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We use density-functional theory (DFT) calculations to quantitatively address the superconducting properties of bulk and monolayer NbSe₂ [1]. We demonstrate that NbSe₂ is close to a ferromagnetic instability, and analyze our results in the context of experimental measurements of the spin susceptibility in NbSe₂. We show how this magnetic instability, which is pronounced in a single monolayer, can enable sizable singlet-triplet mixing of the superconducting order parameter, contrary to contemporary considerations of the pairing symmetry in monolayer NbSe₂, Our calculations also enable a quantitative description of the large anisotropy of the superconducting critical field, using DFT calculations of monolayer NbSe₂ in the normal state.

[1] D.Wickramaratne, S. Khmelevskyi, D.F. Agterberg, and I. I. Mazin, Phys. Rev. X 10, 041003 (2020).