Monolayer ${\mathrm{NbSe}}_2$, as an Ising superconductor, has attracted considerable attention. Experimentally, its samples inevitably contain defects, yet the effects of these defects on superconductivity remain largely unexplored. Here, we demonstrate that selenium (Se) monovacancies, the most common defects in monolayer ${\mathrm{NbSe}}_2$, critically affect superconductivity. Specifically, Se monovacancies disrupt both the out-of-plane mirror symmetry and the $C_{3z}$ rotational symmetry in pristine monolayer ${\mathrm{NbSe}}_2$, resulting in an asymmetric spin-orbit coupling (SOC). While pristine monolayer ${\mathrm{NbSe}}_2$, which possesses Ising SOC, exhibits an isotropic in-plane upper critical field $\left(B_{c2}\right)$, the asymmetric SOC induced by Se monovacancies leads the in-plane $B_{c2}$ to display an anisotropic twofold symmetry in response to the direction of the in-plane magnetic field. Furthermore, Se monovacancies significantly reduce the in-plane spin susceptibility of the superconducting state, causing $B_{c2}$ to exceed the Pauli limit only moderately, consistent with experimental observations. These findings provide valuable insights into the role of defects in tuning superconductivity of two-dimensional materials.

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• Spin-orbit coupling
• Superconductivity
• 2-dimensional systems
• Transition metal dichalcogenides
• Density functional theory
• First-principles calculations
• Mean field theory
• Wannier function methods