Metastability and topology in the magnetic topological insulator MnBi₂Te₄

MnBi₂Te₄ (MBT) has generated great interest as a first possible realization of an intrinsic topological insulator with A-type antiferromagnetic order. However, consistent observation of the surface gap formation with synthesized MBT samples has remained a challenge. Motivated by the observation of stacking faults, we have explored the possible existence of metastable local stacking modes at the MBT surface and their influence on the topological surface states. To this end, we used PBE+U calculations where Hubbard U was tuned with diffusion Monte Carlo techniques. We find that spin-orbit and van der Waals interactions are intertwined, which stabilizes the experimentally observed interlayer distance, and stacking. This stabilization can be understood in terms of interlayer hybridization among p_z orbitals of Bi and Te. We further demonstrate that deviation from this optimal interlayer distance leads to a topological phase transition from a non-trivial to a trivial topology. The topological properties of a metastable stacking mode at the MBT surface, which directly induces a larger interlayer separation, will also be discussed.