  Layered van der Waals (vdW) topological materials, especially the recently
discovered MnBi$_2$Te$_4$-family magnetic topological insulators (TIs), have
aroused great attention. However, there has been a serious debate about whether
the surface states are gapped or gapless for antiferromagnetic (AFM) TI
MnBi$_2$Te$_4$, which is crucial to the prospect of various magnetic
topological phenomena. Here, a minimal three-Dirac-fermion approach is
developed to generally describe topological surface states of
nonmagnetic/magnetic vdW TIs under the modulation of the interlayer vdW gap. In
particular, this approach is applied to address the controversial issues
concerning the surface states of vdW AFM TIs. Remarkably, topologically
protected gapless Dirac-cone surface states are found to arise due to a small
expansion of the interlayer vdW gap on the surface, when the Chern number
equals zero for the surface ferromagnetic layer; while the surface states
remain gapped in all other cases. These results are further confirmed by our
first-principles calculations on AFM TI MnBi$_2$Te$_4$. The theorectically
discovered gapless Dirac-cone states provide a unique mechanism for
understanding the puzzle of the experimentally observed gapless surface states
in MnBi$_2$Te$_4$. This work also provides a promising way for experiments to
realize the intrinsic magnetic quantum anomalous Hall effect in MnBi$_2$Te$_4$
films with a large energy gap.
