Magnetic Form Factor of Elastic Neutron Scattering Expected for Octupolar Phases in Ce_1−xLa_xB₆ and NpO₂

抄録

Elastic neutron scattering spectra expected for ordered magnetic octupoles are studied theoretically for Ce_1−xLa_xB₆ and NpO₂. Assuming the order parameter deduced from the resonant X-ray scattering and NMR experiments, we have calculated spin and orbital parts of the magnetic form factors using the radial wave function for the single f-electron state in the Ce and Np ions. The octupolar contributions to the form factor at the superlattice spots are shown to be quite small in the long wave-length region, whereas it becomes markedly enhanced for short wave lengths. We discuss how and to what extent the intensity due to octupoles in short wave length is developed, and whether it is observable in experiments. The anisotropy of the form factor is discussed in connection with the symmetry aspect of the neutron cross section due to octupoles. In the case of Ce_1−xLa_xB₆, it is shown that the calculated results are consistent with recent observations by Kuwahara et al., thus confirming for the first time the existence of the ordered octupole by neutron scattering.