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Title: Why the MnBi2Te4 surface Dirac cone appears gapless
Plain: Magnetism should open an energy gap in the surface electrons of MnBi2Te4, which is what allows zero-field quantized Hall effects, but several photoemission experiments see no gap.
Precise: Antiferromagnetic MnBi2Te4, 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.
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.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: contested
Status note: Gapless and gapped ARPES reports coexist as of 2026, to this survey's knowledge.
Posed since: 2019