Surface-chemistry-engineered strong-coupling superconductivity in halogen-functionalized Mo₂C MXenes

Two-dimensional MXenes provide a versatile platform for engineering emergent quantum phases through surface chemistry. Here, we demonstrate via first-principles calculations that halogen functionalization enables strong-coupling superconductivity in Mo₂C MXene monolayers. Among the systems considered (X = F, Cl, Br, I), only Br- and I-functionalized Mo₂C are both dynamically and mechanically stable, exhibiting positive-definite phonon spectra and satisfying elastic stability criteria. Electronic structure analysis reveals metallic states dominated by Mo d orbitals with enhanced density of states near the Fermi level, providing favorable conditions for strong electron-phonon coupling (EPC). The calculated Eliashberg spectral functions show that low- and intermediate-frequency Mo-dominated phonon modes contribute predominantly to the EPC, driving the systems into the strong-coupling regime. Superconducting transition temperatures of 13.1 K for Mo₂CBr₂ and 18.1 K for Mo₂CI₂ are obtained within the Allen-Dynes formalism. Halogen functionalization simultaneously modifies lattice dynamics and redistributes electronic states, leading to a substantial enhancement of EPC compared to pristine Mo₂C. Furthermore, electron doping serves as an effective tuning parameter, increasing the coupling strength and elevating the superconducting transition temperature above 27.5 K. In contrast, biaxial tensile strain induces competing effects between phonon softening and coupling enhancement, highlighting the interplay between lattice stiffness and pairing strength. These findings establish surface functionalization as a nanoscale design strategy for realizing tunable strong-coupling superconductivity in MXenes.