Evidence for octupole order in Ce_0.7La_0.3B ₆ from resonant X-ray scattering

The azimuthal angle dependence observed in the resonant X-ray scattering in phase IV of Ce_0.7La_0.3B₆ is analyzed theoretically. It is shown that the peculiar angle dependence observed in the E2 channel is consistent with the Λ_5u-type octupole order with principal axis along [111] and equivalent directions. Under the assumption that the four equivalent octupole domains are nearly equally populated in the sample, the observed angle dependences are reproduced by calculation for both ωω′ and ωπ′ polarizations. The calculation for various symmetries of order parameters unambiguously excludes order parameters other than the Λ_5u-type octupole.