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Strong electron-phonon coupling in 3D tungsten nitride and coexistence of intrinsic superconductivity and topological nodal line in its 2D limit

Three-component fermion beyond the conventional Dirac Weyl Majorana classification attracts extensive attentions recently and many efforts have been paid to explore their superconductivity. Based on first-principles calculations, we systematically investigate the electron-phonon coupling (EPC) in the three-component fermion materials WN, WC and TaN. The EPC in pristine and pressured WC and TaN are to small to induce superconductivity. Electron doping can efficiently enhance the EPC strength and the predicted Tcs reach the value of experiments. Upon 0.6 electron/unitcell doping, the EPC strength of TaN is boosted by two orders of magnitude and Tc can even be as high as 27 K, revealing the crucial role of charge doping in the formation of superconductivity observed in WC and TaN. In stark contrast, pristine WN exhibits overwhelmingly strong EPC and can be a good superconductor with a high transition temperature Tc of 31 K. The strong EPC in WN are dictated by a synergistic effect of strong Fermi nesting and large deformation potential. Going down from three-dimension (3D) to three-dimension (2D), WN thin film (i.e. monolayer W3N4) is also an intrinsic superconductor with Tc of 11 K. Most importantly, monolayer W3N4 host Dirac nodal lines protected by mirror symmetry in the absence of spin-orbit coupling (SOC), Including SOC, the Dirac nodal lines split into three pairs of spinful Weyl rings. These nodal lines lies closely near the Fermi level, they are pure and clean without other nontrivial bands, which is scarce in real materials and making the exotic topological properties easily accessible in experiment. The coexistence of superconductivity with high transition temperature and topological states in WN and its 2D film provide a promising platform for exploring topological superconductivity.