Electron-Phonon Coupling and Charge Density Wave Instabilities in W2N and Halogen-Functionalized W2N Monolayers

The interplay between charge-density-wave (CDW) order and superconductivity is a central problem in condensed-matter physics because both phenomena often originate from the same electron-phonon coupling (EPC) mechanism. Here, we investigate the structural, electronic, vibrational, and superconducting properties of monolayer W2N and halogen-functionalized W2N (W2NF2 and W2NCl2) using first-principles calculations. Pristine W2N exhibits pronounced phonon instabilities near the M and K points driven by exceptionally strong EPC associated with softened low-frequency phonons. The coincidence between phonon softening and enhanced phonon linewidths identifies the instability as EPC-driven and indicative of a CDW tendency. Inclusion of van der Waals interactions stabilizes the lattice and yields strong-coupling superconductivity with {\lambda} = 1.00 and Tc = 13.2 K, while fluorination further weakens the soft-phonon anomaly, resulting in a moderate-coupling superconductor with {\lambda} = 0.67 and Tc = 5.3 K. In contrast, W2NCl2 exhibits a re-emergence of CDW-related phonon softening that can be continuously suppressed by compressive strain or electron doping. Under -3% compressive strain, the EPC constant decreases from {\lambda} = 1.35 to {\lambda} = 0.71, giving rise to superconductivity with Tc = 5.8 K. Across the entire W2N family, the low-energy physics is governed by softened ZA phonons near the M point, establishing a unified framework in which CDW order and superconductivity emerge as competing manifestations of the same soft-phonon-driven EPC mechanism.