{"record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"2aaf4d4d40dc40217d12008746cf5b3d190f7bd95d3e97ae23c3565e07d03113","created":"2026-10-15T23:43:22Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["ce59fb6cd812fc05906d0c60a71c75ea22bc686bcf38e7c3ef7010834d98ee7b","f003b33b273a1bcbd7188f048d36eb26077ee9f359fde30ad8127c19c74a7fe7"],"salt":"d2361645e2ccca74e025677d58a6f764e6d9af7fd35e92a91eca247e70842216","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"mechanism","assisted_by":[],"external_id":"cm.magnetic-topological-insulators.mnbi2te4-surface-gap","kind":"phenomenon","literature_status":"contested","n":"2","parents":[],"plain":"Magnetism should open an energy gap in the surface electrons of $\\mathrm{MnBi_2Te_4}$, which is what allows zero-field quantized Hall effects, but several photoemission experiments see no gap.","posed_since":"2019","precise":"Antiferromagnetic $\\mathrm{MnBi_2Te_4}$, whose outermost septuple layer is ferromagnetic, should have an exchange gap of tens of meV in its surface Dirac cone, yet ARPES (Chen et al., PRX 9, 041040, 2019; Nevola et al., PRL 125, 117205, 2020) found a gapless or nearly gapless surface cone, while other studies report gaps. Determine the origin: surface magnetic reconstruction (canted or disordered top layer), Mn-Bi antisite defects, or relocation of the surface state away from the magnetic top layer. An answer links the measured surface magnetic structure to the measured gap on the same samples.","problem_ref":"f003b33b273a1bcbd7188f048d36eb26077ee9f359fde30ad8127c19c74a7fe7","references":"","settled_by":"Spin-polarized STM or surface-sensitive X-ray magnetic circular dichroism of the top layer combined with ARPES gap measurement on identical, defect-characterized surfaces.","status_note":"Gapless and gapped ARPES reports coexist as of 2026, to this survey's knowledge.","title":"Why the $\\mathrm{MnBi_2Te_4}$ surface Dirac cone appears gapless","topic_ref":null},"attested":{"attestation":{"batch":null,"client_id":null,"id_token_sha256":null,"kind":"platform"},"record_hash":"e346d847ff6bd1cf2bcc7091ab933ab62753507911c5e53520ca449ba861a392","schema":"pubphys.attested/1"},"envelope":{"attested_hash":"b92567ebf3f5536f2574380cf647ecf356d672150574647e44dd1fd1e9e0fa77","platform_signature":{"key_id":"c6afc19b31429869751f06879c75cd64ea92654423d15b44be775bf1310a60da","sig":"zlr7oAhEYYw5VCUhhjodHQAQgvjm_HEVHyxpfm1Jk4u2gwVzZbx65J44kYHf-xTeSqhBHvdR0kVBj_eCrZbJBw"},"schema":"pubphys.envelope/1"},"record_hash":"e346d847ff6bd1cf2bcc7091ab933ab62753507911c5e53520ca449ba861a392","leaf_index":2920}