Microscopic analysis of multipole susceptibility of actinide dioxides: A scenario of multipole ordering in AmO$_2$

TL;DR

This study uses a numerical renormalization group approach to analyze multipole susceptibilities in actinide dioxides, proposing a scenario for multipole ordering in AmO₂ based on possible ground states and high-order multipoles.

Contribution

It introduces a detailed analysis of multipole states in AmO₂ using a seven-orbital impurity Anderson model and suggests a new scenario for multipole ordering involving high-order multipoles.

Findings

AmO₂ may have a Γ₈⁻ quartet ground state.

High-order multipoles like quadrupole and octupole are relevant in AmO₂.

The ground state can be influenced by competition between spin-orbit coupling and Coulomb interactions.

Abstract

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 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 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 AmO$_2$.