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Two-dimensional (2D) NbSe₂ is a recently-developed non-centrosymmetric superconductor with the giant upper critical field H_c2 for the parallel magnetic fields (H_c2||) originating from the unique Ising paring. Here we report the angle dependence of H_c2 in 2D NbSe₂, which turned out to exhibit the cusp-like behavior around the parallel fields even far below the superconducting critical temperature despite that H_c2|| is dominated purely by the paramagnetic effect^[1]. This behavior is well understood within the Ginzburg-Landau framework by considering different origins of H_c2 for the parallel and perpendicular fields. The proposed model is widely applicable to all the superconductors, offering a powerful approach for a comprehensive understanding of the origin of H_c2.
[1] H. Matsuoka et al., Phys. Rev. Research, 2, 012064(R) (2020)