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

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.

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

Leverage3 / 5

A method or resource many groups across the field will adopt within a year.

18% of this desk’s rubric

Magnitude2 / 5

Solid incremental gain on a meaningful problem.

20% of this desk’s rubric

Evidence3 / 5

Solid: multiple benchmarks or cohorts, ablations, fair baselines, released code or data.

22% of this desk’s rubric

Novelty3 / 5

A genuinely new approach to an open problem.

22% of this desk’s rubric

Trajectory2 / 5

Some room to improve with obvious engineering.

10% of this desk’s rubric

Stakes2 / 5

Benefits a professional community (practitioners, clinicians, engineers).

8% of this desk’s rubric

• Citations0 (reference 20, via openalex, Oct 27, 2026, 07:29 UTC)

• Reviewed by heuristic-v10 on Oct 27, 2026, 07:29 UTC. Paper type: method.
• Categories: cond-mat.mtrl-sci, cond-mat.str-el
• TOP, No.4 in the Physics edition of October 27, 2026.