Why the $\mathrm{MnBi_2Te_4}$ surface Dirac cone appears gapless
In plain words
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.
Precise statement
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.
What would settle it
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 in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
Gapless and gapped ARPES reports coexist as of 2026, to this survey's knowledge.
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