Mengke Liu1,2,Chao Lei1,Hyunsue Kim1,Yanxing Li1,Lisa Frammolino1,Jiaqiang Yan3,Allan Macdonald1,Chih-Kang Shih1
The University of Texas at Austin1,Harvard University2,Oak Ridge National Laboratory3
Mengke Liu1,2,Chao Lei1,Hyunsue Kim1,Yanxing Li1,Lisa Frammolino1,Jiaqiang Yan3,Allan Macdonald1,Chih-Kang Shih1
The University of Texas at Austin1,Harvard University2,Oak Ridge National Laboratory3
<b>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<sub>2</sub>Te<sub>4 </sub>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. </b><b>We find that in high defect regions, the Dirac mass gap collapses. <i>Ab initio</i> 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<sub>2</sub>Te<sub>4</sub></b><b>, and by revealing the microscopic origin of its gap variation, establishes a material design principle for realizing exotic states in intrinsic magnetic topological insulators. </b>