Highly correlated electron behavior in NpCu5−x(Pd,Ni)x-a contrast to UCu5−x(Pd,Ni)x

We report the low-temperature specific heats of NpCu5−xPdx (x=0, 0.5, 1.0, and 1.5), the “plus one f-electron” analogs to the well-studied non-Fermi liquid (nFl) system UCu5−xPdx. The Np materials were shown to form in the same AuBe5 structure type as the U pseudobinary alloys. A number of differences between the Np and U materials are found. The magnetic transition at 15 K in the U system is evidently suppressed to lower temperatures in the Np analog, and the low-temperature C/T values are strongly enhanced in the NpCu5−xPdx (x=1.0 and 1.5) alloys vs. their U counterparts. Finally, NpCu4Ni appears to show non-Fermi liquid behavior similar to that of the NpCu5−xPdx system, just as UCu4Ni is quite similar to UCu4Pd in its nFl behavior.