitive or SQUID detection of field-induced polarization, or electric-field-induced magnetization, in an insulating sample free of dissipative edge or side-wall conduction.","status_note":"Quantized Faraday and Kerr rotations, a terahertz-frequency form of the effect in an applied field, were reported in topological insulator films in 2016-2017; a static (dc) measurement of $P = (\\alpha/4\\pi) B$ in an axion insulator is lacking as of 2026.","title":"Direct measurement of the quantized topological magnetoelectric 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