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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":null,"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 MnBi2Te4 surface Dirac cone appears 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