Topological semimetal in honeycomb lattice LnSI

We show that fruitful topological states can be realized in a special honeycomb lattice model. Guided by this model, we find that GdSI, the long perceived ideal Weyl semimetal, has only two pairs of Weyl nodes and that LuSI (YSI) is a topological insulator with the right-handed helical surface states. Our work provides a mechanism to study topological semimetals and proposes a platform for exploring the physics of Weyl semimetals as well as related device designs.