Spin-fluctuation mediated superconductivity in monolayer NbSe₂

The Ising spin-orbit coupling (SOC) generated by the lack of inversion symmetry in monolayer NbSe₂ generally causes an admixture between the spin-singlet and spin-triplet components of the superconducting gap function. The resulting Ising superconducting state has attracted significant attention, as it has been proposed to display a wide range of unusual properties, including topologically nontrivial ones. Recent experiments suggest that monolayer NbSe₂ has a subleading unconventional pairing state in close proximity to the leading s-wave instability. However, the microscopic mechanism responsible for this subleading state remains under debate. Here, motivated by density functional theory results demonstrating that monolayer NbSe₂ is near to a ferromagnetic instability. We investigate unconventional pairing in NbSe₂ mediated by ferromagnetic fluctuations. Focusing first on the Γ pocket, we show that upon increasing the Ising SOC, the pairing state changes from a (p+d) symmetry to an (s+f) symmetry. We also solve the coupled gap equations between the pockets at Γ and K/K' points, discussing the impact of the ferromagnetic correlation length in promoting a robust pairing state.