  Nodal-line semimetals (NLSs) represent a new type of topological semimetallic
beyond Weyl and Dirac semimetals in the sense that they host closed loops or
open curves of band degeneracies in the Brillouin zone. Parallel to the
classification of type-I and type-II Weyl semimetals, there are two types of
NLSs. The conventional NLS phase, in which the two bands forming the nodal line
have opposite signs for their slopes along any direction perpendicular to the
nodal line, has been proposed and realized in many compounds, whereas the
exotic type-II NLS is very rare. Our first-principles calculations show that
Mg$_3$Bi$_2$ is a material candidate that hosts a single type-II nodal loop
around $\Gamma$. The band crossing is close to the Fermi level and the two
crossing bands have the same sign in their slopes along the radial direction of
the loop, indicating the type-II nature of the nodal line. Spin-orbit coupling
generates only a small energy gap ($\sim$35 meV) at the nodal points and does
not negate the band dispersion of Mg$_3$Bi$_2$ that yields the type-II nodal
line. Based on this prediction we have synthesized Mg$_3$Bi$_2$ single crystals
and confirmed the presence of the type-II nodal lines in the material. Our
angle-resolved photoemission spectroscopy (ARPES) measurements agree well with
our first-principles results and thus establish Mg$_3$Bi$_2$ as an ideal
materials platform for studying the exotic properties of type-II nodal line
semimetals.
