Defect-induced displacement of topological surface state in quantum magnet MnBi$_2$Te$_4$

TL;DR

This study investigates how antisite defects in MnBi₂Te₄ displace topological surface states into the bulk, reducing surface gaps and impacting quantum material development.

Contribution

It provides experimental validation that high defect concentrations cause surface states to shift into the bulk, explaining gap suppression.

Findings

High defect density displaces surface states into the bulk

Displacement correlates with reduced surface gap

Combining STM, ARPES, and DFT elucidates defect effects

Abstract

The topological magnet MnBi$_2$Te$_4$ (MBT), with gapped topological surface state, is an attractive platform for realizing quantum anomalous Hall and Axion insulator states. However, the experimentally observed surface state gaps fail to meet theoretical predictions, although the exact mechanism behind the gap suppression has been debated. Recent theoretical studies suggest that intrinsic antisite defects push the topological surface state away from the MBT surface, closing its gap and making it less accessible to scanning probe experiments. Here, we report on the local effect of defects on the MBT surface states and demonstrate that high defect concentrations lead to a displacement of the surface states well into the MBT crystal, validating the theorized mechanism. The local and global influence of antisite defects on the topological surface states are studied with samples of varying defect densities by combining scanning tunneling microscopy (STM), angle-resolved photoemission (ARPES), and density functional theory (DFT). Our findings identify a combination of increased defect density and reduced defect spacing as the primary factors underlying the displacement of the surface states and suppression of surface gap, guiding further development of topological quantum materials.