Anisotropic spin fluctuations in the heavy fermion systems; Case studies of CePdAl and NpPdAl

Sakai, Hironori   ; Chudo, Hiroyuki ; Tokunaga, Yo  ; Kambe, Shinsaku  ; Haga, Yoshinori   ; Ronning, F.*; Bauer, E. D.*; Thompson, J. D.*; Homma, Yoshiya*; Aoki, Dai*; Nakano, Yasunori*; Honda, Fuminori*; Settai, Rikio*; Onuki, Yoshichika

In the so-called 115 family members with chemical formulae of CeIn (=Co, Rh, Ir) and Ga (=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 -wave superconductivity with higher . In order to examine this working hypothesis, the anisotropy ratios of antiferromagnetic fluctuations are derived from our NMR results for the superconductor NpPdAl and antiferromagnet CePdAl, which are tetragonal compounds with a 115-related structure. CePdAl shows Ising-type anisotropy, which is the same anisotropy as in the ordered state. On the other hand, NpPdAl shows XY-type anisotropy just above .