Interplay of charge ordering and superconductivity in two-dimensional 2H group V transition-metal dichalcogenides

Layered 2H-TX2 with T = Nb, Ta and X = S, Se exhibit rich dimensionality effects on charge density wave (CDW) order and superconductivity, but comprehensive understanding of these correlated quantum phases in the two-dimensional limit of 2H-TX2 is still lacking, hindering their practical applications. Here, I calculate from first principles the phonon linewidth and bare susceptibility to study the origin of CDW formation in NbSe2, TaS2, TaSe2, and NbS2 monolayers, analyze their relative CDW strength, and evaluate electron-phonon superconductivity within the fully anisotropic Migdal-Eliashberg theory. A peaked linewidth of the longitudinal acoustic branch and no Fermi-surface nesting around qCDW indicate CDW instability driven by the wave -vector-dependent electron-phonon coupling. The 3 x 3 CDW ground state favors a distinct hollow-centered clustering of transition-metal atoms, with larger distortion amplitude in NbSe2 and TaSe2 than in TaS2 and NbS2. Superconducting results of the monolayer CDW phase prove that strong spin-orbit coupling effects are dominant in determining the critical temperature Tc of each system via modifying both the strength and anisotropic extent of electron-phonon interactions. I also recapitulate measured opposite thickness dependencies of Tc in NbX2 and TaX2, and show that whether superconductivity is weakened or enhanced in monolayer limit relies on specific evolution of CDW-modulated Fermi-level density of states and electron-phonon matrix elements under reduced dimensionality. This work lays the foundation for applications of 2H group V transition-metal dichalcogenides in versatile nanoscale quantum devices.