Topological semimetal in honeycomb lattice LnSI

Recognized as elementary particles in the standard model,Weyl fermions in condensed matter have received growing attention. However, most of the previously reportedWeyl semimetals exhibit rather complicated electronic structures that, in turn, may have raised questions regarding the underlying physics. Here, we report for the first time promising topological phases that can be realized in specific honeycomb lattices, including ideal Weyl semimetal structures, 3D strong topological insulators, and nodal-line semimetal configurations. In particular, we highlight a novel semimetal featuring both Weyl nodes and nodal lines. Guided by this model, we demonstrated that GdSI the long perceived ideal Weyl semimetal has two pairs ofWeyl nodes residing at the Fermi level, and that LuSI (YSI) is a 3D strong topological insulator with the right-handed helical surface states. Our work provides a new mechanism to study topological semimetals, and proposes a platform towards exploring the physics of Weyl semimetals as well as related device designs.