Title: Two-dimensional topological nodal line semimetal in layered $X_2Y$ ($X$ = Ca, Sr, and Ba; $Y$ = As, Sb, and Bi),
Abstract: In topological semimetals the Dirac points can form zero-dimensional and
one-dimensional manifolds, as predicted for Dirac/Weyl semimetals and
topological nodal line semimetals, respectively. Here, based on
first-principles calculations, we predict a topological nodal line semimetal
phase in the two-dimensional compounds $X_2Y$ ($X$=Ca, Sr, and Ba; $Y$=As, Sb,
and Bi) in the absence of spin-orbit coupling (SOC) with a band inversion at
the M point. The mirror symmetry as well as the electrostatic interaction, that
can be engineered via strain, are responsible for the nontrivial phase. In
addition, we demonstrate that the exotic edge states can be also obtained
without and with SOC although a tiny gap appears at the nodal line for the bulk
states when SOC is included.