  Dirac line node (DLN) semimetals are a class of topological semimetals that
feature band-crossing lines in momentum space. We study the type-I and type-II
classification of DLN semimetals by developing a criterion that determines the
type using band velocities. Using first-principles calculations, we also
predict that Na3N under an epitaxial tensile strain realizes a type-II DLN
semimetal with vanishing spin-orbit coupling (SOC), characterized by the Berry
phase that is Z2-quantized in the presence of inversion and time-reversal
symmetries. The surface energy spectrum is calculated to demonstrate the
topological phase, and the type-II nature is demonstrated by calculating the
band velocities. We also develop a tight-binding model and a low-energy
effective Hamiltonian that describe the low-energy electronic structure of
strained Na3N. The occurrence of a DLN in Na3N under strain is captured in the
optical conductivity, which we propose as a means to experimentally confirm the
type-II class of the DLN semimetal.
