Topological nodal-line semimetals in alkaline-earth stannides, germanides and silicides

TL;DR

This study uses first-principles calculations to explore the electronic and topological properties of alkaline-earth compounds, revealing their potential as topological nodal-line semimetals and insulators depending on spin-orbit coupling effects.

Contribution

It provides a systematic analysis of alkaline-earth $AX_2$ compounds, identifying conditions under which they exhibit topological nodal-line semimetal or insulator phases.

Findings

Nodal-line semimetals with snake-like loops in $AX_2$ compounds.

Surface states vary with surface termination and orientation.

Spin-orbit coupling gaps the nodal line, turning the material into a topological insulator.

Abstract

Based on first-principles calculations and an effective Hamiltonian analysis, we systematically investigate the electronic and topological properties of alkaline-earth compounds $AX_2$ ($A$=Ca, Sr, Ba; $X$=Si, Ge, Sn). Taking BaSn$_2$ as an example, we find that when spin-orbit coupling is ignored, these materials are three-dimensional topological nodal-line semimetals characterized by a snake-like nodal loop in three-dimensional momentum space. Drumhead-like surface states emerge either inside or outside the loop circle on the (001) surface depending on surface termination, while complicated double-drumhead-like surface states appear on the (010) surface. When spin-orbit coupling is included, the nodal line is gapped and the system becomes a topological insulator with Z$_2$ topological invariants (1;001). Since spin-orbit coupling effects are weak in light elements, the nodal-line semimetal phase is expected to be achievable in some alkaline-earth germanides and silicides.