Analysis of f-p model for octupole ordering in NpO2

TL;DR

This paper develops a microscopic f-p orbital model for NpO2 to investigate the origin of octupole ordering, revealing a tendency toward 9bau; antiferro-octupole order through numerical analysis.

Contribution

It introduces a new microscopic f-p model and derives effective multipole interactions using fourth-order perturbation theory to explain octupole ordering in NpO2.

Findings

Tendency toward 9bau; antiferro-octupole ordering identified

Effective multipole interactions derived from the f-p model

Numerical analysis supports octupole ordering hypothesis

Abstract

In order to examine the origin of octupole ordering in NpO2, we propose a microscopic model constituted of neptunium 5f and oxygen 2p orbitals. To study multipole ordering, we derive effective multipole interactions from the f-p model by using the fourth-order perturbation theory in terms of p-f hopping integrals. Analyzing the effective model numerically, we find a tendency toward \Gamma_{5u} antiferro-octupole ordering.