Irremovable Mn-Bi Site Mixing in MnBi2Te4

MnBi₂Te₄, an antiferromagnetic topological insulator, was theoretically predicted to have a gapped surface state on its (111) surface. However, a much smaller gapped or even gapless surface state has been observed experimentally, which is thought to be caused by the defects in MnBi₂Te₄. Here, we have theoretically identified the antisite Mn_Bi and Bi_Mn as dominant defects and revealed their evolution during the phase transition from MnTe/Bi₂Te₃ to MnBi₂Te₄. We found that the complete elimination of Mn_Bi and Bi_Mn defects in MnBi₂Te₄ by simple annealing is almost impossible due to the high migration barrier in kinetics. Moreover, the gap of the Dirac point-related bands in a MnBi₂Te₄ monolayer would be eliminated with an increasing concentration of Mn_Bi and Bi_Mn defects, which could explain the experimentally unobserved large-gap surface state in MnBi₂Te₄. Our results provide an insight into the theoretical understanding of the quality and the experimentally measured topological properties of the synthesized MnBi₂Te₄.