Magnetic Field Induced 4f Octupole in CeB6 Probed by Resonant X-Ray Diffraction

A mysterious antiferroquadrupolar ordered phase of CeB6 is considered as originating from the T-xyz-type magnetic octupole moment in magnetic fields. By resonant x-ray diffraction, we have verified that the T-xyz-type octupole is indeed induced in the 4f orbital of Ce with a propagation vector (1/2, 1/2, 1/2), thereby supporting the theory. We observed an asymmetric field dependence of the intensity for an electric quadrupole (E2) resonance when the field was reversed and extracted a field dependence of the octupole by utilizing the interference with an electric dipole (E1) resonance. The result is in good agreement with that of the NMR-line splitting, which reflects the transferred hyperfine field at the boron nucleus from the anisotropic spin distribution of Ce with an O-xy-type quadrupole.