Saddle-like topological surface states on the TT'X family of compounds (T, T' = Transition metal, X= Si, Ge)

Based on first-principles calculations and an effective model analysis, we identify the presence of topological nodal-line semimetal states in the low crystalline symmetric TT'X family of compounds (T, T' = transition metal, X= Si, or Ge) in the absence of spin-orbit coupling (SOC). Taking ZrPtGe as an exemplar system, we show that owing to lowered lattice symmetry this material harbors a single nodal line on the k_y=0 plane with large energy dispersion and a unique drumhead surface state with a saddle-like energy dispersion. When the SOC is included, the nodal line gaps out and the system transitions to a strong topological insulator state with Z₂=(1;000). The topological surface state evolves from the drumhead surface state via the sharing of its saddle-like energy dispersion within the bulk energy gap. These features differ remarkably from those of the currently known topological surface states in topological insulators such as Bi₂Se₃ with Dirac-cone-like energy dispersions.
1. B. Singh et al, arXiv:1703.04048