Angle dependence of Hc2 with a crossover between the orbital and paramagnetic limits

A pair-breaking mechanism of a superconductor under magnetic fields, namely, the origin of the upper critical field Hc2, can be categorized into the orbital effect and the paramagnetic effect, which have been separately discussed so far because the physical pictures are totally different. Here we propose a model that unifies these two origins into one formalism with a generalized physical picture. The obtained formula well describes the experimental results on the angle dependence of Hc2 in a recently developed noncentrosymmetric superconductor, two-dimensional (2D) NbSe2, providing essential information on the spin states of Cooper pairs in 2D NbSe2. The proposed model is widely applicable to all superconductors, offering a powerful approach for comprehensive understanding of the origin of Hc2.