# Two-dimensional topological nodal line semimetal in layered $X_2Y$ ($X$ = Ca, Sr, and Ba; $Y$ = As, Sb, and Bi)

## TL;DR

This paper predicts a new two-dimensional topological nodal line semimetal phase in layered $X_2Y$ compounds using first-principles calculations, highlighting the role of symmetry and strain in stabilizing the phase and its edge states.

## Contribution

It introduces a novel 2D topological nodal line semimetal phase in layered compounds, emphasizing the effects of mirror symmetry and strain without prior experimental evidence.

## Key findings

- Nodal line phase exists without spin-orbit coupling in $X_2Y$ compounds.
- Edge states are present with and without SOC, with a tiny gap when SOC is included.
- Band inversion occurs at the M point in these materials.

## 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.