Anisotropic Spin Fluctuations in the Heavy Fermion Systems: Case studies of CePd5Al2 and NpPd5Al2

In the so-called 115 family members with chemical formulae of CeTIn5 (T=Co, Rh, Ir) and AnTGa(5) (An=U, Np, Pu), our analysis of the Knight shift and spin-lattice relaxation rates suggests that larger XY-type anisotropy in the antiferromagnetic spin fluctuations in the normal states would favor d-wave superconductivity with higher T-c. In order to examine this working hypothesis, the anisotropy ratios of antiferromagnetic fluctuations are derived from our NMR results for the superconductor NpPd5Al2 and antiferromagnet CePd5Al2, which are tetragonal compounds with a 115-related structure. CePd5Al2 shows Ising-type anisotropy, which is the same anisotropy as in the ordered state. On the other hand, NpPd5Al2 shows XY-type anisotropy just above T-c.