Material realization of spinless, covalent-type Dirac semimetals in three dimensions

Realization of a three-dimensional (3D) analogue of graphene has been a central challenge in topological materials science. Graphene is stabilized by covalent bonding unlike conventional spin-orbit type 3D Dirac semimetals (DSMs). In this study, we demonstrate the material realization of covalent-type 3D DSMs $R_8$Co$X_3$ stabilized by covalent bonding. We observe that the carrier mobility $\mu$ of Dirac fermions reaches 3,000$\mathrm{c}{\mathrm{m}}^2\mathrm{/}\mathrm{V}\mathrm{s}$ even in polycrystalline samples, and $\mu$ increases with the inverse of the Fermi energy, evidencing significant contributions to charge transport from Dirac electrons. $R_8$Co$X_3$ provides a material platform for exploration of Dirac electrons in three dimensions with wide chemical tunability.