Abstract
Possible order parameters of the phase IV in Ce x La 1- x B 6 are discussed with special attention to the lattice distortion recently observed. A Γ 5 u -type octupole order with a finite wave number is proposed as the origin of the distortion along the [111] direction. The Γ 8 crystalline electric field (CEF) level splits into three levels by a mean field with the Γ 5 u symmetry. The ground and highest singlets have the same quadrupole moment, while the intermediate doublet has an opposite sign. It is shown that any collinear order of the Γ 5 u -type octupole moment accompanies the Γ 5 g -type ferro-quadrupole order, and the coupling of the quadrupole moment with the lattice induces the distortion. The cusp in the magnetization at the phase transition is reproduced, but the internal magnetic field due to the octupole moment is smaller than the observed one by an order of magnitude.
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