Native point defects and their implications for the Dirac point gap at MnBi 2 Te 4 (0001)

We study the surface crystalline and electronic structures of the antiferromagnetic topological insulator MnBi 2 Te 4 using scanning tunneling microscopy/spectroscopy (STM/S), micro( μ )-laser angle-resolved photoemission spectroscopy (ARPES), and density functional theory calculations. Our STM images reveal native point defects at the surface that we identify as Bi Te antisites and Mn Bi substitutions. Bulk X-ray diffraction further evidences the presence of the Mn-Bi intermixing. Overall, our characterizations suggest that the defects concentration is nonuniform within crystals and differs from sample to sample. Consistently, the ARPES and STS experiments reveal that the Dirac point gap of the topological surface state is different for different samples and sample cleavages, respectively. Our calculations show that the antiparallel alignment of the Mn Bi moments with respect to those of the Mn layer can indeed cause a strong reduction of the Dirac point gap size. The present study provides important insights into a highly debated issue of the MnBi 2 Te 4 Dirac point gap.