Dirac surface states in intrinsic magnetic topological insulators EuSn2As2 and MnBi2nTe3n+1

Abstract: In magnetic topological insulators (TIs), the interplay between magnetic order and nontrivial topology can induce fascinating topological quantum phenomena, such as the quantum anomalous Hall effect, chiral Majorana fermions and axion electrodynamics. Recently, a great deal of attention has been focused on the intrinsic magnetic TIs, where disorder effects can be eliminated to a large extent, which is expected to facilitate the emergence of topological quantum phenomena. In despite of intensive efforts, experimental evidence of the topological surface states (SSs) remains elusive. Here, by combining first-principles calculations and angle-resolved photoemission spectroscopy (ARPES) experiments, we have revealed that EuSn2As2 is an antiferromagnetic TI with observation of Dirac SSs consistent with our prediction. We also observe nearly gapless Dirac SSs in antiferromagnetic TIs MnBi2nTe3n+1 (n = 1 and 2), which were absent in previous ARPES results. These results provide clear evidence for nontrivial topology of these intrinsic magnetic TIs. Furthermore, we find that the topological SSs show no observable changes across the magnetic transition within the experimental resolution, indicating that the magnetic order has quite small effect on the topological SSs, which can be attributed to weak hybridization between the localized magnetic moments, from either 4f or 3d orbitals, and the topological electronic states. This provides insights for further research that the correlations between magnetism and topological states need to be strengthened to induce larger gaps in the topological SSs, which will facilitate the realization of topological quantum phenomena at higher temperatures.

Dirac Surface States in Intrinsic Magnetic Topological Insulators

The paper Dirac surface states in intrinsic magnetic topological insulators reports the observation and theoretical understanding of Dirac surface states (SSs) in intrinsic magnetic topological insulators (TIs) such as EuSn$_2$As$_2$ and MnBi$_{2n}$Te$_{3n+1}$ (where $n = 1, 2$). This work crucially combines angle-resolved photoemission spectroscopy (ARPES) experiments with first-principles calculations to elucidate the complex interplay between magnetism and nontrivial topological states in these materials.

Key Findings

• Antiferromagnetic EuSn$_2$As$_2$ Characterization: The research establishes EuSn$_2$As$_2$ as an antiferromagnetic TI exhibiting Dirac SSs. Using ARPES, nearly gapless Dirac SSs are observed, providing substantial evidence for its topological nature.
• Near-Gapless Dirac SSs in MnBi TIs: In MnBi$_{2n}$Te$_{3n+1}$, nearly gapless Dirac SSs are detected, contradicting previous ARPES studies which suggested a significant energy gap in these states, thus revising the earlier understanding of the electronic structure in these compounds.
• Influence of Magnetic Transition: The paper demonstrates that in these intrinsic magnetic TIs, Dirac SSs exhibit negligible sensitivity to magnetic transitions within the experimental resolution. This indicates a weak hybridization between localized magnetic moments from 4f or 3d orbitals and the topological electronic states.

Theoretical and Practical Implications

The outcomes offer insightful contributions to the understanding of intrinsic magnetic TIs. The weak coupling between magnetic order and topological states implies that further modifications are necessary to enhance hybridization. Strengthening these interactions could lead to the realization of more substantial energy gaps in the topological SSs, which is crucial for the manifestation of topological quantum phenomena, such as the quantum anomalous Hall effect, at higher temperatures.

The theoretical characterization of EuSn$_2$As$_2$, which transitions from a strong topological insulator (TI) in the paramagnetic phase to an axion insulator in the antiferromagnetic phase, reinforces the correlation between magnetic order and topological properties. The study emphasizes the critical role of magnetic topological coupling and the need to control the electronic states contributing to these phenomena.

Future Directions

This work suggests several avenues for future research. Efforts should be concentrated on identifying intrinsic magnetic TIs with stronger coupling between magnetic and topological states. This could unlock potential for achieving topologically protected states at technologically relevant temperatures. Moreover, the discrepancies between theoretical predictions and experimental observations call for refined theoretical models, potentially incorporating advanced computation techniques to accurately capture the subtle interactions within these systems.

Overall, this study advances the understanding of intrinsic antiferromagnetic TIs and highlights critical steps toward harnessing topological quantum effects, potentially paving the way for novel applications in quantum computing and spintronics.