Spin-flop transition and Zeeman effect of defect-localized bound states in the antiferromagnetic topological insulator MnBi₂Te₄

The correlation of surface impurity states with the antiferromagnetic ground states is crucial for understanding the formation of the topological surface state in the antiferromagnetic topological insulators MnBi₂Te₄. By using low-temperature scanning tunneling microscopy and spectroscopy, we observed a localized bound state around the Mn-Bi antisite defect at the Te-terminated surface of the antiferromagnetic topological insulator MnBi₂Te₄. When applying a magnetic field perpendicular to the surface (B_z) from −1.5 to 3.0 T, the bound state shifts linearly to a lower energy with increasing B_z, which is attributed to the Zeeman effect. Remarkably, when applying a large range of B_z from −8.0 to 8.0 T, the magnetic field induced reorientation of surface magnetic moments results in an abrupt jump in the local density of states (LDOS), which is characterized by LDOS-change-ratio d σ˜ / d B quantitatively. Interestingly, two asymmetric critical field, −2.0 and 4.0 T determined by the two peaks in d σ˜ / d B are observed, which is consistent with simulated results according to a Mills-model, describing a surface spin flop transition (SSF). Our results provide a new flatform for studying the interplay between magnetic order and topological phases in magnetic topological materials.[Figure not available: see fulltext.]