NMR studies on UPt 3

Abstract

A complete set of the 195Pt Knight-shift (KS) data on the superconducting (SC) state in UPt 3 identified the spin structure of the Cooper pair corresponding to the multiple SC phases. UPt 3 was acclaimed as the first odd-parity superconductor including a non-unitary pairing state characterized by the two-component d vector like d b+ i d 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 CeCu 2Si 2 and UPd 2Al 3. 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 δ χ obs=( N A μ B/ A hf)δ K 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 T 1 TK s 2=const. This gives direct evidence that the χ s of heavy quasiparticles in CeCu 2Si 2 and UPd 2Al 3 is rather isotropic and decreases to zero as T→0 due to the Cooper-pair formation. On the other hand in UPt 3, the δ χ obs b, c s 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 δ χ obs b, c s 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 [J. Phys. Soc. Japan 66 (1997) 3714] to be possible if 5f electrons in the non-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 UPt 3.