Direct observation of a spin-polarized surface Dirac gap in the antiferromagnetic topological insulator NdBi

Abstract

A gapped Dirac cone in antiferromagnetic topological insulators (AFM-TIs) is essential for realizing the quantized anomalous Hall effect, yet its intrinsic magnitude remains controversial. In the prototypical AFM-TI MnBi2Te4, however, the observed Dirac gap is significantly smaller than theoretical predictions or nearly absent, highlighting a long-standing discrepancy. Here we demonstrate that NdBi hosts a spin-polarized topological surface Dirac cone with a sizable gap of 17 meV at the Dirac point, in excellent agreement with density functional theory predictions (12 meV). This direct observation is enabled by high-resolution 7 eV laser-based spin- and angle-resolved photoemission spectroscopy, which unambiguously disentangles the spin-polarized surface Dirac cone from bulk-derived states. Furthermore, through temperature dependence and controlled surface contamination, we establish a direct linkage between surface magnetism and the Dirac gap, demonstrating that the gap opens only in the presence of surface magnetic order. These results provide definitive spectroscopic evidence for an intrinsic Dirac gap in AFM-TIs and establish NdBi as a model platform to resolve the long-standing discrepancy between experiment and theory in AFM-TIs.

Versions

10.1038/s41467-026-77571-4Published version10.1038/s41467-026-77571-4

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BibTeX RIS

BibTeX

@misc{fukushima2026direct,
  title = {{Direct observation of a spin-polarized surface Dirac gap in the antiferromagnetic topological insulator NdBi}},
  author = {Yuto Fukushima and Hiroyuki S. Suzuki and Masayuki Ochi and Takuma Nishida and Toshiya Ikenobe and Hiraku Saito and Yuto Kinoshita and Kaishu Kawaguchi and Hiroaki Tanaka and Ayumi Harasawa and Takushi Iimori and Koichiro Yaji and Masashi Tokunaga and Taro Nakajima and Kiyohisa Tanaka and Takahiro Morimoto and Ryo Mori and Takeshi Kondo},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2610.35353},
  eprint = {2610.35353},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2610.35353},
  note = {CC BY 4.0},
}

RIS

TY  - UNPB
TI  - Direct observation of a spin-polarized surface Dirac gap in the antiferromagnetic topological insulator NdBi
AU  - Yuto Fukushima
AU  - Hiroyuki S. Suzuki
AU  - Masayuki Ochi
AU  - Takuma Nishida
AU  - Toshiya Ikenobe
AU  - Hiraku Saito
AU  - Yuto Kinoshita
AU  - Kaishu Kawaguchi
AU  - Hiroaki Tanaka
AU  - Ayumi Harasawa
AU  - Takushi Iimori
AU  - Koichiro Yaji
AU  - Masashi Tokunaga
AU  - Taro Nakajima
AU  - Kiyohisa Tanaka
AU  - Takahiro Morimoto
AU  - Ryo Mori
AU  - Takeshi Kondo
PY  - 2026
DA  - 2026/10/26
PB  - arXiv
DO  - 10.48550/arxiv.2610.35353
UR  - https://arxiv.org/abs/2610.35353
AB  - A gapped Dirac cone in antiferromagnetic topological insulators (AFM-TIs) is essential for realizing the quantized anomalous Hall effect, yet its intrinsic magnitude remains controversial. In the prototypical AFM-TI MnBi2Te4, however, the observed Dirac gap is significantly smaller than theoretical predictions or nearly absent, highlighting a long-standing discrepancy. Here we demonstrate that NdBi hosts a spin-polarized topological surface Dirac cone with a sizable gap of 17 meV at the Dirac point, in excellent agreement with density functional theory predictions (12 meV). This direct observation is enabled by high-resolution 7 eV laser-based spin- and angle-resolved photoemission spectroscopy, which unambiguously disentangles the spin-polarized surface Dirac cone from bulk-derived states. Furthermore, through temperature dependence and controlled surface contamination, we establish a direct linkage between surface magnetism and the Dirac gap, demonstrating that the gap opens only in the presence of surface magnetic order. These results provide definitive spectroscopic evidence for an intrinsic Dirac gap in AFM-TIs and establish NdBi as a model platform to resolve the long-standing discrepancy between experiment and theory in AFM-TIs.
ER  -

Abstract

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