A complete set of the 195Pt Knight-shift (KS) data on the superconducting (SC) state in UPt3 identified the spin structure of the Cooper pair corresponding to the multiple SC phases. UPt3 was acclaimed as the first odd-parity superconductor including a non-unitary pairing state characterized by the two-component d vector like db+idc at low T and low H [H. Tou et al., Phys. Rev. Lett. 77 (1996) 1374; 80 (1998) 3129]. We have shed further light on these novel results through a comparison with the singlet even-parity anisotropic superconductors CeCu2Si2 and UPd2Al3. In the singlet pairing state, the fractional decrease in KS below Tc,δKobs is independent of the crystal direction. We have found that δχobs=(NAμB/Ahf)δKobs where Ahf is the hyperfine coupling constant, is in good agreement with spin susceptibilities χγel calculated from an enhanced electronic specific heat γel and χnmr from the quasiparticle Korringa relation T1TKs2=const. This gives direct evidence that the χs of heavy quasiparticles in CeCu2Si2 and UPd2Al3 is rather isotropic and decreases to zero as T→0 due to the Cooper-pair formation. On the other hand in UPt3, the δχobsb,cs along the b- and c-axis in the non-unitary-pairing state (B phase) are two orders of magnitude smaller than χγel and χnmr. These anomalously small values for δχobsb,cs may suggest either that the spin degree of freedom in the B phase is not perfectly locked to the a-axis or that χs is not enhanced although γel is. The latter is theoretically pointed out by Ikeda and Miyake [J. Phys. Soc. Japan 66 (1997) 3714] to be possible if 5f electrons in the non-Kramerse singlet ground state for 5f2 are hybridized with conduction electrons. We need further effort towards coherent understanding of a microscopic mechanism leading to the occurrence of the odd-parity superconductivity in UPt3.
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