Theoretical prediction and discovery of antiferromagnetic topological insulators

Magnetic topological insulators (MTIs) are narrow gap semiconductor materials that combine non-trivial band topology and magnetic order. Unlike their nonmagnetic counterparts, MTIs may have some of the surfaces gapped due to breaking the time-reversal symmetry, which enables exotic phenomena having potential applications in spintronics. Previously, MTIs were only created by means of doping nonmagnetic TIs with 3d transition metal atoms, however such an approach leads to strongly inhomogeneous magnetic and electronic properties of these materials, restricting the observation of important effects to very low temperatures. Finding intrinsic MTI, i.e. a stoichiometric well-ordered compound, could be an ideal solution to these problems. Using ab initio calculations, we predicted the van der Waals layered compound MnBi₂Te₄ to be the first antiferromagnetic TI (AFMTI) [1]. To date, many experimental groups confirmed the AFMTI state in MnBi₂Te₄, with the first observation reported in [1]. In the 2D limit, we predicted MnBi₂Te₄ to show a unique set of thickness- and field-dependent magnetic and topological transitions, which drive it through the intrinsic quantum anomalous Hall state, its zero plateau (axion insulator), and the Chern insulator state, achieved under external magnetic field, but without Landau levels [2]. These predictions have been confirmed in recent experiments [3,4]. The discovery of the first AFMTI MnBi₂Te₄ opens a new field that focuses on intrinsically magnetic stoichiometric compounds: several MnBi₂Te₄-derived MTIs were synthesized right away [5], that will be discussed in the talk along with other AFMTI candidates predicted.

[1] M. Otrokov et al. Nature 576, 416 (2019), arXiv:1809.07389
[2] M. Otrokov et al. Phys. Rev. Lett. 122, 107202 (2019), arXiv:1810.05289
[3] Y. Deng et al. Science 367, 895 (2020)
[4] C. Liu et al. Nature Mater. 19, 522 (2020)
[5] I. Klimovskikh, M. Otrokov et al. npj Quantum Mater. 5, 54 (2020)