Signatures of both the quantum anomalous Hall effect and axion electrodynamics have been recently observed to exist in thin films of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, a stoichiometric antiferromagnetic topological insulator. Direct evidence of the bulk topological magnetoelectric response in an axion insulator requires an energy gap at its topological surface state (TSS). However, independent spectroscopic experiments revealed that such a surface gap is much smaller than previously thought. Here we utilize angle resolved photoemission spectroscopy and density functional theory calculations to demonstrate that a sizable TSS gap unexpectedly exists in Sb-doped ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ where the bulk system remains topologically nontrivial. This gap is found to be insensitive to the bulk antiferromagnetic-paramagnetic transition, while it enlarges along with increasing Sb concentration, enabling simultaneous tunability of the Fermi level and the TSS gap size (up to $>100$ meV). Our work shows that Sb dopants in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ can not only control the Fermi level but also induce a tunable surface gap, providing a potential platform to observe the key features of the high-temperature axion-insulator phase.

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• Dirac fermions
• Surface states
• Topological phases of matter
• Antiferromagnets
• Chern insulators
• Magnetic multilayers
• Magnetic semiconductors
• Topological materials
• Angle-resolved photoemission spectroscopy
• Crystal growth
• Density functional theory
• Electronic structure
• Hall bar
• Resistivity measurements
• Scanning electron microscopy
• Scanning tunneling microscopy
• Spin-resolved photoemission spectroscopy