Irremovable Mn-Bi Site Mixing in MnBi 2 Te 4 .

MnBi 2 Te 4 , 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 MnBi 2 Te 4 . 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/Bi 2 Te 3 to MnBi 2 Te 4 . We found that the complete elimination of Mn Bi and Bi Mn defects in MnBi 2 Te 4 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 MnBi 2 Te 4 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 MnBi 2 Te 4 . Our results provide an insight into the theoretical understanding of the quality and the experimentally measured topological properties of the synthesized MnBi 2 Te 4 .