NMR studies on UPt₃

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 d_b+id_c at low T and low H. 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 T_c, δK_obs is independent of the crystal direction. We have found that δχ_obs = (N_Aμ_B/A_hf)δK_obs where A_hf is the hyperfine coupling constant, is in good agreement with spin susceptibilities χ_γ(el) calculated from an enhanced electronic specific heat γ_el and χ_nmr from the quasiparticle Korringa relation T₁TK_s² = const. This gives direct evidence that the χ_s 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 δχ_obs^b,c along the b- and c-axis in the non-unitary-pairing state (B phase) are two orders of magnitude smaller than χ_γ(el) and χ_nmr. These anomalously small values for δχ_obs^b,c may suggest either that the spin degree of freedom in the B phase is not perfectly locked to the a-axis or that χ_s is not enhanced although γ_el is. The latter is theoretically pointed out by Ikeda and Miyake 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.