Evidence for weak spin–orbit interaction experienced by cooper pairs in the spin-triplet superconductor UPT₃: ¹⁹⁵PT-NMR study

¹⁹⁵Pt-NMR measurements for H ∥ c in the vicinity of the upper critical field H_c2 ^kc have been carried out on the heavy-fermion superconductor UPt₃. The¹⁹⁵Pt NMR line does not change its profile or its resonance frequency across the superconducting transition temperature T_c. This behavior is consistent with the previous experimental results showing that the spin susceptibility remains unchanged across T_c in the superconducting C phase [H. Tou et al., Phys. Rev. Lett. 77, 1374 (1996) and Gannon et al., Phys. Rev. B 96, 041111(R) (2017)]. The lack of the first-order phase transition across T_c(H) strongly indicates that the Pauli limiting effect inherent in the ∣S_z = 0〉 component does not occur near H_c2 ^kc. This experimental finding is supported by the analysis of the Pauli limiting field based on the superconducting thermodynamics. We also discuss the field dependence of the quasiparticle spin susceptibility in the superconducting state. The field dependence of the Knight shift cannot be explained by the behavior of the spin-triplet superconducting state having ∣S_z = 0〉.