Strong electron-phonon coupling and predicted highest known $T_c$ of MXenes revealed in 2H-Mo${}_2$N under biaxial stress

This letter reports the unexpectedly strong electron-phonon coupling (EPC) and the highest $T_c$ record ($\approx$ 38 K) among the MXenes revealed in the 2H-Mo${}_2$N under biaxial stress. At first, its excellent mechanical properties are demonstrated with ideal strength of 37 GPa and elastic modulus of 438 GPa. Subsequently, EPC and corresponding $T_c$ are elucidated upon the dynamically stable range of strain. For strain-free 2H-Mo${}_2$N, the EPC constant ($\lambda$) and $T_c$ are 1.3 and 22.7 K, respectively. This $T_c$ is higher than those of 2H-Mo${}_2$C (4.3 K), 1T-Mo${}_2$N (16.8 K), and other pristine MXenes. The material exhibits remarkable enhancement in $\lambda$ and $T_c$ when subject to compressive and tensile stresses. The obvious strong EPC with $\lambda$ over 2.0 occurs at strains of -4%, -2.5%, and 5%, yielding $T_c$s of 37.8, 35.4, and 28.9 K, respectively. Our findings suggest that the strain-dependent feature and energy levels of electronic bands play an essential role in enhancing EPC. Moreover, the stronger EPC in Mo${}_2$N compared with Mo${}_2$C is clarified based on lattice vibrations. Therefore, this work paves a practical way for designing 2D superconducting materials using tuning atomic recipes and strain-dependent engineering.