Microscopic analysis of multipole susceptibility of actinide dioxides: A scenario of multipole ordering in AmO2

By evaluating multipole susceptibility of a seven-orbital impurity Anderson model with the use of a numerical renormalization group method, we discuss possible multipole states of actinide dioxides at low temperatures. In particular, here we point out a possible scenario for multipole ordering in americium dioxide. For Am⁴⁺ ion with five 5f electrons, it is considered that the ground state is Γ₇^- doublet and the first-excited state is Γ₈^- quartet, but we remark that the f⁵ ground state is easily converted due to the competition between spin-orbit coupling and Coulomb interactions. Then, we find that the Γ₈^- quartet can be the ground state of AmO₂ even for the same crystalline electric-field potential. In the case of Γ₈^- quartet ground state, the numerical results suggest that high-order multipoles such as quadrupole and octupole can be relevant to AmO₂ .