Axion insulator in HgTe superlattices

Continuing our study on HgTe heterostructures in the search for novel topological phases, we included a magnetic component into our superlattice setup. In particular, we studied short-period HgTe/MnTe, where MnTe is an antiferromagnetic insulator in the bulk, by means of density-functional theory. Our results show the evolution of the magnetic topological phases with respect to the different possible magnetic configurations in the MnTe layers. 

Most notably, we found the elusive axion insulator phase for out-of-plane antiferromagnetic order below a critical MnTe thickness. Such a phase gives rise to exotic electromagnetic properties typically dubbed axion electrodynamics. Switching the magnetic orientation into the plane, the superlattice realizes different antiferromagnetic topological insulators depending on the thickness of the MnTe layers. These phases feature gapless surface Dirac cones shifted away from high-symmetry points, which is in contrast to non-magnetic topological insulators. For ferromagnetic order, the system realizes a ferromagnetic Weyl semimetal. Interestingly, we also observed a large anomalous Hall conductivity in this case indicating the presence of large Berry curvature. 

All in all, we identified a novel promising material platform for the study of magnetic topological phases. 

R. Islam, S. Mardanya, A. Lau, G. Cuono, T.-R. Chang, B. Singh, C. M. Canali, T. Dietl, and C. Autieri,
Engineering axion insulator and other topological phases in superlattices with inversion symmetry breaking,
Physical Review B 107, 125102 (2023), arXiv:2211.05152.