Topological phonons and electronic structure of Li 2 BaSi class of semimetals.

Extension of the topological concepts to the bosonic systems has led to the prediction of topological phonons in materials. Here we discuss the topological phonons and electronic structure of Li 2 BaX (X = Si, Ge, Sn, and Pb) materials using first-principles theoretical modelling. A careful analysis of the phonon spectrum of Li 2 BaX reveals an optical mode inversion with the formation of nodal line states in the Brillouin zone. Our electronic structure results reveal a double band inversion at the Γ point with the formation of inner nodal-chain states in the absence of spin-orbit coupling (SOC). Inclusion of the SOC opens a materials-dependent gap at the band crossing points and transitions the system into a trivial insulator state. We also discuss the lattice thermal conductivity and transport properties of Li 2 BaX materials. Our results show that coexisting phonon and electron nontrivial topology with robust transport properties would make Li 2 BaX materials appealing for device applications.