First-Principles Theory of Multipolar Order in Neptunium Dioxide

TL;DR

This paper develops a first-principles theoretical framework for understanding complex multipolar order in NpO$_2$, revealing the significant roles of higher-order multipoles in its ordered phase.

Contribution

It introduces a materials-specific, first-principles LDA+U approach to describe multipolar order and superexchange in NpO$_2$, including higher-order multipoles often neglected.

Findings

Active non-dipolar degrees of freedom in NpO$_2$'s ordered phase.

Higher-order multipoles like hexadecapoles are as significant as lower-order ones.

The theory accurately describes the triple-q antiferro ordered phase without dipolar moments.

Abstract

We provide a first-principle, materials-specific theory of multipolar order and superexchange in NpO$_2$ by means of a non-collinear local-density approximation +$U$ (LDA+$U$) method. Our calculations offer a precise microscopic description of the triple-$q$-antiferro ordered phase in the absence of any dipolar moment. We find that, while the most common non-dipolar degrees of freedom (e.g., electric quadrupoles and magnetic octupoles) are active in the ordered phase, both the usually neglected higher-order multipoles (electric hexadecapoles and magnetic triakontadipoles) have at least an equally significant effect.