  We have performed angle-resolved photoemission spectroscopy (ARPES) on
layered ternary compounds ZrGeXc (Xc = S, Se, and Te) with square Ge lattices.
ARPES measurements with bulk-sensitive soft-x-ray photons revealed a
quasi-two-dimensional bulk-band structure with the bulk nodal loops protected
by glide mirror symmetry of the crystal lattice. Moreover, high-resolution
ARPES measurements near the Fermi level with vacuum-ultraviolet photons
combined with first-principles band-structure calculations elucidated a
Dirac-node-arc surface state traversing a tiny spin-orbit gap associated with
the nodal loops. We found that this surface state commonly exists in ZrGeXc
despite the difference in the shape of nodal loops. The present results suggest
that the spin-orbit coupling and the multiple nodal loops cooperatively play a
key role in creating the exotic Dirac-node-arc surface states in this class of
topological line-node semimetals.
