Computational study 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. So far, 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 (MBT) to be the first antiferromagnetic TI (AFMTI) [1,2]. The interlayer AFM ordering makes MBT invariant with respect to the combination of the time-reversal (Θ) and primitive-lattice translation (T_1/2) symmetries, S=ΘT_1/2, which gives rise to the Z₂ classification of AFM insulators. We find Z₂=1 for MnBi₂Te₄, which confirms its topologically nontrivial nature. To date, many experimental groups confirmed the AFMTI state in MBT, with the first observation reported in Ref. [1].
In the 2D limit, MBT is expected to show a unique set of thickness-dependent magnetic and topological transitions, which drive it through FM and (un)compensated AFM phases, as well as quantum anomalous Hall (QAH) and zero plateau QAH states [2]. The latter was predicted to host the axion insulator phase.
The discovery of the first AFMTI opens a new field that focuses on intrinsically magnetic stoichiometric compounds: several MBT-derived MTI-candidates were experimentally synthesized right away, that will be discussed in the talk along with other AFMTI candidates predicted ab initio.

[1] M. Otrokov et al., arXiv:1809.07389.
[2] M. Otrokov et al., Phys. Rev. Lett. 122, 107202 (2019).