Quasparticle Spin Susceptibility in Heavy-Fermion Superconductors: An NMR Study Compared with Specific Heat Results

Quasi-particle spin susceptibility (χqp) for various heavy-fermion (HF) superconductors are discussed on the basis of the experimental results of electronic specific heat (γₑₗ), NMR Knight shift (K) and NMR relaxation rate (1/T₁) within the framework of the Fermi liquid model for a Kramers doublet crystal electric field (CEF) ground state. χqpγ is calculated from the enhanced Sommerfeld coefficient γₑₗ and χqpT₁ from the quasi-particle Korringa relation T₁T(KqpT₁)²=const. via the relation of χqpT₁=(NAμB/Ahf)KqpT₁ where Ahf is the hyperfine coupling constant, NA the Abogadoro's number and μB the Bohr magneton. For the even-parity (spin-singlet) superconductors CeCu₂Si₂, CeCoIn₅ and UPd₂Al₃, the fractional decrease in the Knight shift, δ Kᵒᵇˢ, below the superconducting transition temperature (Tc) is due to the decrease of the spin susceptibility of heavy quasi-particle estimated consistently from χqpγ and χqpT₁. This result allows us to conclude that the heavy quasi-particles form the spin-singlet Cooper pairs in CeCu₂Si₂, CeCoIn₅ and UPd₂Al₃. On the other hand, no reduction in the Knight shift is observed in UPt₃ and UNi₂Al₃, nevertheless the estimated values of χqpγ and χqpT₁ are large enough to be probed experimentally. The odd-parity superconductivity is therefore concluded in these compounds. The NMR result provides a convincing way to classify the HF superconductors into either even- or odd- parity paring together with the identification for the gap structure, as long as the system has Kramers degeneracy.