  Antiferromagnetic (AF) topological materials offer a fertile ground to
explore a variety of quantum phenomena such as axion magnetoelectric dynamics
and chiral Majorana fermions. To realize such intriguing states, it is
essential to establish a direct link between electronic states and topology in
the AF phase, whereas this has been challenging because of the lack of a
suitable materials platform. Here we report the experimental realization of the
AF topological-insulator phase in NdBi. By using micro-focused angle-resolved
photoemission spectroscopy, we discovered contrasting surface electronic states
for two types of AF domains; the surface having the out-of-plane component in
the AF-ordering vector displays Dirac-cone states with a gigantic energy gap,
whereas the surface parallel to the AF-ordering vector hosts gapless Dirac
states despite the time-reversal-symmetry breaking. The present results
establish an essential role of combined symmetry to protect massless Dirac
fermions under the presence of AF order and widen opportunities to realize
exotic phenomena utilizing AF topological materials.
