Neutron scattering study on the field-induced O_xy-type antiferroquadrupolar ordering of the heavy-fermion superconductor PrOs₄Sb₁₂

Neutron scattering experiments have revealed a field-induced antiferroquadrupolar order parameter in the Pr-based heavy-fermion superconductor PrOs₄Sb₁₂. We observed the field-induced antiferromagnetic dipole moment with the propagation vector q=(100) for the applied field direction both H‖[110] and [001]. For H‖[110] at 8 T, it should be noted that the induced antiferromagnetic moment of 0.16(10) μ_B∕Pr orients parallel to the field within our experimental accuracy. This observation is strong evidence that the O_xy electric quadrupole is the primary order parameter for H‖[110]. A mean-field calculation, based on Γ₁ singlet and Γ₄⁽²⁾ excited triplet with the O_xy quadrupolar interaction, reproduces the induced moment direction and its field response. These facts indicate the predominance of O_xy-type antiferroquadrupolar interaction in PrOs₄Sb₁₂.