Multiple Topological Electronic Phases in Superconductor MoC

The search for a superconductor with non-s-wave pairing is important not only for understanding unconventional mechanisms of superconductivity but also for finding new types of quasiparticles such as Majorana bound states. Materials with both topological band structure and superconductivity are promising candidates as $p+ip$ superconducting states can be generated through pairing the spin-polarized topological surface states. In this work, the electronic and phonon properties of the superconductor molybdenum carbide (MoC) are studied with first-principles methods. Our calculations show that nontrivial band topology and superconductivity coexist in both structural phases of MoC, namely, the cubic $\alpha$ and hexagonal $\gamma$ phases. The $\alpha$ phase is a strong topological insulator and the $\gamma$ phase is a topological nodal line semimetal with drumhead surface states. In addition, hole doping can stabilize the crystal structure of the $\alpha$ phase and elevate the transition temperature in the $\gamma$ phase. Therefore, MoC in different structural forms can be a practical material platform for studying topological superconductivity and elusive Majorana fermions.