Quasparticle spin susceptibility in heavy-fermion superconductors: An NMR study compared with specific heat results

Quasiparticle spin susceptibility (χqp) for various heavy-fermion (HF) superconductors is discussed on the basis of the experimental results of NMR Knight shift (K), NMR relaxation rate (1/T1), and electronic specific heat (γel) within the framework of the Fermi liquid model for a Kramers doublet crystal electric field (CEF) ground state. χγ is calculated from the enhanced Sommerfeld coefficient γel, and χT1 from the quasiparticle Korringa relation T1T(KT1)2=const. via the relation of χT1=(NAμB/Ahf)KT1 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, δKobs=Kobs(Tc)-Kobs(T→0), below the superconducting transition temperature (Tc) is due to the decrease of the spin susceptibility of heavy quasiparticle estimated consistently from χγ and χT1. This result allows us to conclude that the heavy quasiparticles 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 χγ and χT1 are large enough to be probed experimentally. The odd-parity superconductivity is therefore concluded in these compounds. the NMR Knight shift results provide a convicting way to classify the HF superconductors into either even- or odd-parity pairing. (author)