Microscopic theory of multipole ordering in NpO2

TL;DR

This paper develops a microscopic model for NpO2, revealing that multipole interactions lead to a specific octupole order, enhancing understanding of its mysterious ordered phase.

Contribution

It introduces a new f-electron model based on a j-j coupling scheme and demonstrates the emergence of b3b5b5 octupole order through numerical and mean-field analysis.

Findings

Identification of relevant b3b4b5b4 and b3b5b5b5 moments for ground state

Confirmation of longitudinal triple-q b3b5b5 octupole order in NpO2

Insight into multipole interactions and anisotropy effects

Abstract

In order to examine the mysterious ordered phase of NpO2 from a microscopic viewpoint, we investigate an f-electron model on an fcc lattice constructed based on a j-j coupling scheme. First, an effective model with multipole interactions is derived in the strong-coupling limit. Numerical analysis of the model clearly indicates that the interactions for \Gamma_{4u} and \Gamma_{5u} moments are relevant to the ground state. Then, by applying mean-field theory to the simplified model including only such interactions, we conclude that longitudinal triple-q \Gamma_{5u} octupole order is realized in NpO2 through the combined effects of multipole interactions and anisotropy of the \Gamma_{5u} moment.