Phase stability and superconductivity in hydrogenated and lithiated Janus Ga <i>X</i> S2 ( <i>X</i>  = Ga, In) monolayers

Hydrogen and lithium functionalization of two-dimensional (2D) materials offers a promising route to enhance electronic properties and induce superconductivity. Here, we employ first-principles calculations to explore the phase stability and superconducting behavior of hydrogenated and lithiated Janus GaXS2 (X=Ga,In) monolayers. Among Ga2SH, Ga2SLi, GaInSH, and GaInSLi, only the 2H-GaInSLi structure is dynamically, thermally, and mechanically stable, as confirmed by phonon dispersion, ab initio molecular dynamics, and elastic constants. This monolayer adopts a hexagonal lattice, exhibits metallic behavior, and has a negative formation energy, suggesting experimental feasibility. Anisotropic Migdal–Eliashberg analysis reveals phonon-mediated superconductivity with a critical temperature (Tc) of 4.8 K. Notably, three distinct superconducting gaps emerge, linked to specific atomic orbitals and phonon modes. Electron doping of 0.2 e per cell increases Tc to nearly 6.2 K while maintaining the three-gap character. These results highlight the effectiveness of selective functionalization in engineering superconductivity and identify GaInSLi as a promising platform for next-generation multi-gap 2D superconducting devices.