  One of key challenges in current material research is to search for new
topological materials with inverted bulk-band structure. In topological
insulators, the band inversion caused by strong spin-orbit coupling leads to
opening of a band gap in the entire Brillouin zone, whereas an additional
crystal symmetry such as point-group and nonsymmorphic symmetries sometimes
prohibits the gap opening at/on specific points or line in momentum space,
giving rise to topological semimetals. Despite many theoretical predictions of
topological insulators/semimetals associated with such crystal symmetries, the
experimental realization is still relatively scarce. Here, using angle-resolved
photoemission spectroscopy with bulk-sensitive soft x-ray photons, we
experimentally demonstrate that hexagonal pnictide CaAgAs belongs to a new
family of topological insulators characterized by the inverted band structure
and the mirror reflection symmetry of crystal. We have established the bulk
valence-band structure in three-dimensional Brillouin zone, and observed the
Dirac-like energy band and ring-torus Fermi surface associated with the line
node, where bulk valence and conducting bands cross on a line in the momentum
space under negligible spin-orbit coupling. Intriguingly, we found that no
other bands cross the Fermi level and therefore the low-energy excitations are
solely characterized by the Dirac-like band. CaAgAs provides an excellent
platform to study the interplay among low-energy electron dynamics, crystal
symmetry, and exotic topological properties.
