Hidden order and multipolar exchange striction in a correlated f-electron system

TL;DR

This paper uncovers the hidden multipolar order in NpO₂ using a low-energy Hamiltonian derived from ab initio methods, revealing complex multipolar interactions and an unconventional exchange-striction mechanism.

Contribution

It provides a detailed theoretical description of the hidden order in NpO₂, identifying rank-5 multipolar order and a novel exchange-striction mechanism through first-principles calculations.

Findings

Identification of rank-5 multipolar order in NpO₂

Discovery of an unconventional exchange-striction mechanism

Explanation of volume contraction in the hidden order phase

Abstract

The nature of order in low-temperature phases of some materials is not directly seen by experiment. Such "hidden orders" (HO) may inspire decades of research to identify the mechanism underlying those exotic states of matter. In insulators, HO phases originate in degenerate many-electron states on localized f or d shells that may harbor high-rank multipole moments. Coupled by inter-site exchange, those moments form a vast space of competing order parameters. Here, we show how the ground state order and magnetic excitations of a prototypical HO system, neptunium dioxide NpO$_2$, can be fully described by a low-energy Hamiltonian derived by a many-body ab initio force-theorem method. Superexchange interactions between the lowest crystal-field quadruplet of Np$^{4+}$ ions induce a primary non-collinear order of time-odd rank-5 (triakontadipolar) moments with a secondary quadrupole order preserving the cubic symmetry of NpO$_2$. Our study also reveals an unconventional multipolar exchange-striction mechanism behind the anomalous volume contraction of the NpO$_2$ HO phase.