${}^{195}\mathrm{Pt}$ Knight shift (KS) measurements covering the superconducting multiple phases for major field ( $H$) orientations have been carried out on the high-quality single crystal ${\mathrm{UPt}}_3$. For $H>\mathrm{}5\mathrm{}$ 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 $H_b\parallel b$ axis and $H_b<5\mathrm{}$ kOe and for $H_c\parallel c$ axis and $H_c<2.3\mathrm{}$ kOe, whereas the KS for $H_a\parallel a$ axis is $T$ independent across $T_c$ down to $H_a\sim 1.764$ kOe. These novel findings entitle ${\mathrm{UPt}}_3$ as the first spin-triplet odd-parity superconductor including a nonunitary pairing characterized by the two-component $\mathbf{d}$ vector like ${\mathbf{d}}_b+i{\mathbf{d}}_c$ at low $T$ and low $H$.

• Received 4 September 1997

DOI:https://doi.org/10.1103/PhysRevLett.80.3129