Angle dependence of H_{c2} with a crossover between the orbital and paramagnetic limits

Abstract

A pair-breaking mechanism of a superconductor under magnetic fields, namely, the origin of the upper critical field H_{c2}, 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 H_{c2} in a recently developed noncentrosymmetric superconductor, two-dimensional (2D) NbSe_{2}, providing essential information on the spin states of Cooper pairs in 2D NbSe_{2}. The proposed model is widely applicable to all superconductors, offering a powerful approach for comprehensive understanding of the origin of H_{c2}.