  Topological Dirac semimetals (TDSs) represent a new state of quantum matter
recently discovered that offers a platform for realizing many exotic physical
phenomena. A TDS is characterized by the linear touching of bulk (conduction
and valance) bands at discrete points in the momentum space (i.e. 3D Dirac
points), such as in Na3Bi and Cd3As2. More recently, new types of Dirac
semimetals with robust Dirac line-nodes (with non-trivial topology or near the
critical point between topological phase transitions) have been proposed that
extends the bulk linear touching from discrete points to 1D lines. In this
work, using angle-resolved photoemission spectroscopy (ARPES), we explored the
electronic structure of the non-symmorphic crystals MSiS (M=Hf, Zr).
Remarkably, by mapping out the band structure in the full 3D Brillouin Zone
(BZ), we observed two sets of Dirac line-nodes in parallel with the kz-axis and
their dispersions. Interestingly, along directions other than the line-nodes in
the 3D BZ, the bulk degeneracy is lifted by spin-orbit coupling (SOC) in both
compounds with larger magnitude in HfSiS. Our work not only experimentally
confirms a new Dirac line-node semimetal family protected by non-symmorphic
symmetry, but also helps understanding and further exploring the exotic
properties as well as practical applications of the MSiS family of compounds.
