Six- or four-fold band degeneration in CoAs3, RhAs3 and RhSb3 topological semimetals.

Searching for new topological phases of matter has long been a hot topic in condensed matter physics and materials science based on its fundamental physics and promising device applications. Here we report a systematic ab initio study on the topological electronic properties of CoAs3, RhAs3, and RhSb3 binary compounds. Without spin-orbit coupling, there is a six-fold band crossing node at the high-symmetric Γ point with topological charge , which is denoted as "six-fold excitation". This nodal point is formed by the highest occupied band and two of the lowest unoccupied bands, and protected by time-reversal symmetry, and spatial-inversion symmetry and stabilized by the two-fold rotational symmetry. Detailed band structure and elementary band representation analysis shows that the six-fold band degeneracy at the Γ point near the Fermi level is formed by the bands of Ag@8c originating from the d-orbital of metal atoms. Meanwhile, with spin-orbit coupling, the six-fold nodal point becomes a four-fold degenerate quadratic Dirac point with the topological charge conserved as . These results provide a systematic understanding of the electronic properties of the skutterudite compounds and enrich the families of topological fermions in condensed matter systems.