  The recent discovered antiferromagnetic topological insulators in Mn-Bi-Te
family with intrinsic magnetic ordering have rapidly drawn broad interest since
its cleaved surface state is believed to be gapped, hosting the unprecedented
axion states with half-integer quantum Hall effect. Here, however, we show
unambiguously by using high-resolution angle-resolved photoemission
spectroscopy that a gapless Dirac cone at the (0001) surface of MnBi$_2$Te$_4$
exists between the bulk band gap. Such unexpected surface state remains
unchanged across the bulk N\'eel temperature, and is even robust against severe
surface degradation, indicating additional topological protection. Through
symmetry analysis and $\textit{ab}$-$\textit{initio}$ calculations we consider
different types of surface reconstruction of the magnetic moments as possible
origins giving rise to such linear dispersion. Our results reveal that the
intrinsic magnetic topological insulator hosts a rich platform to realize
various topological phases such as topological crystalline insulator and
time-reversal-preserved topological insulator, by tuning the magnetic
configurations.
