Evidence for magnetic-field-induced quadrupolar ordering in the heavy-fermion superconductor PrOs4Sb12

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 parallel to z). The analysis, based on the Gamma(1) 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 Gamma(1) singlet and the quadrupolar ordering is induced due to level crossing with an excited state under a magnetic field.