Microscopic theory of the insulating electronic ground states of actinide dioxides AnO2 (An=U, Np, Pu, Am, and Cm)

TL;DR

This paper uses first-principles calculations to analyze the electronic ground states of actinide dioxides, revealing how spin-orbit and Coulomb interactions lead to insulating states with compound-specific mechanisms.

Contribution

It provides a systematic microscopic understanding of the insulating mechanisms in AnO2 compounds through detailed analysis of 5f states and multipolar moments.

Findings

Nonmagnetic PuO2 and CmO2 are insulating without symmetry breaking.

UO2, NpO2, and AmO2 require magnetic transitions for insulating states.

Occupancy of An-f orbitals is key to the insulating mechanisms.

Abstract

The electronic ground states of the actinide dioxides AnO2 (with An=U, Np, Pu, Am, and Cm) are investigated employing first-principles calculations within the framework of the local density approximation +U (LDA+U) approach, implemented in a full-potential linearized augmented plane-wave scheme. A systematic analysis of the An-5f states is performed which provides intuitive connections between the electronic structures and the local crystalline fields of the f states in the AnO2 series. Particularly the mechanisms leading to the experimentally observed insulating ground states are investigated. These are found to be caused by the strong spin-orbit and Coulomb interactions of the 5f orbitals; however, as a result of the different configurations, this mechanism works in distinctly different ways for each of the AnO2 compounds. In agreement with experimental observations, the nonmagnetic states of plutonium and curium dioxide are computed to be insulating, whereas those of uranium, neptunium, and americium dioxides require additional symmetry breaking to reproduce the insulator ground states, a condition which is met with magnetic phase transitions. We show that the occupancy of the An-f orbitals is closely connected to each of the appearing insulating mechanisms. We furthermore investigate the detailed constitution of the noncollinear multipolar moments for transverse 3q magnetic ordered states in UO2 and longitudinal 3q high-rank multipolar ordered states in NpO2 and AmO2.