Low Temperature Properties of Pr-based Compounds with Non-magnetic Ground State

Abstract

Praseodymium-based compounds with a Pr³⁺ ion of 4f² configuration have effective magnetic moments of 3.6 μ_B/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 T_Q=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.