Phononic and electronic properties of the superconducting topological metal ThMo2Si2C

The phononic and electronic properties of newly discovered quaternary superconductor ${\mathrm{ThMo}}_{2}{\mathrm{Si}}_{2}\mathrm{C}$ are reported based on the results of first-principles calculations. The phonon spectrum contains soft phonon modes around the zone corners that lead to a $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}\ifmmode\times\else\texttimes\fi{}1$ charge-density-wave (CDW) instability, which is related to the momentum-dependent electron-phonon coupling. Intriguingly, these soft phonon modes can be removed by considering anharmonic effects. The electronic band structure without spin-orbit coupling (SOC) hosts multiple band crossing points near the Fermi level (${E}_{f}$), giving rise to multiple bulk nodal lines in this system. When SOC is considered, the nodal lines are gapped out and a pair of anisotropic Dirac points protected by ${C}_{4}$ rotational symmetry emerge along the $\mathrm{\ensuremath{\Gamma}}\ensuremath{-}Z$ direction near the ${E}_{f}$. These results suggest that superconductor ${\mathrm{ThMo}}_{2}{\mathrm{Si}}_{2}\mathrm{C}$ is a topological Dirac metal on the verge of a CDW instability, making it a realistic material platform to investigate the interplay among superconductivity, CDW quantum criticality, and bulk Dirac fermions.