Low temperature properties of Pr-based compounds with non-magnetic ground state

Praseodymium-based compounds with a Pr3+ ion of 4f2 configuration have effective magnetic moments of 3.6 μB/Pr at high temperature. However, the ground state is often non-magnetic when the Pr3+ ion has a nonmagnetic doublet ground state under a cubic crystal field. The twofold degeneracy is lifted by the ordering of the multipolar degrees of freedom. Such an example is PrPb3, whose CEF ground state is the non-magnetic Γ3 doublet. It was believed to involve an antiferroquadrupolar (AFQ) ordering at TQ=0.4 K, where electric quadrupoles align alternatively. We have determined the order parameter and the quadrupole ordering structure in the AFQ phase by angle-resolved magnetization and neutron diffraction measurements; the quadrupole O20 is aligned with a sinusoidally modulated structure, whereas O22 appears only at the fields higher than 6.5 T applied along the [110] direction. These suggest that the anisotropic interaction between the quadrupoles is an indirect RKKY-type interaction mediated by conduction electrons. Recently, superconductivity, structural transition and heavy fermion behavior have been observed in caged compounds PrT2Zn20 (T=Ru, Ir) with non-magnetic CEF ground states. (author)