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.

The spin polarization in nonmagnetic materials is conventionally attributed to the outcome of spin-orbit coupling when the global inversion symmetry is broken. The recently discovered hidden spin polarization indicates that a specific atomic site asymmetry could also induce measurable spin polarization, leading to a paradigm shift in research on centrosymmetric crystals for potential spintronic applications. Here, combining spin- and angle-resolved photoemission spectroscopy and theoretical calculations, we report distinct spin-momentum-layer locking phenomena in a centrosymmetric, layered material, BiOI. The measured spin is highly polarized along the Brillouin zone boundary, while the same effect almost vanishes around the zone center due to its nonsymmorphic crystal structure. Our work demonstrates the existence of momentum-dependent hidden spin polarization and uncovers the microscopic mechanism of spin, momentum, and layer locking to each other, thus shedding light on the design metrics for future spintronic materials.