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 MnBi 2 Te 4 , 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 ± 2 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.

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Publication Details

Journal
Nature Communications
Published
2026-09-15
DOI
https://doi.org/10.1038/s41467-026-77571-4
Primary Topic
Topological Materials and Phenomena
Type
article
Field-Weighted Citation Impact
0.00

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article

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

Kaishu Kawaguchi, Hiroki Tanaka, Koichiro Yaji, Y. Fukushima et al.
Nature Communications
Topological Materials and Phenomena
article

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

Kaishu Kawaguchi, Hiroki Tanaka, Koichiro Yaji, Y. Fukushima, Ayumi Harasawa, Hiroyuki Suzuki, Toshiya Ikenobe, Kiyohisa Tanaka, Masayuki Ochi, Takeshi Kondo, Masashi Tokunaga, Takahiro Morimoto, Hiraku Saito, Yuto Kinoshita, Taro Nakajima, Ryo Mori, Takuma Nishida, Takushi Iimori
article en

Abstract

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 MnBi 2 Te 4 , 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 ± 2 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.

Nature Communications
High Energy Accelerator Research Organization (JP), Osaka University of Economics (JP), The Graduate University for Advanced Studies, SOKENDAI (JP), Tohoku University (JP), National Institute for Materials Science (JP), Kyoto University (JP), Osaka College of Music (JP), Osaka Prefectural Toyonaka Support School (JP), RIKEN Center for Emergent Matter Science (JP), Institute for Molecular Science (JP), University of Maryland, College Park (US), The University of Tokyo (JP)
Asahi Glass Foundation, Ministry of Education, Culture, Sports, Science and Technology, Mitsubishi Foundation, University of Tokyo, Murata Science Foundation, Toray Science Foundation, Japan Society for the Promotion of Science, Precursory Research for Embryonic Science and Technology
Openalex Percentile: Top 13%
Topological Materials and Phenomena
DOIOpen Access PDFOpenAlex
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