  We predict a new family of two-dimensional (2D) rare earth monochalcogenide
materials MX (M = Sc, Y; X = S, Se, Te). Based on first-principles
calculations, we confirm their stability and systematically investigate their
mechanical properties. We find that these materials are metallic and
interestingly, they possess nodal lines in the low-energy band structure
surrounding the whole Brillouin zone, protected by nonsymmorphic crystal
symmetries in the absence of spin-orbit coupling (SOC). SOC opens small energy
gaps at the nodal line, except for two high-symmetry points, at which fourfold
degenerate 2D spin-orbit Dirac points are obtained. We show that these
topological band features are robust under uniaxial and biaxial strains, but
can be lifted by the shear strain. We also investigate the optical
conductivities of these materials, and show that the transformation of the band
structure under strain can be inferred from the optical absorption spectrum.
Our work reveals a new family of 2D topological metal materials with
interesting mechanical and electronic properties, which will facilitate the
study of nonsymmorphic symmetry enabled nodal features in 2D.
