Discovery of an antiferromagnetic nodal line

While topological states have been observed in many non-magnetic materials, magnetic topological compounds with gapless crossings are rarely studied. Through state-of-the-art angle-resolved photoemission spectroscopy and density functional theory, we visualize an antiferromagnetic nodal line in an antiferromagnet. This fourfold degenerate nodal line is gapless even when spin-orbit coupling and magnetism are considered. We further demonstrate that it is enforced by the combination of magnetism, space-time inversion symmetry and nonsymmorphic lattice symmetry. Finally, we clearly observe the corresponding drumhead surface states. Our work greatly expand beyond the existing horizon of topological materials, introducing a new family with novel magnetic topological phases.