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

Abstract

Based on first-principles calculations and an effective Hamiltonian analysis, we systematically investigate the electronic and topological properties of alkaline-earth compounds $A{X}_{2}$ $(A=\text{Ca}$, Sr, Ba; $X=\text{Si}$, Ge, Sn). Taking ${\mathrm{BaSn}}_{2}$ as an example, we find that when spin-orbit coupling is ignored, these materials are three-dimensional topological nodal-line semimetals characterized by a snakelike nodal loop in three-dimensional momentum space. Drumheadlike 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 ${\mathbb{Z}}_{2}$ 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.