Evidence For Weak Spin-Orbit Interaction Experienced By Cooper Pairs In The Spin-Triplet Superconductor Upt3: Pt-195-Nmr Study

Pt-195-NMR measurements for H parallel to c in the vicinity of the upper critical field H-c2(parallel to c) have been carried out on the heavy-fermion superconductor UPt3. The Pt-195 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 Tc 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 vertical bar S-z = 0 > component does not occur near H-c2(parallel to c). 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 vertical bar S-z = 0 >. It is strongly indicated that the d-vector rotates freely above 5 kOe and that the spin-orbit interaction experienced by the Cooper pairs is weak in UPt3.