First-principles study of bulk and two-dimensional structures of the AMnBi family of materials (A=K,Rb,Cs)

Magnetic materials with high mobilities are intriguing subjects of research from both fundamental and application perspectives. Based on first-principle calculations, we investigate the physical properties of the already synthesized $A\mathrm{MnBi}$ ($A=\mathrm{K}$, Rb, Cs) family materials. We show that these materials are antiferromagnetic (AFM), with N\'eel temperatures above 300 K. They contain AFM ordered Mn layers, while the interlayer coupling changes from ferromagnetic (FM) for KMnBi to AFM for RbMnBi and CsMnBi. We find that these materials are narrow gap semiconductors. Owing to the small effective mass, the electron carrier mobility can be very high, reaching up to ${10}⁵\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}²/(\mathrm{V}\phantom{\rule{0.16em}{0ex}}\mathrm{s}$) for KMnBi. In contrast, the hole mobility is much suppressed, typically lower by two orders of magnitude. We further study their two-dimensional (2D) single layer structures, which are found to be AFM with fairly high mobility $\ensuremath{\sim}{10}³\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}²/(\mathrm{V}\phantom{\rule{0.16em}{0ex}}\mathrm{s})$. Their N\'eel temperatures can still reach room temperature. Interesting, we find that the magnetic phase transition is also accompanied by a metal-insulator phase transition, with the paramagnetic metal phase possessing a pair of nonsymmorphic-symmetry-protected 2D spin-orbit Dirac points. Furthermore, the magnetism can be effectively controlled by the applied strain. When the magnetic ordering is turned into FM, the system can become a quantum anomalous Hall insulator with gapless chiral edge states.