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

摘要

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