Charge Density Wave Order and Superconductivity in Janus MoXH Monolayers

Abstract

Two-dimensional Janus hydrogenated transition metal chalcogenides provide an unusual platform where lattice instabilities, electron-phonon coupling, and superconductivity are strongly intertwined. Using first-principles calculations, we demonstrate that Janus 2H and 1T MoXH (X = S, Se) monolayers host an intrinsic, commensurate charge density wave (CDW) ground state originating from soft phonon modes at the Brillouin zone M point. Real-space supercell optimizations confirm that the CDW reconstruction lowers the total energy and fully stabilizes the lattice, eliminating the imaginary phonon modes present in the high-symmetry metallic structures. Analysis of the electronic susceptibility shows that the CDW instability is not driven by Fermi surface nesting, but instead arises from strong electron-phonon coupling. We further reveal a material-dependent interplay between CDW order and superconductivity. In 1T MoSH, CDW formation enhances low-energy phonon contributions and strengthens electron-phonon coupling, leading to an increased superconducting transition temperature. In contrast, for 1T MoSeH and 2H MoSeH, the CDW phase suppresses electron-phonon coupling and reduces superconductivity. Finally, we show that thermal fluctuations, compressive strain, and carrier doping can selectively suppress CDW order and restore superconductivity. These results establish Janus MoXH monolayers as a tunable two-dimensional system for exploring lattice-driven charge ordering and its competition with superconductivity.