{"record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"f8c244715ba34232452684273e7ccb8e483d4c17824e2b2ac819023fb0893750","created":"2026-10-15T23:43:22Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["a6567dbaa2dcfb8b91065890ac46a5cc5c4cf8dc1cc8a616fb538366fd5c942b","ce59fb6cd812fc05906d0c60a71c75ea22bc686bcf38e7c3ef7010834d98ee7b"],"salt":"453482746d22c9247bf2601f89e4f3d8d44f779d744c171f4e4d8b2d7b04d9b4","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":"2","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 $\\mathrm{MnBi_2Te_4}$), 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":"a6567dbaa2dcfb8b91065890ac46a5cc5c4cf8dc1cc8a616fb538366fd5c942b","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 effect","topic_ref":null},"attested":{"attestation":{"batch":null,"client_id":null,"id_token_sha256":null,"kind":"platform"},"record_hash":"3b7441227b8f5eac1049d0413941f96936ce27e9558fe2636db2f0e2c7d677e0","schema":"pubphys.attested/1"},"envelope":{"attested_hash":"7e8eef6baa387a7ad8bcdf28f49d0ffbb805b4e0f1c7e20526f7ed2621473aaf","platform_signature":{"key_id":"c6afc19b31429869751f06879c75cd64ea92654423d15b44be775bf1310a60da","sig":"zrY2pQyIQ-ASE_JLfdG7zBvjSGYi-k8r9sJlerVeE5rrSz2JQJisVtQNbIBqBOXGDcYfRhYTNCVGQO1usKPGCQ"},"schema":"pubphys.envelope/1"},"record_hash":"3b7441227b8f5eac1049d0413941f96936ce27e9558fe2636db2f0e2c7d677e0","leaf_index":2924}