Nonunitary Spin-Triplet Superconductivity in UPt3: Evidence from 195Pt Knight Shift Study

¹⁹⁵Pt Knight shift (KS) measurements covering the superconducting multiple phases for major field ( H) orientations have been carried out on the high-quality single crystal UPt₃. For H>5 kOe, the KS does not change below the superconducting transition temperature T_c down to 28 mK, regardless of major crystal orientations, which provides evidence that the odd-parity superconductivity with the parallel spin pairing is realized. By contrast, the KS decreases below T_c for H_b∥b axis and H_b<5 kOe and for H_c∥c axis and H_c<2.3 kOe, whereas the KS for H_a∥a axis is T independent across T_c down to H_a~1.764 kOe. These novel findings entitle UPt₃ as the first spin-triplet odd-parity superconductor including a nonunitary pairing characterized by the two-component d vector like d_b+id_c at low T and low H.