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 ${\text{Am}}^{4+}$ ion with five $5f$ electrons, it is considered that the ground state is ${\Gamma }_7^{-}$ doublet and the first-excited state is ${\Gamma }_8^{-}$ quartet, but we remark that the $f^5$ ground state is easily converted due to the competition between spin-orbit coupling and Coulomb interactions. Then, we find that the ${\Gamma }_8^{-}$ quartet can be the ground state of ${\text{AmO}}_2$ even for the same crystalline electric-field potential. In the case of ${\Gamma }_8^{-}$ quartet ground state, the numerical results suggest that high-order multipoles such as quadrupole and octupole can be relevant to ${\text{AmO}}_2$.

• Received 1 June 2009

DOI:https://doi.org/10.1103/PhysRevB.80.024408