Nonunitary spin-triplet superconductivity in UPt3:: Evidence from 195Pt Knight shift study

Pt-195 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 parallel to b axis and H-b < 5 kOe and for H-c parallel to c axis and H-c < 2.3 kOe, whereas the KS for H-a parallel to a axis is T independent across T-c down to H-a similar to 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 liked(b) + id(c) at low T and low H.