Topological phonons and electronic structure of LiBaSi 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 LiBaX (X = Si, Ge, Sn, and Pb) materials using first-principles theoretical modelling. A careful analysis of the phonon spectrum of LiBaX 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 LiBaX materials. Our results show that coexisting phonon and electron nontrivial topology with robust transport properties would make LiBaX materials appealing for device applications.