Paramagnetic pair-breaking in spin-triplet superconductors with spin–orbit coupling: Application to Sr₂RuO₄

Paramagnetic pair-breaking effects by in-plane magnetic fields are studied in the presence of the spin–orbit coupling, based on the tight-binding three-orbital model for Sr₂RuO₄. The spin–orbit coupling interfere with the paramagnetic pair-breaking effects in the spin-singlet pairing and the spin-triplet helical p-wave superconductors. We show how the superconducting transition temperature T_c depends on the field orientation and the pairing symmetry. In the spin-triplet chiral p-wave superconductors, as the condition for that the paramagnetic pair-breaking works even to the in-plane fields, we need the mixing of different d-orbital electrons by the c-direction hopping, in addition to the spin–orbit coupling. We also study a case of the change from the helical to chiral p-wave states by the paramagnetic effect.