Nonunitary Spin-Triplet Superconductivity in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>UPt</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>: Evidence from<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>P</mml:mi></mml:mrow><mml:mprescripts/><mml:mrow/><mml:mrow><mml:mn>195</mml:mn></mml:mrow><mml:mrow/><mml:mrow/></mml:mmultiscripts></mml:mrow><mml:mi>t</mml:mi></mml:math>Knight Shift Study

${}^{195}\mathrm{Pt}$ 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 $H&gt;5$ 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 ${H}_{b}\ensuremath{\parallel}b$ axis and ${H}_{b}&lt;5$ kOe and for ${H}_{c}\ensuremath{\parallel}c$ axis and ${H}_{c}&lt;2.3$ kOe, whereas the KS for ${H}_{a}\ensuremath{\parallel}a$ axis is $T$ independent across ${T}_{c}$ down to ${H}_{a}\ensuremath{\sim}1.764$ 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 $H$.