Real Chern Insulator in Monolayer Decorated Transition Metal Nitrides

A trend in recent research is to push the study of topological phases to the spin-splitting antiferromagnetic (SSAFM) systems, which has become a research forefront of spintronics. Here, the realization of SSAFM real Chern insulator (RCI) in the monolayer transition metal nitride A(BN)₂ (A = Mg, Ca, Zn and B = Co) is revealed. Taking monolayer Mg(CoN)₂ (ML-MgCoN) as an example. In the absence of spin-orbit coupling, the ML-MgCoN is an altermagnetic (AM) material, and each spin channel of the material possesses a nontrivial real Chern number protected by $C_{2z}\mathcal{T}$ symmetry. Unlike conventional RCIs, the multi-atomic-layer structure and altermagnetism of ML-MgCoN lead to the emergence of two sets of corner states exhibiting opposite layer and spin polarizations. Under out-of-plane electric field (E_z) and uniaxial strain, the ML-MgCoN is no longer an AM material but still a SSAFM material with $C_{2z}\mathcal{T}$ symmetry in each spin channel. Moreover, the degeneracy of the two sets of topological corner states is lifted, suggesting a selective control of layer and spin polarizations under external fields. This work not only significantly broadens the material database of AM RCI, but also reveals a new mechanism to control the spin in spintronics.