Native point defects and their implications for the Dirac point gap at MnBi$_2$Te$_4$(0001)

Abstract

The Dirac point gap at the surface of the antiferromagnetic topological insulator MnBi2Te4 is a highly debated issue. While the early photoemission measurements reported on large gaps in agreement with theoretical predictions, other experiments found vanishingly small splitting of the MnBi2Te4 Dirac cone. Here, we study the crystalline and electronic structure of MnBi2Te4(0001) using scanning tunneling microscopy/spectroscopy (STM/S), micro(µ)-laser angle resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) calculations. Our topographic STM images clearly reveal features corresponding to point defects in the surface Te and subsurface Bi layers that we identify with the aid of STM simulations as BiTe antisites (Bi atoms at the Te sites) and MnBi substitutions (Mn atoms at the Bi sites), respectively. X-ray diffraction (XRD) experiments further evidence the presence of cation (Mn-Bi) intermixing. Altogether, this affects the distribution of the Mn atoms, which, inevitably, leads to a deviation of the MnBi2Te4 magnetic structure from that predicted for the ideal crystal structure. Our transport measurements suggest that the degree of this deviation varies from sample to sample. Consistently, the ARPES/STS experiments reveal that the Dirac point gap of the topological surface state is different for different samples/sample cleavages. Our DFT surface electronic structure calculations show that, due to the predominant localization of the topological surface state near the Bi layers, MnBi defects can cause a strong reduction of the MnBi2Te4 Dirac point gap, given the recently proved antiparallel alignment of the MnBi moments with respect to those of the Mn layer. Our results provide a key to puzzle out the MnBi2Te4 Dirac point gap mystery.

Vapor transport growth of MnBi2Te4 and related compounds

“…The nonstoichiometry of Te and the site mixing between Bi and Te should also be considered depending on the growth conditions. These are confirmed by the experimental observation of lattice defects via single crystal x-ray and neutron diffraction, scanning tunneling microscopy(STM), scanning transmission electron microscopy(STEM) [10][11][12][13][14][15][16][17][18][19][20]. We noticed in our flux-grown MnBi 2 Te 4 crystals, while the concentration of Mn Bi (Mn at Bi site) is in the range of 2-4% in crystals from different batches, the concentration of Bi M n (Bi at Mn site) can vary in a wide range 4-15% which is rather sensitive to growth parameters.…”

“…These lattice defects induce electronic inhomogeneities, a complex ferrimagnetism in each septuple layer, and can affect and even change the sign of inter-septuple-layer magnetic interactions. [15][16][17]21] Recently, lattice disorder in MnBi 2 Te 4 was proposed to account for the experimentally observed high Chern number state in MnBi 2 Te 4 flakes [22] and affect the gap size of the surface states [18][19][20]. Careful investigations on MnTe.mBi 2 Te 3 with m >1 found the above mentioned lattice defects in both the quintuple and septuple layers, which lead to diverse magnetic ground states [6,23].…”

The elusive quantum anomalous Hall effect in MnBi2Te4: a materials perspective

“…Interestingly, quantum phenomena were observed despite those lattice defects and resulting electronic inhomogeneities. Recently, lattice disorder in MnBi 2 Te 4 was proposed to induce the quantum Hall edge state coexisting with the QAH edge state in magnetic fields [14] and to modulate the gap size of the surface states [15][16][17].…”