Native point defects and their implications for the Dirac point gap at MnBi2Te4(0001)

Abstract

We study the surface crystalline and electronic structures of the antiferromagnetic topological insulator MnBi2Te4 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 BiTe antisites and MnBi 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 MnBi 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 MnBi2Te4 Dirac point gap.

Factors influencing the energy gap in topological states of antiferromagnetic MnBi$_2$Te$_4$

“…The change in the effective magnetic moment can be developed, for instance, due to the substitution defects like Mn/Bi or Mn/Te [46], when the magnetic Mn atoms are replaced by nonmagnetic ones. On the other hand, Mn atoms on the Te atomic sites have opposite magnetic moment orientation that also leads to a decrease of total exchange field acting on TSSs.…”

“…The resulting shift of the TSSs localization towards the second lower-lying SL, which is characterized by the opposite orientation of the Mn magnetic moments, can lead to compensation of the effective out-of-plane magnetic moment in the TSSs localization region and the corresponding shrinking of the Dirac gap. Similarly, a change in the Dirac gap size can occur due to the presence and accumulation of various surface defects of different concentrations [36,39,46], including possible adsorption of residual gas molecules, which can cause the changes in the TSSs localization resulting in modulation of the energy gap at the DP [36,39].…”

“…The calculation results will be analyzed in comparison with the experimental dispersions measured by ARPES. The experimental basis for such variation of calculation parameters can be: (i) surface relaxation processes leading to a change in the SvdW interval [38], (ii) the formation of Mn/Bi type substitution defects, which reduce the exchange field in the region of localization of the TSSs [46], and (iii) the formation of various types of defects in the near-surface region that may change the surface potential gradient, which can be taken into account by artificial variation of the SOC strength for the surface atoms [36,39]. At the same time, such a change in the parameters of the theoretical model can also include the situation of changing the composition of the sample surface by replacing some of the atoms in the near-surface layer with atoms possessing a lower atomic SOC, which may be important in the formation of synthetic layered topological structures.…”

Delicate Ferromagnetism in MnBi6Te10

“…In contrast, the TSS electrons on the 1-BT termination are localized to the top BT layer and spatially separate from the magnetic MBT layer, leading to a gapless Dirac point. Gap opening in the MnBi 2 n Te 3 n +1 compound family has been highly controversial. ,,,,,,,,,,,− Specifically on MnBi 6 Te 10 , a 60 meV gap was reported in the AFM phase in contrast to the results from all other ARPES studies on MnBi 6 Te 10 ,,, including ours. However, this gap persists well above the magnetic ordering temperature and is likely due to extrinsic reasons such as local impurities.…”

Momentum-inversion symmetry breaking on the Fermi surface of magnetic topological insulators

“…To address the discrepancy between experiment and theory, several candidate mechanisms based on either surface magnetic reconstruction, surface structural relaxation, or surface-bulk band hybridization have been proposed [15,18,20,22,[28][29][30][31][32][33][34], among which the surface magnetic reconstruction is most extensively discussed. The ground state of bulk materials has the A-type anti-ferromagnetic configuration [intralayer ferromagnetic (FM) and interlayer anti-ferromagnetic (AFM)] for n = 0, 1, 2 and FM configuration for n ≥ 3 due to the intensely weakening of the interlayer AFM supersuperexchange coupling [35][36][37].…”