Visualizing the interplay of Dirac mass gap and magnetism at nanoscale in intrinsic magnetic topological insulators

Abstract

In intrinsic magnetic topological insulators, Dirac surface-state gaps are prerequisites for quantum anomalous Hall and axion insulating states. Unambiguous experimental identification of these gaps has proved to be a challenge, however. Here, we use molecular beam epitaxy to grow intrinsic MnBi 2 Te 4 thin films. Using scanning tunneling microscopy/spectroscopy, we directly visualize the Dirac mass gap and its disappearance below and above the magnetic order temperature. We further reveal the interplay of Dirac mass gaps and local magnetic defects. We find that, in high defect regions, the Dirac mass gap collapses. Ab initio and coupled Dirac cone model calculations provide insight into the microscopic origin of the correlation between defect density and spatial gap variations. This work provides unambiguous identification of the Dirac mass gap in MnBi 2 Te 4 and, by revealing the microscopic origin of its gap variation, establishes a material design principle for realizing exotic states in intrinsic magnetic topological insulators.

Role of Magnetic Defects in Tuning Ground States of Magnetic Topological Insulators

“…[ 22 ] The exchange coupling between antisite defects and surface electronic bands has been shown to be a pivotal factor that controls the surface magnetism and consequently the Chern gap. [ 20,23–26 ] These studies reveal the important role of Mn defects in MBT and MST, and the prospect that defect‐engineering can control and tailor the global magnetic ground state that affects topological surface states. This accentuates the importance of unraveling the physics of dilute magnetic TIs, such as (Bi 1− x Mn x ) 2 Te 3 or (Sb 1− x Mn x ) 2 Te 3 , where, in a similar fashion, Mn randomly occupies the Bi/Sb sites.…”

Towards comprehension of the surface state properties in the intrinsic magnetic topological insulators

“…The scanning tunneling microscopy and spectroscopy revealed that MnBi 2 Te 4 usually contains a few percent of Mn Bi and Bi Mn antisite defects, while the average density of Bi Te defects is several tenths of a percent [30,[43][44][45], although Ref. [46] exhibits that, in thin film MnBi 2 Te 4 , the defective areas with high local concentrations of Mn Bi (∼9%) are adjacent to defect-free areas. It should also be borne in mind that the as-grown bulk MnBi 2 Te 4 samples are usually heavily degenerate n-type semiconductors, with the Fermi level located inside the conduction band [12,34,36,43,47].…”

Recent progress in MnBi2nTe3n+1 intrinsic magnetic topological insulators: crystal growth, magnetism and chemical disorder

“…The MBE allows one to access more metastable phases such as MnBi 2 n Se 3 n +1 and Mn 4 Bi 2 Te 7 [ 52–54 ] and control local defects, although it is difficult to get rid of the Mn-doped Bi 2 Te 3 impurities [ 55 ]. The in situ measurements such as in situ scanning tunneling microscopy (STM) and ARPES allow the study of the band structure, topology and defects without sample degradation that may occur during the device fabrication process [ 56 ].…”