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

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