The multipole ordered phase in Ce 0.7 La 0.3 B 6, emerging below 1.5 K and named phase IV, has been studied by resonant x-ray diffraction in magnetic fields. By utilizing diamond x-ray phase plates to rotate the incident linear polarization and a conventional crystal analyzer system, full linear polarization analysis has been performed to identify the order parameters. The analysis shows that the Γ 5g (O yz, O zx, O xy) quadrupoles are more induced by the field than the Γ 3g (O 20 and O 22) quadrupoles on the Γ 5u (T β x+y+z) antiferro-octupole order in phase IV. The problem is that this result is contradictory to a mean-field calculation, which inevitably gives the Γ 3g quadrupole as the main induced moment. This result indicates that the Γ 5g quadrupole order is close in energy. We consider that a large fluctuation of the Γ 5g quadrupole is hidden behind the primary ordering of the Γ 5u octupole and that the multipolar fluctuation significantly affects the ordering phenomenon.

This work was supported by Grants-in-Aid for Scientific Research (Nos. 21204456 and 24340087) from the Japan Society for the Promotion of Science (JSPS), by a Grant-in-Aid for Scientific Research on Innovative Areas “Heavy Electrons” (No. 21102515) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), and by a Basic Science Research Grant from the Sumitomo Foundation.