The interplay between magnetism and nontrivial topology in magnetic topological insulators (MTIs) is expected to give rise to exotic topological quantum phenomena like the quantum anomalous Hall effect and the topological axion states. A key to assessing these novel properties is to realize gapped topological surface sates. $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ possesses nontrivial band topology with an intrinsic antiferromagnetic state. However, the highly electron-doped nature of the $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ crystals obstructs the exhibition of the surface band gap. Here, we tailor the material through Sb substitution to reveal the gapped surface states in $\mathrm{Mn}{\mathrm{Bi}}_{2\text{−}x}{\mathrm{Sb}}_x{\mathrm{Te}}_4$. By shifting the Fermi level into the bulk band gap, we access the surface states and show a band gap of 50 meV at the Dirac point from quasiparticle interference measured by scanning tunneling microscopy (STM). Surface-dominant conduction is confirmed through transport spectroscopy measured by multiprobe STM below the Néel temperature. The surface band gap is robust against the out-of-plane magnetic field despite the promotion of field-induced ferromagnetism. The realization of bulk-insulating MTIs with the large exchange gap offers a promising platform for exploring emergent topological phenomena.

• Received 11 March 2020
• Revised 12 August 2020
• Accepted 14 August 2020

DOI:https://doi.org/10.1103/PhysRevB.102.115402