Evidence for Magnetic-Field-Induced Quadrupolar Ordering in the Heavy-Fermion Superconductor PrOs₄Sb₁₂

Abstract

Neutron diffraction experiments on the heavy-fermion superconductor PrOs₄Sb₁₂ revealed that a small antiferromagnetic moment, parallel to [010] (y-direction), is induced in the field-induced phase (H||z). The analysis, based on the Γ₁ singlet ground state crystal field model, shows that antiferro-order of O_yz-type quadrupole moments of Pr ions is formed in the field-induced phase. This strongly suggests that the crystal field ground-state of Pr ions in PrOs₄Sb₁₂ is the Γ₁ singlet and the quadrupolar ordering is induced due to level crossing with an excited state under a magnetic field.