NMR studies on UPt3

A complete set of the Pt-195 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 palling state characterized by the two-component d vector like d(b) + id(c) 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 T-c, delta K-obs is independent of the crystal direction. We have found that delta chi(obs) = (N(A)mu(B)/A(hf))delta K-obs where A(hf) is the hyperfine coupling constant. is in good agreement with spin susceptibilities chi(gamma A) calculated from an enhanced electronic specific heat gamma(el) and chi(nmr) from the quasiparticle: Korringa relation T1TKs2 = const. This gives direct evidence that the chi(A) 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 delta chi(u)(bx)(h)(c)s along the b- and c-axis in the non-unitary-pairing stare (B phase) are two orders of magnitude smaller than chi(yet) and chi(nmr). These anomalously small values for delta chi(o)(bs)(h)(e)s may suggest either that the spin degree of freedom in the B phase is not perfectly locked to the a-axis or that chi(s) is not enhanced although gamma(el) is. The latter is theoretically pointed out by Ikeda and Miyake [J. Phys. See. Japan 66 (1997) 3714] to be possible if 5f electrons in the mon-Kramerse singlet ground state for 5f(2) 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. (C) 2000 Elsevier Science B.V. All rights reserved.